Language：English   Publishing type：Research paper (scientific journal)  

Measurement of the Electron Cyclotron Emission (ECE) spectrum is one of the most popular electron temperature diagnostics in nuclear fusion plasma research. A 2-dimensional ECE imaging systemwas developed with an adaptive-Array approach. Aradio-frequency (RF) heterodyne detection system with Software Defined Radio (SDR) devices and a phased-Array receiver antenna was used to measure the phase and amplitude of the ECE wave. The SDR heterodyne system could continuously measure the phase and amplitude with sufficient accuracy and time resolution while the previous digitizer system could only acquire data at specific times. Robust streaming phase measurements for adaptive-Arrayed continuous ECE diagnostics were demonstrated using Fast Fourier Transform (FFT) analysis with the SDR system. The emission field pattern was reconstructed using adaptive-Array analysis. The reconstructed profileswere discussed using profiles calculated from coherent single-frequency radiation from the phase array antenna.

DOI： 10.1088/1748-0221/11/04/C04010

Language：English   Publishing type：Research paper (scientific journal)  

A phased array antenna and Software-Defined Radio (SDR) heterodyne-detection systems have been developed for adaptive array approaches in reflectometry on the QUEST. In the QUEST device considered as a large oversized cavity, standing wave (multiple wall-reflection) effect was significantly observed with distorted amplitude and phase evolution even if the adaptive array analyses were applied. The distorted fields were analyzed by Fast Fourier Transform (FFT) in wavenumber domain to treat separately the components with and without wall reflections. The differential phase evolution was properly obtained from the distorted field evolution by the FFT procedures. A frequency derivative method has been proposed to overcome the multiple-wall reflection effect, and SDR super-heterodyned components with small frequency difference for the derivative method were correctly obtained using the FFT analysis.

Language：English   Publishing type：Research paper (international conference proceedings)  

The broadband fast frequency-switching heterodyne radiometers have been developed to measure simultaneously three-band components of electron cyclotron emission with high time and frequency resolutions in the large helical device (LHD). The fast frequency-switching system was prepared as a local oscillator part of the heterodyne radiometers. The frequency switching time was 10 mu s. The fundamental electron cyclotron emission spectrum was successfully measured using the radiometer in the LHD experiment.

Language：English   Publishing type：Research paper (international conference proceedings)  

The amplitude modulation reflectometer system has been developed to measure the density profile in detail on the Compact PWI ( plasma-wall interaction) Device (CPD) experiments. The probing frequency range was 6-18 GHz, and its frequency-switching time was 10 mu s in an arbitrary step. Using the broadband fast-scanning reflectometer, the wide-range density profile can be measured with high time resolution. In X-mode reflectometory, the developed system successfully measured the phase delay of propagating waves at 6.6, 8.8 and 13.2 GHz, reflected at the vessel wall of the CPD and the R-cutoff layers in the CPD plasma.

Language：English   Publishing type：Research paper (international conference proceedings)  

The fields radiated from the waveguide were used to analyze propagating mode content in an oversized circular corrugated waveguide. The radiated fields were calculated by the Kirchhoff Integral in the paraxial approximation. The irradiant waveguide modes of the radiated field are orthogonal within the paraxial approximation. The waveguide mode content was evaluated from the irradiant waveguide modes. The radiated intensity and phase profiles were directly measured with a vector network analyzer system at 84GHz for the analysis. The analyzed waveguide mode content was consistent with that excited in the waveguide, predicted by the coupling theory.

Language：English   Publishing type：Research paper (scientific journal)  

The accuracy of a phase-retrieval procedure for microwave beams was analyzed and verified using an 84-GHz quasi-optical beam transmitted by a corrugated waveguide. The output intensity and phase profiles of the beam were directly measured with a microwave receiver. The beam intensity was also measured at several planes and the data were used with a phase-retrieval algorithm to yield an independent estimate of the phase. Moment and matching coefficient methods were used to analyze the retrieved profiles. In the higher order moment analysis, the intensity profiles, which were calculated from the retrieved phase profiles, coincided well with the measured profiles in terms of the beam size, asymmetric property (skewness), and flatness/peakedness (kurtosis) over the entire profile. Analysis indicated that the mode content of the beam obtained by phase retrieval matched that of the direct measurement with a matching coefficient of 0.97 using both the intensity and phase profiles.

DOI： 10.1109/TMTT.2006.884632

Language：English   Publishing type：Research paper (scientific journal)  

A remote steering antenna was recently developed for electron cyclotron heating and current drive (ECH/ECCD) experiments on the TRIAM- I M tokamak. This is the first application of the remote steering antenna concept for ECH/ECCD experiments, which have conditions relevant to the International Thermonuclear Experimental Reactor (ITER). Fundamental ECH and ECCD experiments were conducted in the ITER frequency from the low field using this antenna system. In addition to the angles near 0 degrees, the launcher was a symmetric direction antenna with an extended steering-angle capability of +/- (8 degrees-19 degrees). The output beam from the antenna was a well-defined Gaussian with a proper steering angle. The Gaussian content and the steering-angle accuracy were 0.85 and -0.5 degrees, respectively. The high power tests measured the antenna transmission efficiency at 0.90-0.94. The efficiencies obtained in the low and high power tests were consistent with the calculations using higher-order modes. In order to excite the pure O/X-modes in the oblique injection, two polarizers were used to control the elliptical polarization of the incident beam for the ECCD experiments. The fundamental O/X-mode ECH/ECCD was applied to lower hyrid current drive plasmas at the optimized incident polarization. In the X-mode experiment, at medium density (similar to 1 x 10(19) m(-3)), clear differences in the plasma current and the hard x-ray intensity were observed between the co- and counter-steering injections due to the ECCD effect on the coupling of forward fast electrons.

DOI： 10.1088/0029-5515/46/5/001

Language：English   Publishing type：Research paper (international conference proceedings)  

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Language：English   Publishing type：Research paper (scientific journal)  

Publisher：EPJ Web of Conferences  

ECCD system design activities for JA DEMO is in progress. Activities contains physical analysis, launcher design, and RF power plant design. The physical analysis activity explores various approaches to enhance ECCD efficiency. The RF power plant design activity produced a conceptual design of a 100 MW class RF power plant. The launcher design activity presented the upper and equatorial launcher port of the JA DEMO tokamak vacuum vessel and analyzed the ECCD operation of each launcher. The result is the achievement of RF power deposition at the core of plasma for ECCD by selecting the proper injection angle and frequency.

DOI： 10.1051/epjconf/202431301001

Scopus

Publisher：Nuclear Fusion  

Superconducting (SC) tokamak JT-60SA plays an essential role in fusion research and development by supporting and complementing the ITER project, providing directions to the DEMO design activity and fostering next generation scientists and engineers. Since the short circuit incident at the terminal joints of equilibrium field coil #1 during the integrated commissioning (IC) in March 2021, both EU and JA implementing agencies (IAs) have examined how to ensure safe operation of JT-60SA by mitigating the risk of possible discharge occurrence inside the cryostat. Based on the experience of the global Paschen tests, the IAs have established a strategy of risk mitigation measures, which is a combination of (i) reinforcement of insulation, (ii) avoiding unnecessary voltage application to the coil systems and (iii) immediate de-energization of the coils when deteriorated vacuum conditions are detected. Thanks to the considerable efforts of the Integrated Project Team members, the IC restarted in May 2023. After confirmation of the SC state of the coil systems (TF, EF and CS), the coil energization test and the plasma operation phase 1 (OP-1) started. The first plasma was successfully achieved on 23 October 2023 with a limited value of voltage and current applied to the coils. The plasma configuration control was also confirmed with low plasma current and low auxiliary heating power conditions. Based on the IO-F4E-QST collaboration, activities of JT-60SA have been shared with the IO and provided an important lesson for ITER assembly and commissioning, and will provide an outstanding contribution to fusion research at large. After OP-1, maintenance & enhancement phase 1 (M/E-1) starts from January 2024, in which in-vessel components are installed, and heating and diagnostic systems are extensively upgraded to allow a high power heating experiment planned in OP-2. In order to make the best use of JT-60SA, a newly organized JT-60SA experiment team will refine the research plan for the future high heating power operation phase.

DOI： 10.1088/1741-4326/ad34e4

Web of Science

Scopus

Publisher：Plasma Physics and Controlled Fusion  

A comparative investigation was conducted to assess the impact of high-field side (HFS) extraordinary (X) mode (HFS X) injection and low-field side (LFS) ordinary (O) mode (LFS O) injection of 8.2 GHz radio frequency (RF) power in the Q-shu University experimental steady-state spherical tokamak. In the case of the HFS X injection, the ratio of electron plasma frequency and RF frequency f pe / f RF was 1.3, indicating an overdense plasma, whereas in the LFS O injection, this ratio was 0.9. Distinctive features emerged in the electron temperature and density profiles between the two injection scenarios. In the HFS X injection, the profiles peaked between the fundamental electron cyclotron resonance (ECR) layer and the upper hybrid resonance layer. Conversely, in the LFS O injection, the peaks were situated near the 1st ECR layer. This implies effective excitation of the electron Bernstein wave (EBW) in the case of the HFS X injection, with the wave power being absorbed through the doppler-shifted ECR of the EBW. Density and temperature oscillations were observed only in the start-up phase of the HFS X injection, which may correlate with the presence of the left-hand cutoff of the X-mode. These findings will be contributed to understand the distinctive characteristics associated with plasma heating through EBW excitation in the HFS X injection.

DOI： 10.1088/1361-6587/ad802e

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Scopus

Publisher：Nuclear Fusion  

The plasma current start-up experiment is conducted through electron cyclotron (EC) heating in the QUEST spherical tokamak. During the EC heating, the application of a toroidal electric field in the opposite direction to the plasma current effectively inhibits the growth of energetic electrons. Observations show rapid increases in plasma current and hard x-ray count immediately following the cancellation of the retarding electric field. When a compact tokamak configuration maintains equilibrium on the high field side, along with the retarding field, it leads to effective bulk electron heating. This heating achieved an electron temperature of Te ≈ 1 keV at electron density ne > 1.0 × 1018 m−3. Ray tracing of the EC wave verifies that more power absorption into plasma through a single-pass occurs around the second resonance layer with higher values of electron density and temperature.

DOI： 10.1088/1741-4326/ad6914

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Scopus

Publisher：Nuclear Fusion  

The elimination of the need for an Ohmic heating solenoid may be the most impactful design driver for the realization of economical compact fusion tokamak reactor systems. However, this would require fully non-inductive start-up and current ramp-up from zero plasma current and low electron temperature of sub-keV to the full plasma current of ∼10-15 MA at 20-30 keV electron temperature. To address this challenge, an efficient solenoid-free start-up and ramp-up scenario utilizing a low-field-side-launched extraordinary mode at the fundamental electron cyclotron harmonic frequency (X-I) is proposed, which has more than two orders of magnitude higher electron cyclotron current drive (ECCD) efficiency than the conventional ECCD for the sub-keV start-up regime. A time dependent model was developed to simulate the start-up scenarios. For the Spherical Tokamak Advanced Reactor (STAR) (Menard et al 2023 Next-Step Low-Aspect-Ratio Tokamak Design Studies (IAEA)), it was found that to fully non-inductively ramp-up to 15 MA, it would take about 25 MW of EC power at 170 GHz. Because of the relatively large plasma volume of STAR, radiation losses must be considered. It is important to make sure that high Z impurities are kept sufficiently low during the early current start-up phase where the temperature is sub-keV range. Since the initial current ramp up takes place at a factor of ten lower density compared to the sustained regimes, it is important to transition into a higher bootstrap fraction discharge at lower density to minimize the ECCD power requirement during the densification. For the sustainment phase an array of eight gyrotron launchers with a total of about 60 MW of fundamental O-mode was found to be sufficient to provide the required axis-peaked external current drive. High efficiencies between 19-57 kA MW−1 were found with optimal aiming, and these were resilient to small changes in aiming angles and density and temperature profiles.

DOI： 10.1088/1741-4326/ad556f

Web of Science

Scopus

Publisher：Nuclear Fusion  

Electron cyclotron wall conditioning with neon gas (Ne-ECWC) has been performed on the normal conducting spherial tokamak QUEST with metal walls under a trapped particle configuration with O-mode EC waves including X-mode polarization with a frequency of 8.2 GHz and an injection power of 16 kW. The Ne-ECWC removes hydrogen from the wall with small neon retention. The Ne-ECWC decreases hydrogen recycling at the following tokamak discharges, contributing to an improvement of the following tokamak plasma start-up: the plasma current increases and the start-up timing of the plasma current shifts forward. However, defects such as voids and bubbles are formed on tungsten surface exposed to the Ne-ECWC plasma.

DOI： 10.1088/1741-4326/ad3d6e

Web of Science

Scopus

Language：English   Publisher：Transdisciplinary Research and Education Center for Green Technologies, Kyushu University  

During the early phase of tokamak plasma start-up prior to the formation of a closed magnetic surface, various plasma current profiles are expected to be observed. Especially, the noninductive current drive (CD) by the electron Bernstein wave (EBW) is likely to be the most dominant, and the current profile is expected to focus on a local area predicted by the theoretical model. In addition, the EBWCD direction could be affected by a horizontal magnetic field induced by a toroidal magnetic field coil and feed-through. The magnetic flux data cannot be fitted by using a previous model described by Yoshinaga23), and hence, this model needs improvement. Consequently, an improved model is introduced in this paper based on the theoretical prediction of EBWCD. This model can be applied to reproduce the complex plasma current profile accurately during the early phase of the tokamak plasma start-up.

DOI： 10.5109/7183306

CiNii Research

Publisher：Plasma Physics and Controlled Fusion  

The electron temperature and density profiles in the midplane of a spherical tokamak plasma produced by electron cyclotron heating (ECH) in Q-shu University experiment with steady-state spherical tokamak (QUEST) are measured by the helium line intensity ratio method. The measured profiles are compared with those obtained by the Thomson scattering method, and the measured temperatures and densities are found to agree within factors of ∼2 and ∼6, respectively. Taken together with the previous results of comparisons performed in the scrape-off layers of several toroidal devices, the same degree of agreement between the helium line intensity ratio method and other methods is obtained in the ranges of 7-100 eV for temperature and 4 × 1016-1 × 1019 m−3 for density.

DOI： 10.1088/1361-6587/ad2c2a

Web of Science

Scopus

Publisher：Nuclear Fusion  

Results from the successful solenoid-free plasma startup using the method of transient coaxial helicity injection (transient CHI) in the QUEST spherical tokamak (ST) are reported. Unlike previous applications of CHI on HIT-II and on NSTX which required two toroidal insulating breaks to the vacuum vessel, QUEST uses a first of its kind, floating single biased electrode configuration, which does not use such a vacuum break. Instead, the CHI electrode is simply insulated from the outer lower divertor plate support structure. This configuration is much more suitable for implementation in a fusion reactor than the previous configurations. Transient CHI generated toroidal currents of 135 kA were obtained. The toroidal current during the formation of a closed flux configuration was over 50 kA. These results bode well for the application of transient CHI in a new generation of compact high-field STs and tokamaks in which the space for the central solenoid is very restricted.

DOI： 10.1088/1741-4326/ad0dd6

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Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Nuclear Fusion  

A novel divertor biasing by four biasing plates distributed toroidally (TDDB) on the upper divertor target plate is applied to low density tokamak plasmas started-up by 28 GHz 2nd harmonic electron cyclotron current drive (ECCD) in a quasi-double null configuration of the QUEST spherical tokamak (ST). In the ST plasmas of line averaged electron density < n e > ∼0.7-1 × 1018 m−3, about 20%-40% of the current I bias driven by ∼85 V sawtooth bias voltage reaches the lower divertor plate along the biased scrape-off-layer (SOL) flux tube as the SOL current I SOL . The fact I SOL is noticeably lower than I bias indicates an appreciable leakage of parallel current from the biased SOL flux tube. The leakage currents in the toroidal and radial directions are confirmed by detection of them using the unbiased plates in the TDDB experiments. From the ion saturation current density profile obtained by a divertor Langmuir probe array, the fall-off lengths of divertor particle flux are estimated together with the strike line position. Total particle flux to the upper divertor, evaluated by the integrated ion saturation current density profile is reduced by up to 45% during positive biasing of the TDDB, depending on the position of the strike line to the biased plate. In addition, the TDDB also induces a noticeable loss of fast electrons produced by ECCD, leading to an ∼2% reduction in the maximum toroidal current of the ST plasma compared to a shot without the TDDB. Reduction of the divertor particle flux and enhancement of the fast electron losses are thought to be dominantly caused by E × B drift induced by the TDDB. In the present experimental conditions, the effects of magnetic perturbations produced by the SOL currents on the fast electron losses can be neglected because of a too small SOL current.

DOI： 10.1088/1741-4326/acee13

Web of Science

Scopus

Publisher：AIP Conference Proceedings  

JA DEMO is a steady-state tokamak which produces several hundreds of MW of electric power. Electron cyclotron heating and current drive (ECH/CD) system is one of the major components of JA DEMO tokamak, and it is expected to play an important role in its plasma operation scenario. Design activities for JA DEMO ECH/CD are in progress regarding both physics and engineering. Analysis of ECCD physics for current drive efficiency improvement shows that an upper-side injection can produce sufficient driven current by avoiding absorption at second harmonic resonance. RF power for ECCD operation of JA DEMO is expected to be 100 MW at the frequency of >200 GHz range. The conceptual design of the RF power plant represents the location and scale of the building for the RF power plant. The transmission Line (TL) for RF power delivery from the RF power plant to the tokamak exceeds 200 m, and studies of TL components for highly efficient RF transmission are in progress. The concept of the launcher, which combines remote steering technology and phased array, is evaluated by a simulation. A low-power experiment and preliminary results demonstrate that the large oblique angle injection necessary for ECCD operation is possible.

DOI： 10.1063/5.0162701

Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：The Japan Society of Plasma Science and Nuclear Fusion Research  

<p>Non-inductive plasma current start-up experiments have been conducted using 2^ndharmonic electron-cyclotron resonance heating (ECRH) on the QUEST spherical tokamak. Sudden or abrupt changes in plasma current, referred to as sudden change events (SCEs), have been frequently observed during these experiments. To investigate these events, fast magnetic activity was measured globally by installing arrays of magnetic pick-up coils inside the vacuum vessel on QUEST. Analysis revealed that all observed SCEs could be classified into three categories based on their magnetic characteristics. In one type of event, called SCE–III, the plasma current suddenly decreases by up to 50% instantaneously, occurring in a wide discharge region achieved using 2^ndECRH start-up on QUEST. These SCE–III events may potentially lead to disruptions in future discharges at higher powers.</p>

DOI： 10.1585/pfr.18.2402066

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Scopus

CiNii Research

Language：English   Publishing type：Research paper (scientific journal)  

DOI： https://doi.org/10.1016/j.fusengdes.2023.113794

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Nuclear Fusion  

Spatially resolved rotational temperature of ground state hydrogen molecules desorbed from plasma-facing surface was measured in QUEST, LTX-β, and DIII-D tokamaks, and the increases of the rotational temperature with the surface temperature and due to collisional-radiative processes in the plasmas were evaluated. The increase due to collisional-radiative processes was calculated by solving rate equations considering electron and proton collisional excitation and deexcitation and spontaneous emission. The calculation results suggest a high sensitivity for the rotational temperature to electron and proton densities, but a negligible sensitivity to the electron, proton, and surface temperatures. In the three tokamaks with different plasma parameters and plasma-facing surface materials, the spatial profile of the rotational temperature was estimated using Fulcher-α emission lines (600-608 nm). In QUEST, the spatial profile of the rotational temperature was estimated from spatially resolved spectra. In the other tokamaks, the rotational temperature was evaluated assuming a single point emission with a location determined from the Fulcher-α emission profile as measured with a filtered camera. In metal-walled devices QUEST and LTX-β, the rotational temperature increased with the surface temperature, and the calculated collisional-radiative increase is consistent with measured increase assuming that the rotational temperature at the surface is approximately 500-600 K higher than the surface temperature. In DIII-D with carbon walls, a larger collisional-radiative increase than the other tokamaks was observed because of the higher density leading to a large difference from the calculated increase compared to the other smaller tokamaks. Measurement of the Fulcher-α emission profile with higher spatial resolution in DIII-D may reduce the difference and reveal the effect of the surface temperature on the rotational temperature. These results show the increases in the rotational temperature with the surface temperature and due to the collisional-radiative processes.

DOI： 10.1088/1741-4326/acd4d1

Web of Science

Scopus

Language：English   Publisher：The Japan Society of Plasma Science and Nuclear Fusion Research  

<p>A database has been developed to provide a highly efficient data analysis environment by registering various types of data obtained in the QUEST (Q-shu University Experiment with Steady-State Spherical Tokamak) experiments and providing the data to researchers through standard SQL query output. In conventional experiments, measurements are usually obtained using several instruments, and the results are often stored separately in proprietary formats. An analysis to collect distributed files and convert them into appropriate formats individually was necessary, which led to a decrease in analysis efficiency. In order to build a system to centrally store and provide data, we constructed new servers, each for registering data in a database and for extracting data from the database and visualizing the data. As a result, various real-time monitoring data can be registered in the database, and data retrieval for analysis can be performed easily with a unified user interface. The construction of this database has made it possible to easily retrieve real-time monitoring data and a wide variety of table data, enabling analysis from a new perspective, and improving analysis efficiency.</p>

DOI： 10.1585/pfr.18.1305048

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Scopus

CiNii Research

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Fusion Engineering and Design  

Here, a versatile power supply system, consisting of two current sources, insulated-gate bipolar transistor stacks, and a capacitor bank, is proposed for application in various operations of the QUEST spherical tokamak. The central solenoid coil in this system can carrying a maximum current and voltage of ±8 kA and ±375 V, respectively. In plasma start-up experiments based on ohmic heating through double flux swing, a plasma current of over 100 kA is stably obtained. Moreover, the feedback control of the thyristor power supply is utilized to control the plasma current, plasma shape, and position. The oscillating central-solenoid current applies a bipolar loop voltage at constant time intervals, and the capacitor bank discharge facilitates a co-axial helicity injection as well as amplifies the toroidal current.

DOI： 10.1016/j.fusengdes.2023.113648

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Scopus

Language：English   Publisher：The Japan Society of Plasma Science and Nuclear Fusion Research  

<p>A motor-generator slowly converts low electrical power to the large rotating energy of the induction motor, which is then converted to the higher electrical power of the synchronous generator through a pulse. Such a cycle is repeated in a tokamak plasma experiment. In the transient phase at the start of the synchronous generator,high transient voltage appears in the armature windings repeatedly. Due to the repeatability, it is imperative to estimate the high voltage and control it so that the maximum voltage is kept under a tolerable value. A quaternion is a four-dimensional hypercomplex number that is good at describing three-dimensional rotation. Utilizing the quaternion capability, a three-phase motor-generator is analyzed using three-dimensional rotation. The mechanical rotation was anlyzed by the rotation quaternion. The salient pole-type rotating field can be manipulated by direct-quadrature conversion even in quaternion analysis. The rotating dynamics and electrical phenomena of a motor-generator can be analyzed by considering the quaternion power from the motor-generator and the electrical load for plasma control.</p>

DOI： 10.1585/pfr.18.2405035

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Scopus

CiNii Research

Language：English   Publishing type：Research paper (scientific journal)  

DOI： https://doi.org/10.1088/2058-6272/acafc2

Language：English   Publisher：The Japan Society of Plasma Science and Nuclear Fusion Research  

<p>The Thomson scattering control system has been modified to measure the time evolution of the electron density and temperature profiles during long duration discharges in the QUEST spherical tokamak. The system consists of a signal generator and a control circuit. The former accepts a QUEST main trigger and provides multiple triggers, each of which starts a short-term (e.g. 15 s) measurement. The latter provides triggers to synchronize the oscilloscope and laser oscillation during a short-term measurement. The system was used in 1000 s long duration discharges in QUEST, and the temporal evolutions of density and temperature profiles were obtained successfully. It was found the profiles are stationary after about 300 s.</p>

DOI： 10.1585/pfr.18.1405012

Web of Science

Scopus

CiNii Research

Publisher：Journal of Fusion Energy  

Transient coaxial helicity injection (CHI) is a promising candidate for solenoid-free plasma current start-up in a low-aspect-ratio tokamak in support of developing fully non-inductive scenarios. The aim of the transient CHI research on QUEST is to develop a reactor-relevant CHI design. On QUEST, a CHI discharge is initiated by driving current along magnetic field lines that connect an electrically insulated electrode plate (which is referred to as a bias electrode) to a vessel component at the bottom region of the spherical tokamak. In the first application of CHI on QUEST, the electrically insulated electrode plate was biased with respect to the outer vessel in a configuration referred to as low-field-side (LFS) injection. To maintain a narrow injector flux footprint width throughout the discharge, QUEST is now developing a high-field-side (HFS) injection configuration, in which the electrically insulated electrode plate is biased with respect to the inner vessel components. Through the implementation of a CHI-dedicated gas injection system, studies in the HFS injection configuration have now demonstrated good magnetic flux evolution into the vacuum vessel. Toroidal currents up to 43 kA have been achieved, and the generated current has increased in proportion to the magnetic flux connecting the electrodes. These results which show agreement with the CHI-scaling suggest that much higher levels of toroidal current can be generated on QUEST in an optimized CHI system in which the magnetic flux connecting the CHI electrodes is further increased.

DOI： 10.1007/s10894-022-00338-4

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Scopus

Language：English   Publisher：Review of Scientific Instruments  

Measuring the time variation of the wavenumber spectrum of turbulence is important for understanding the characteristics of high-temperature plasmas, and the application of a Doppler reflectometer with simultaneous multi-frequency sources is expected. To implement this diagnostic in future fusion devices, the use of a phased array antenna (PAA) that can scan microwave beams without moving antennas is recommended. Since the frequency-scanning waveguide leaky-wave antenna-type PAA has a complex structure, we have investigated its characteristics by modeling it with 3D metal powder additive manufacturing (AM). First, a single waveguide is fabricated to understand the characteristics of 3D AM techniques, and it is clear that there are differences in performance depending on the direction of manufacture and surface treatment. Then, a PAA is made, and it is confirmed that the beam can be emitted in any direction by frequency scanning. The plasma flow velocity can be measured by applying the 3D manufacturing PAA to plasma measurement.

DOI： 10.1063/5.0101723

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Scopus

PubMed

Language：English   Publisher：Review of Scientific Instruments  

A heavy ion beam probe (HIBP) has been designed for the QUEST spherical tokamak to measure plasma turbulence and the profiles of electric potential profiles. Using a cesium ion beam with an energy of several 10 keV, the observable region covers most of the upper half of the plasma. Although the probe beam is deflected by the poloidal magnetic field produced by plasma current and poloidal coil currents, it can be detected under plasma current up to 150 kA by modifying the trajectories with two electrostatic sweepers. According to the numerical estimation of the intensity of the detected beam, sufficient signal intensity for measuring plasma turbulence can be obtained over almost the measurable area when the electron density is up to 1 × 1019 m-3, which is larger than the cut-off density of electron cyclotron heating in QUEST. The performance of the designed HIBP is sufficient to explore the mechanisms of heat and particle transport in magnetically confined plasmas, including the influence of plasma wall interactions, which is a goal of the QUEST project.

DOI： 10.1063/5.0101770

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Scopus

PubMed

Publisher：IEEE Transactions on Plasma Science  

Transient coaxial helicity injection (T-CHI), a method first developed on the small helicity injected torus (HIT-II) experiment and then validated on the much larger National Spherical Torus Experiment (NSTX) device, is a method to initiate an inductive-like tokamak plasma discharge without reliance on the central solenoid. A CHI discharge is initiated by driving current along magnetic flux that connects the inner and outer divertor plates on one end of the tokamak. To permit this, on both HIT-II and NSTX, toroidal ceramic insulators were used to electrically separate the inner and outer vessel components. The use of such large toroidal vacuum insulators may not be easy to implement in reactors. To address this issue, the Q-shu University experiments with steady-state spherical tokamak (ST) (QUEST) is developing a reactor-relevant CHI configuration in which one of the divertor plates is electrically insulated from the rest of the vessel. The first application of T-CHI on QUEST biased the CHI electrode to the outer vessel. While the CHI discharges could be easily generated, it was found that as the discharge filled the vessel, the separation distance between the injector magnetic flux footprints widened, a condition that is not favorable for the generation of closed flux surfaces. Biasing the electrode to the inner wall is a configuration similar to that used on NSTX and HIT-II, but initial testing in this configuration has proved to be challenging. The design described here overcomes the present limitation by locating the CHI electrode much closer to the CHI injector flux coil and using an NSTX-like gas injection manifold to enable high-field-side T-CHI startup on QUEST. The concepts described in this article should also benefit the future implementation of T-CHI systems in other tokamaks and spherical tokamaks.

DOI： 10.1109/TPS.2022.3193069

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Scopus

Language：English   Publisher：Physical Review E  

An accessibility enhanced efficient fundamental X-mode electron cyclotron heating (ECH) current start-up regime was identified for a reactorlike toroidal magnetic field range which has more than 100 times higher current drive efficiency compared to more conventional ECH methods for the relevant start-up temperature range. Very high current drive efficiency is possible due to the strong cyclotron interaction only with unidirectional passing electrons constrained by the wave accessibility conditions. This efficient electron cyclotron current drive regime may help facilitate the design of innovative economical solenoid-free tokamak fusion reactor systems.

DOI： 10.1103/PhysRevE.106.L023201

Scopus

PubMed

Language：English   Publisher：Elsevier B.V.  

In order to generate high-performance plasma for future fusion power generation, it is desirable to keep high quality vacuum during experiment. Mass spectrometer is commonly used to monitor the vacuum quality and to record the amount of atoms and molecules in the vacuum vessel. Leak is the most serious accident to avoid that can nullify an experiment and even harm researchers. Detecting leaks are ever more important since it can be easily overlooked, e.g., when the deterioration in the vacuum degree is modest. This forces the researcher to carefully observe the vacuum and mass spectrometer data. This article presents a way to suggest potential leaks in the vacuum vessel by analyzing mass spectrometer data. This is done by utilizing the Euclidean distance between composition ratios at different times for the clustering using the daily composition ratio. We show that our cluster analysis is an effective way of separating these two cases, which results in a semi-automatic determination of leaks is more efficient than the current norm, which is to check many measures to find a small abnormality in the data manually. We plan further model improvements for long-term evaluation.

DOI： 10.1016/j.fusengdes.2022.113199

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Scopus

CiNii Research

Publisher：Nuclear Fusion  

Construction of the JT-60SA tokamak was completed on schedule in March 2020. Manufacture and assembly of all the main tokamak components satisfied technical requirements, including dimensional accuracy and functional performances. Development of the plasma heating systems and diagnostics have also progressed, including the demonstration of the favourable electron cyclotron range of frequency (ECRF) transmission at multiple frequencies and the achievement of long sustainment of a high-energy intense negative ion beam. Development of all the tokamak operation control systems has been completed, together with an improved plasma equilibrium control scheme suitable for superconducting tokamaks including ITER. For preparation of the tokamak operation, plasma discharge scenarios have been established using this advanced equilibrium controller. Individual commissioning of the cryogenic system and the power supply system confirmed that these systems satisfy design requirements including operational schemes contributing directly to ITER, such as active control of heat load fluctuation of the cryoplant, which is essential for dynamic operation in superconducting tokamaks. The integrated commissioning (IC) is started by vacuum pumping of the vacuum vessel and cryostat, and then moved to cool-down of the tokamak and coil excitation tests. Transition to the super-conducting state was confirmed for all the TF, EF and CS coils. The TF coil current successfully reached 25.7 kA, which is the nominal operating current of the TF coil. For this nominal toroidal field of 2.25 T, ECRF was applied and an ECRF plasma was created. The IC was, however, suspended by an incident of over current of one of the superconducting equilibrium field coil and He leakage caused by insufficient voltage holding capability at a terminal joint of the coil. The unique importance of JT-60SA for H-mode and high-β steady-state plasma research has been confirmed using advanced integrated modellings. These experiences of assembly, IC and plasma operation of JT-60SA contribute to ITER risk mitigation and efficient implementation of ITER operation.

DOI： 10.1088/1741-4326/ac10e7

Scopus

Language：English   Publishing type：Research paper (scientific journal)  

DOI： DOI: 10.1585/pfr.17.2405025

Language：English   Publishing type：Research paper (scientific journal)  

DOI： https://doi.org/10.1088/1741-4326/ac29cf

Language：English   Publishing type：Research paper (scientific journal)  

DOI： https://doi.org/10.1088/1361-6587/ac1838

Language：English   Publishing type：Research paper (scientific journal)  

DOI： https://doi.org/10.1585/pfr.16.2402090

Language：English   Publishing type：Research paper (scientific journal)  

DOI： https://doi.org/10.1016/j.nme.2021.101013

Language：English   Publishing type：Research paper (scientific journal)  

DOI： https://doi.org/10.1585/pfr.16.2402048

Language：English   Publishing type：Research paper (scientific journal)  

DOI： https://doi.org/10.1016/j.fusengdes.2021.112362

Language：English   Publishing type：Research paper (scientific journal)  

DOI： https://doi.org/10.1016/j.nme.2021.100905

Language：English   Publishing type：Research paper (scientific journal)  

DOI： https://doi.org/10.1016/j.nme.2020.100872

Language：English   Publishing type：Research paper (scientific journal)  

DOI： https://doi.org/10.1063/5.0031357

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1063/5.0013966

Language：English   Publishing type：Research paper (scientific journal)  

DOI： https://doi.org/10.1585/pfr.15.2402063

Language：English   Publishing type：Research paper (scientific journal)  

DOI： https://doi.org/10.3390/atoms8030044

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1016/j.heliyon.2020.e04214

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1585/pfr.15.2402013

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1088/1361-6587/ab6903

Language：English   Publishing type：Research paper (scientific journal)  

A high voltage DC power supply for the cathode of the 8.56 GHz CW klystron has been set up for electron cyclotron heating (ECH) in steady state tokamak operation on QUEST spherical tokamak. The power supply is equipped with an IGBT array and a reactor for fast shutoff of the voltage in 10 μs, where the influx of electric energy at the short circuit is limited to 5 J. AC switches also have been installed in the three-phase power lines. High voltage relays are useful to save electric energy consumption. Fast three-phase AC switching by IGBT-stack is applicable to reduce the electric load of the components of the klystron power supply.

DOI： 10.1016/j.fusengdes.2019.04.043

Language：English   Publishing type：Research paper (scientific journal)  

A new transmission line from 28 GHz gyrotron to QUEST spherical tokamak (ST) has been developed for the highly efficient electron cyclotron heating and current drive (ECHCD) experiments. The ECHCD effect depends on the incident mode or polarization states of the injected beam. Although two corrugated-polarizer miter bends were developed and assembled to transmission line in the QUEST, arcing events were frequently detected at the polarizer miter bend when the high-power millimeter-wave were transmitted. A new quasi-optical (QO) concept for the polarizer system composed of two corrugated plate and a QO mirror was proposed to avoid the arcing at polarizer. The QO mirror was designed for coupling of input and output HE₁₁ modes at the polarizer, its coupling property is discussed with Kirchhoff integral. Polarization-setting performance of the developed QO polarizer was examined in a low-power test system, and is discussed with grating-polarizer calculation.

DOI： 10.1016/j.fusengdes.2019.02.099

Language：English   Publishing type：Research paper (scientific journal)  

In the EC-driven (8.2 GHz) steady-state plasma on QUEST, plasma current seems to flow in the open magnetic surface in the outside of the closed magnetic surface in the low-field region according to plasma current fitting method without taking equilibrium into account. In our previous work, plasma equilibrium solution was fitted assuming all plasma current is flowing in the inside of the Last Closed Flux Surface (LCFS). It was solved within isotropic pressure profile by EFIT code. Opposite-polarity current density region appeared in the high-field region. Here in this article, considering the toroidal rotation, the equilibrium is fitted within nested magnetic surfaces by SU-EFIT. Though the plasma magnetic axis shifts outward due to the centrifugal force, the opposite polarity current does not disappear in the high-field region. And relation between the toroidal rotation speed and the poloidal beta value will be discussed.

DOI： 10.1016/j.fusengdes.2019.04.059

Language：Japanese   Publishing type：Research paper (scientific journal)  

DOI： DOI: 10.1585/pfr.14.3402122

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1585/pfr.14.3402111

Language：Japanese   Publishing type：Research paper (scientific journal)  

DOI： DOI: 10.1585/pfr.14.1402128

Language：Japanese   Publishing type：Research paper (scientific journal)  

DOI： https://doi.org/10.1088/1741-4326/ab1858

Language：English   Publishing type：Research paper (scientific journal)  

This research investigated fuel particle balance during long duration discharge in an all-metal plasma facing wall (PFW)through intensive QUEST execution. A simple wall model including the plasma-induced deposition layer that creates hydrogen (H)barriers, called the H barrier model, was established. A simple calculation, based on a combination of H state rate equations and the H barrier model, was applied to real plasma in the early phase of its longest discharge. The model accurately reconstructed the evolutions of electron density and wall-stored H over time, proper values are chosen for the parameters that are difficult to determine experimentally. Comparative calculations that used the H barrier and a fully reflective models, predicted significant impacts of wall models on the plasma density time response and value of electron density, indicating that a proper wall model should be developed for all-metal PFW devices.

DOI： 10.1016/j.nme.2019.03.015

Language：English   Publishing type：Research paper (scientific journal)  

DOI： https://doi.org/10.1016/j.fusengdes.2019.04.043

Language：English   Publishing type：Research paper (scientific journal)  

Megawatt (MW) gyrotrons, with a wide frequency range from 14 to 300 GHz, are being developed as part of a collaborative electron cyclotron heating (ECH) study for advanced fusion devices and a demonstration power plant (DEMO). (1) Detailed designs for a 14 GHz 1 MW gyrotron are being developed for fabrication. For a 14 GHz radio frequency (RF) beam with high divergence, a calculated transmission efficiency of 94% to the corrugated waveguide coupling position was obtained initially by introducing the design concept (direct RF beam coupling by built-in waveguide) to minimize the RF transmission path. Installing a double-disk sapphire window will make it possible to develop a 1 MW gyrotron with a continuous wave (CW) at 14 GHz. (2) In experimental tests of a new 28/35 GHz dual-frequency gyrotron, the cooling characteristics of an optimal-structure double-disk sapphire window were evaluated. We confirmed that operating at 0.4 MW with a CW at 28 GHz is feasible, reaching twice the output power reported in previous studies. In a 2 ms short-pulse experimental test, maximum powers of 1.65 MW at 28.04 GHz and 1.21 MW at 34.83 GHz were achieved. (3) A design study of a 77/51 GHz dual-frequency gyrotron was performed. Oscillations above 1.5 MW for 77 GHz and 1.3 MW for 51.88 GHz are expected for a beam voltage V _k = 80 kV and beam current I _k = 60 A. (4) In an experiment with a 300 GHz gyrotron, the influence of the wave reflected from the window was reduced by tilting the output window, and mode competition in the cavity was suppressed. An output power of 0.62 MW with a pulse width of 1 ms, which is the new record for this frequency, was obtained. (5) We also performed a trial design study of a 240 GHz gyrotron for DEMO.

DOI： 10.1088/1741-4326/ab0e2c

Language：English   Publishing type：Research paper (scientific journal)  

DOI： https://doi.org/10.1088/1741-4326/ab0e2c

Language：English   Publishing type：Research paper (scientific journal)  

DOI： DOI: 10.1585/pfr.14.1205038

Language：English   Publishing type：Research paper (scientific journal)  

DOI： https://doi.org/10.1016/j.fusengdes.2019.02.099

Language：English   Publishing type：Research paper (scientific journal)  

New polarizer and launcher systems on a 28-GHz electron-cyclotron (EC) heating and current drive (HCD) system have been developed for non-inductive second-harmonic EC-plasma-current ramp-up in the QUEST spherical tokamak. A launcher system with two quasi-optical mirrors providing beam steering capability was designed to focus the incident beam to a small-sized waist of ∼0.05 m at the second-harmonic EC resonance layer. A relatively large focusing mirror was designed based on a Kirchhoff integral code developed to derive wave solutions. The focusing property of the launched beam was first confirmed with a 3-dimensional (3D) electromagnetic-wave simulator. Focusing characteristics were also checked at low-power test facilities, together with the steering capability. The performance of this launcher system was demonstrated to work as designed, and assembled in the QUEST device. The system was applied to the non-inductive second-harmonic EC plasma ramp-up experiments with no optimization required regarding the incident polarization. The results obtained for the non-inductive plasma ramp-up are also presented.

DOI： 10.1016/j.fusengdes.2019.02.027

Language：English   Publishing type：Research paper (scientific journal)  

A two-dimensional (2D) phased-array-antenna (PAA) for electron cyclotron emission is one of the diagnostics for finding the window area of mode conversion from electron cyclotron waves to electron Bernstein wave. Spatial resolution of the reconstructed emission field pattern is improved with a lot of the antenna elements. The study proposes a phased-array patch loop antenna (PAPA) as a promising system to increase the elements of PAA. A 2D image reconstruction of a 6 GHz radio frequency source emission shows that the source position is detected within millimeter range by applying the prototype PAPA. This result indicates that the 2D image reconstruction with multi-element PAPA is feasible.

DOI： 10.1585/PFR.14.3402111

Language：English   Publishing type：Research paper (scientific journal)  

High field side (HFS) injection of eXtra-ordinary X-mode for electron Bernstein wave (EBW) conversion was conducted in the QUEST tokamak. Radio frequency (RF; 8.2 GHz) power was delivered from the low field side (LFS) to the high field side HFS through waveguides, and from the HFS placed 20 cm above the midplane of the vacuum vessel. The aim was to compare the RF launches from the LFS and HFS. The plasma brightness, measured by a fast camera, as well as the H _α signal captured along the mid-plane, was noticeably higher in the HFS launch than in the LFS launch. The HFS injection achieved a plasma current of approximately 130 A, versus 35A in the LFS injection. The electron density n _e predicted from the position of the upper hybrid resonance agreed with the line-averaged n _e measured by an interferometer, confirming the effective conversion and subsequent damping of the EBW mode. The RF leakage of the HFS injection was less than one-sixth that of the LFS injection. These results indicate that HFS delivers better RF coupling and conversion efficiency to EBW than LFS injection. Such efficient plasma heating via EBW will significantly enhance the plasma production.

DOI： 10.1585/pfr.14.1205038

Language：English   Publishing type：Research paper (international conference proceedings)  

A collective scattering method using the sub-THz gyrotron radiation is planned to apply to the direct detection of the electron Bernstein wave in the QUEST where the electron Bernstein wave plays one of the main heating/current drive roles in sustaining steady state spherical tokamak configuration.

DOI： 10.1109/IRMMW-THz.2018.8510425

Language：English   Publishing type：Research paper (scientific journal)  

Initial results from the recently implemented transient coaxial helicity injection (CHI) system on QUEST are reported. QUEST uses a new CHI electrode configuration in which the CHI insulator is not part of the vacuum boundary, making this configuration easier to implement in fusion reactors. Experimental results show that transient CHI startup in this alternate electrode configuration is indeed possible. Reliable gas breakdown was achieved, and toroidal currents up to 45 kA were generated.

DOI： 10.1088/1361-6587/aadcb7

Language：English   Publishing type：Research paper (scientific journal)  

Intensity fluctuations are investigated using the fast camera imaging technique in the slab annular plasma as a function of magnetic shear and connection length in the spherical tokamak QUEST. Note that here QUEST is operated as a simple magnetized torus with a tight aspect ratio. Slab annular plasmas feature open magnetic field lines and can mimic the tokamak edge-scrape off layer (SOL)-like plasma attributes reasonably well. Three magnetic shear regimes are realized using three poloidal magnetic field (PF) coil pairs. A whole range of connection lengths (∼∞ ≥ L _c ≥ 5.5 m) is scanned by varying the PF strength for a given toroidal field for each magnetic shear regime. For the first time a systematic study of the effect of magnetic shear and field line pitch together on edge-SOL-like plasma fluctuations is being reported. Slab plasmas with intermediate magnetic shear are observed to be more susceptible to generate distinct blobs when L _c is reduced by increasing the PF strength. A distinct coherent mode appears only at the lowest magnetic shear slab featuring a deep potential well. Such mode is not apparent at other magnetic shear cases even at the same L _c. Finally, with a combination of PF coil pairs, both the features of intermediate and low magnetic shear slabs are shown to be realizable simultaneously. Significantly stronger blobs are observed with such combination of PF mirror ratios in the presence of a coherent mode. This study may provide better insight into the effect of magnetic configuration in the tokamak edge and SOL turbulence and can help in searching for better tools to control cross-field convective intermittent transport in tokamaks.

DOI： 10.1088/1361-6587/aacb69

Language：English   Publishing type：Research paper (scientific journal)  

In tokamaks, the temperature of the plasma-facing wall is an important parameter for achieving particle balance and therefore steady-state operation. QUEST, which is a middle-sized spherical tokamak, has hot walls that act as plasma-facing walls. They can be actively heated with sheath heaters and actively cooled with water. To control the wall temperature, heating and cooling systems have been developed. These systems adjust the power of the sheath heaters and the motor valves of the cooling system, respectively. The two systems communicate via Ethernet through UDP and control the hot-wall temperature cooperatively. The plasma control system (PCS) in QUEST has also been modified, especially with respect to gas fueling, in order to enable long-duration plasma sustainment. A feedback controller has been installed in the PCS, together with a mass flow controller, allowing Hα emission from the plasma which is used as a reference signal, to be well controlled. Plasma density calculations using a field-programmable gate array are proposed for the feedback control system.

DOI： 10.1016/j.fusengdes.2018.02.069

Language：English   Publishing type：Research paper (scientific journal)  

DOI： https://doi.org/10.1016/j.fusengdes.2018.02.069

Language：English   Publishing type：Research paper (scientific journal)  

Toroidal electron cyclotron resonance (ECR) plasma is an ECR heated plasma in an open toroidal magnetic field. The plasma contains no toroidal current and is used for the pre-ionization and non-inductive startup of a tokamak plasma. To obtain a deeper understanding of the basic properties of the plasmas produced in the spherical tokamak QUEST (Q-shu University Experiment with steady-state Spherical Tokamak), we have developed an optical emission spectroscopy system with multiple viewing chords and have used it to measure the spatial distributions of the toroidal and poloidal flow velocities of C²⁺ ions. We compare the measured velocities with those calculated from the ion drift equations using the plasma parameters reported for a similar spherical tokamak, LATE (the Low Aspect Ratio Torus Experiment).

DOI： 10.1585/PFR.13.3402087

Language：English   Publishing type：Research paper (scientific journal)  

In the present RF-driven (ECCD) steady-state plasma on QUEST, the plasma current seems to flow in the open magnetic surface outside of the closed magnetic surface in the low-field region according to the plasma current fitting (PCF) method. The current in the open magnetic surface is due to the orbit-driven current by high-energy particles in the RF-driven plasma. High-energy particles guiding center orbits are calculated as contour plots of conserved variables in the Hamiltonian formulation considering particles in the initial position with different energies and pitch angles satisfying the resonance condition. A negative current appears near the magnetic axis, and a hollow current profile is expected even if the pressure driven current is considered. The equilibrium is fitted within nested magnetic surfaces by J-EFIT coded by MATLAB using the hollow current profile shift toward the low-field region. Although the plasma boundary shape reflects the plasma current density profile, the equilibrium shape fitted by J-EFIT does not coincide with the orbit-driven current profile. However, introducing an extension of the current profile without assuming nested contours into the J-EFIT code appropriately fits the plasma shape with the hollow current profile to the measured magnetic data.

DOI： 10.1016/j.fusengdes.2017.05.070

Language：English   Publishing type：Research paper (scientific journal)  

Fully non-inductive plasma maintenance was achieved by a microwave of 8.2 GHz and 40 kW for more than 1 h 55 min with a well-controlled plasma-facing wall (PFW) temperature of 393 K, using a hot wall in the middle-sized spherical tokamak QUEST, until the discharge was finally terminated by the uncontrollability of the density. The PFW was composed of atmospheric plasma-sprayed tungsten and stainless steel. The hot wall plays an essential role in reducing the amount of wall-stored hydrogen and facilitates hydrogen recycling. The behaviour of fuel hydrogen in the PFW was investigated by monitoring the injection and evacuation of hydrogen into and from the plasma-producing vessel. A fuel particle balance equation based on the presence of a hydrogen transport barrier between the deposited layer and the substrate was applied to the long-duration discharges. It was found that the model could readily predict the observed behaviour in which a higher wall temperature likely gives rise to faster wall saturation.

DOI： 10.1088/1741-4326/aa8121

Language：English   Publishing type：Research paper (scientific journal)  

After several experimental campaigns in the Kyushu University Experiment with Steady-state Spherical Tokamak (QUEST), the originally stainless steel plasma-facing wall (PFW) becomes completely covered with a deposited film composed of mixture materials, such as iron, chromium, carbon, and tungsten. In this work, an innovative colorimetry-based method was developed to measure the thickness of the deposited film on the actual QUEST wall. Because the optical constants of the deposited film on the PFW were position-dependent and the extinction coefficient k₁ was about 1.0-2.0, which made the probing light not penetrate through some thick deposited films, the colorimetry method developed can only provide a rough value range of thickness of the metal-containing film deposited on the actual PFW in QUEST. However, the use of colorimetry is of great benefit to large-area inspections and to radioactive materials in future fusion devices that will be strictly prohibited from being taken out of the limited area.

DOI： 10.1063/1.5000739

Language：English   Publishing type：Research paper (scientific journal)  

In order to understand the atomic hydrogen distribution in different kinds of plasma and its influence on the recycling, two kinds of plasmas were used: non-confined annular electron cyclotron resonance (ECR) and confined long duration plasmas. The permeation probes are used to measure directly the atomic hydrogen flux at several poloidal positions. The permeation through metals due to the ion and atom component of the hydrogen flux to the wall is indistinguishable. To estimate the contribution of the ions directly, Langmuir probes were used. The Г_inc profile behind the plasma facing components (PFCs) is almost constant, ∼2 ×10¹⁸ H/s/m².

DOI： 10.1016/j.nme.2017.03.027

Language：English   Publishing type：Research paper (scientific journal)  

In tokamak operations, breakdown of plasma is the first step of the plasma build-up. In this paper, we present a combinative investigation of radio frequency (RF)-induced breakdown experiments in QUEST (Q-shu University Experiment with Steady-State Spherical Tokamak) and a one-point model of hydrogen ionization. Experimental results with two different frequencies of 2.45 GHz and 8.2 GHz showed that the clear threshold on connection length, L, existed for breakdown with a negative n-index configuration n=-(R/Bv)·(∂Bv/∂R), where R is the major radius and B_v the is vertical magnetic field. In contrast, breakdown was always obtained with positive n-index when changing L. It indicates that a lifetime of an incubated electron plays a significant role in the plasma breakdown. According to one-point model calculation, the experimental threshold of L is well predicted by the lifetime of the incubated electron estimated by employing the loss term along with L. The model calculation also describes the requirement of the minimum electron temperature T_e for RF-induced breakdown to realize an avalanche of electrons in the tokamak magnetic structure.

DOI： 10.1063/1.4985142

Language：English   Publishing type：Research paper (scientific journal)  

Nationally coordinated research on spherical tokamak is being conducted in Japan. Recent achievements include: (i) plasma current start-up and ramp-up without the use of the central solenoid by RF waves (in electron cyclotron and lower hybrid frequency ranges), (ii) plasma current start-up by AC Ohmic operation and by coaxial helicity injection, (iii) development of an advanced fuelling technique by compact toroid injection, (iv) ultra-long-pulse operation and particle control using a high temperature metal wall, (v) access to the ultra-high-β regime by high-power reconnection heating, and (vi) improvement of spherical tokamak plasma stability by externally applied helical field.

DOI： 10.1088/1741-4326/aa62c1

Language：English   Publishing type：Research paper (scientific journal)  

Megawatt power gyrotrons are being developed for collaborative electron cyclotron heating (ECH) studies of advanced fusion devices and demonstration power plant (DEMO). (1) In the first experiment of a 300 GHz gyrotron, an output power above 0.5 MW in the TE_32,18 single mode was achieved with a pulse width of 2 ms. This was the first observation of MW-scale oscillations in a DEMO-relevant gyrotron mode. It was also found that the reflection at the output window affected the determination of the oscillation mode. Furthermore, several single mode oscillations in the 226-254 GHz range were confirmed, which is important for the step-frequency tunable gyrotron in the sub-THz frequency range. (2) Based on the successful results of the 77 and 154 GHz large helical device (LHD) tubes, a new 154/116 GHz dual-frequency gyrotron with an output of over 1.5 MW is being designed. (3) A new record output of 1.38 MW was obtained using an existing 28 GHz gyrotron. A newly designed tube aimed at achieving a dual-frequency output power of 2 MW at 28 GHz (0.4 MW continuous wave) and 1 MW at 35 GHz was built. In the first experimental test, main mode oscillations were observed at the frequencies of 28.036 and 34.831 GHz with Gaussian-like output beams and output power of 1.27 and 0.48 MW, respectively. A total efficiency of 50% was achieved at 28 GHz operation.

DOI： 10.1088/1741-4326/aa6875

Language：English   Publishing type：Research paper (scientific journal)  

In a high-power long-pulse millimeter-wave transmission line for electron cyclotron heating and current drive (ECH/ECCD), the ohmic loss on the grooved mirror surface of polarizers is one of the important issues for reducing the transmission loss. In this paper, the ohmic loss on the mirror surface is evaluated in simulated real-scale polarizer miter bends for different groove parameters under a linearly-polarized incident wave excitation. The polarizers with low ohmic loss are optimally designed for a new 28 GHz transmission line on the QUEST spherical tokamak. The calculated optimum ohmic loss is restricted to only less than 1.5 times as large as the theoretical loss for a copper flat mirror at room temperature. The copper rounded-rectangular grooves of the polarizers were relatively easy to make smooth in mechanical machining and the resultant surface roughness was not more than 0.15 μm, which is only 0.38 times as large as the skin depth. The combination of the designed elliptical polarizer and the polarization rotator can also realize any polarization state of the reflected wave.

DOI： 10.1016/j.fusengdes.2016.11.019

Language：English   Publishing type：Research paper (scientific journal)  

A typographical error of spelling occurred in author list of the published article. The correct author list is described as above on this page.

DOI： 10.1585/pfr.12.1902026

Language：English   Publishing type：Research paper (scientific journal)  

Coaxial Helicity Injection (CHI) has now been implemented in QUEST. The goals for the first transient CHI experiments were to establish reliable gas breakdown conditions, and to measure CHI-produced toroidal current generation. Both these objectives were successfully met. Toroidal currents up to 29 kA were measured. Interestingly, these first plasmas on QUEST also suggest the formation of small amounts of closed magnetic flux surfaces.

DOI： 10.1585/pfr.12.1202020

Language：English   Publishing type：Research paper (scientific journal)  

Fully non-inductive plasma start-up was successfully achieved by using a well-controlled microwave source on the spherical tokamak, QUEST. Non-inductive plasmas were maintained for approximately 3-5 min, during which time power balance estimates could be achieved by monitoring wall and cooling-water temperatures. Approximately 70%-90% of the injected power could be accounted for by calorimetric measurements and approximately half of the injected power was found to be deposited on the vessel wall, which is slightly dependent on the magnetic configuration. The power distribution to water-cooled limiters, which are expected to be exposed to local heat loads, depends significantly on the magnetic configuration, however some of the deposited power is due to energetic electrons, which have large poloidal orbits and are likely to be deposited on the plasma facing components.

DOI： 10.1088/1009-0630/18/11/03

Language：English   Publishing type：Research paper (scientific journal)  

The plasma control system (PCS) of QUEST is developed according to the progress of QUEST project. Since one of the critical goals of the project is to achieve the steady-state operation with high temperature vacuum vessel wall, the PCS is also required to have the capability to control the plasma for a long period. For the increase of the loads to processing power of the PCS, the PCS is decentralized with the use of reflective memories (RFMs). The PCS controls the plasma edge position with the real-time identification of plasma current and its position. This identification is done with not only flux loops but also hall sensors. The gas fueling method by piezo valve with monitoring the Hα signal filtered by a digital low-pass filter are proposed and suitable for the steady-state operation on QUEST. The present status and prospect of the PCS are presented with recent topics.

DOI： 10.1016/j.fusengdes.2016.04.016

Language：English   Publishing type：Research paper (other academic)  

The high power and long pulse operation of the gyrotron as well as efficient transmission of its output are quite important for achieving improved plasma performances. A 28 GHz 1 MW gyrotron developed for GAMMA 10/PDX achieved an output power of 1.38 MW in 2015 experiment after the power supply was improved. Furthermore, a new 28/35 GHz dual-frequency gyrotron (2 MW 3 s and 0.4 MW CW) for QUEST, NSTX-U, Heliotron J and GAMMA 10/PDX has been fabricated, after the preliminary test of a double-disk sapphire window installed in the gyrotron was performed. In the first experimental test, the oscillation of the main mode was confirmed at a frequency of 28.036 GHz with a Gaussian-like beam and an output power of 1.22 MW.

DOI： 10.1063/1.4964176

Language：English   Publishing type：Research paper (scientific journal)  

In order to evaluate the magnetic surface current density distributions, the electron guiding center orbits were calculated with the equilibrium fitting plasma equilibrium analysis. The current density distributions in electron cyclotron-heated plasma were estimated from the orbit analyses in the equilibrium plasma shaping for the fundamental and the second-harmonic resonant electrons separately. The current density distribution profiles on the equatorial plane were obtained for the electrons with initially positive and negative velocities parallel to the magnetic field direction as well as the trapped electrons. The surface averaged current density profiles of the closed flux surfaces were also evaluated. A significant amount of the positive current density distributions appeared outside the last closed flux surface (LCFS), while all the negative current density distributions were evaluated inside the LCFS. The trapped electrons being the second-harmonic electron cyclotron resonant contributed the negative current inside the LCFS. The positive current density distributions outside the LCFS were significant for the driven current density for both the fundamental and second-harmonic resonances.

DOI： 10.1109/TPS.2016.2570815

Language：English   Publishing type：Research paper (scientific journal)  

The response of the far scrape-off layer (far-SOL) to strong gas puffing (SGP), and its role as the boundary condition for core plasma, are investigated using a two-point Langmuir probe measurement in the high poloidal beta configuration in the QUEST spherical tokamak. The temperature and heat flux behave in an opposite way in the far-SOL and end-plate region after SGP, although SGP increases the density globally. The apparent density decay time in the far-SOL area is much longer than that in the core. Significant co-current flow is driven solely by the electron cyclotron wave in the far-SOL flow. Sheared flow is also observed in the perpendicular velocity profile during the recovered current flat-top phase, and such flow profiles are flattened by SGP. These flow profiles are attributed not only to drift-driven flow but also to transport-driven flow, the sink effect on the end-plate, and the balance of the neutral particle source.

DOI： 10.1088/0741-3335/58/11/115004

Language：English   Publishing type：Research paper (scientific journal)  

Fluctuations are measured in the edge and scrape-off layer (SOL) of QUEST using fast visible imaging diagnostic. Electron cyclotron wave injection in the Ohmic plasma features excitation of low frequency coherent fluctuations near the separatrix and enhanced cross-field transport. Plasma shifts from initial high field side limiter bound (inboard limited, IL) towards inboard poloidal null (IPN) configuration with steepening of the density profile at the edge. This may have facilitated the increased edge and SOL fluctuation activities. Observation of the coherent mode, associated plasma flow, and particle out-flux, for the first time in the IPN plasma configuration in a spherical tokamak may provide further impetus to the edge and SOL turbulence studies in tokamaks.

DOI： 10.1063/1.4960117

Language：English   Publishing type：Research paper (scientific journal)  

Poloidal and radial distributions of the hydrogen flux, both atomic and ion, to the plasma facing materials (PFMs) are measured with four fixed and one movable PdCu membrane probes in the QUEST spherical tokamak. Direct information about hydrogen retention in the areas far from main plasma-wall interaction is essential because the significant part of the global retention is due to atomic hydrogen. Incident hydrogen flux Γ_inc is numerically reconstructed by fitting the experimentally measured hydrogen flux Γ_p, permeated through the membrane. The sensitivity of the probe is improved when compared with other work which allows dynamic changes of the retention flux to be monitored, allowing detection of 10% of the Γ_inc modulation, or ∼10¹⁷ H m⁻²s⁻¹.

DOI： 10.1016/j.vacuum.2016.04.025

Language：English   Publishing type：Research paper (other academic)  

We investigate and calculate particle orbits and the effect of particle orbits on plasma current density for nonrelativistic resonance condition in the present RF-driven divertor plasma on QUEST. We surveyed particle orbits for different values of parallel refractive index, particle initial positions and pitch angles on fundamental and second harmonic resonance conditions. We observed that for fundamental harmonic resonance condition when particle orbits are plotted on the poloidal cross-section for positive values of parallel refractive index, these orbits are started from the resonance surface and produced their orbits around the LCFS (Large Closed Flux Surface). These orbits carry positive current. When particle orbits are plotted for negative values of parallel refractive index, these orbits are started from resonance surface, but remained at the inside of the LCFS. These orbits carry negative current that reduced the overall plasma current. For second harmonic resonance condition when particle orbits are plotted on the poloidal cross-section most of the orbits remained in inside the LCFS and carry positive current. When we consider the value of parallel refractive index-0.4 and +0.4 some particle orbits arrived at the limiter and become lost particles. On the other hand, when we consider particle initial positions 0.16 m or more vertically far from the mid plane some banana orbits are produced. These banana orbits make the current density profile maximum at low field side region. From this calculation we got a hollow current density profile with current density peak at the low field side region outside of the LCFS. From this calculation we can infer that parabolic current density profile is possible, if we set the resonance surface outside of the magnetic axis by increasing the toroidal magnetic field coil current and make the plasma position inward by increasing vertical field coil current.

DOI： 10.1109/SOFE.2015.7482308

Language：English   Publishing type：Research paper (scientific journal)  

Spontaneous toroidal rotation of the plasma is observed in the spherical tokamak QUEST with the help of electron cyclotron resonance (ECR) heating and without the use of any externally injected momentum. Several vertical magnetic field ( B_z) configurations with varying mirror ratio (M) (a measure of field curvature) are applied and evolution of rotation is studied with the help of Doppler spectroscopy of bulk and impurity ions. Significant toroidal rotation (V_φ-6km/s) is initiated in the open magnetic field configuration during the initial plasma breakdown phase, which is later sustained ( V_φ- 20km/s) in a closed magnetic field configuration in steady state. Rotational velocity is primarily along the cocurrent direction and is found to be proportional to the B_z strength and the resulting plasma current. High M and B_z are demonstrated to be the two specific external controls by which rotation can be initiated in the plasma. The rotation in open field lines is found to be initiated at the ECR layer in the slablike plasma, which is evolved to produce a sustained rotation in the natural divertor inboard poloidal field null equilibrium in QUEST.

DOI： 10.1109/TPS.2016.2522765

Language：English   Publishing type：Research paper (scientific journal)  

A phased array antenna and Software-Defined Radio (SDR) heterodyne-detection systems have been developed for adaptive array approaches in reflectometry on the QUEST. In the QUEST device considered as a large oversized cavity, standing wave (multiple wall-reflection) effect was significantly observed with distorted amplitude and phase evolution even if the adaptive array analyses were applied. The distorted fields were analyzed by Fast Fourier Transform (FFT) in wavenumber domain to treat separately the components with and without wall reflections. The differential phase evolution was properly obtained from the distorted field evolution by the FFT procedures. A frequency derivative method has been proposed to overcome the multiple-wall reflection effect, and SDR super-heterodyned components with small frequency difference for the derivative method were correctly obtained using the FFT analysis.

DOI： 10.1088/1748-0221/11/01/C01014

Language：English   Publishing type：Research paper (scientific journal)  

By employing the integral formulation of dielectric tensor, we have introduced the kinetic full wave analysis of ordinary-extraordinary-Bernstein (O-X-B) mode conversion in tokamak plasma in one dimension on the basis of TASK/W1 code using the finite element method. The boundary value problem of Maxwell's equation is solved and the finite Larmor radius effects are represented by integral formulation. The O-X-B mode conversion and the absorption near the cyclotron harmonic resonance are successfully described.

DOI： 10.1585/pfr.11.2403070

Language：English   Publishing type：Research paper (scientific journal)  

With the use of the density scale length and the magnetic field strength around the O-X mode conversion region in the LHD for 77 GHz EC waves, the full wave calculations by the TASK/WF2D code have been performed. The wave patterns obtained by full wave calculation have been compared with the trajectories obtained by multi ray tracing calculations. For the X-mode and the O-mode reflected near each cutoff, wave patterns obtained by full wave calculations and trajectories obtained by multi ray-tracing correspond to each other well. On the contrary, trajectories of the rays restarted at the high field side of the evanescent region between the plasma and the left handed cutoffs do not cover fully the wave patterns after the O-X mode conversion obtained by full wave calculations. Improvement of the manner to restart the ray-tracing is required. This improvement can provide a more precise view of the propagation of the EBWs and may contribute to examining scenarios for electron Bernstein wave heating.

DOI： 10.1585/pfr.11.2403098

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1016/j.fusengdes.2015.11.035

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1016/j.fusengdes.2015.12.022

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1016/j.fusengdes.2015.11.007

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1088/0029-5515/55/10/104018

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1016/j.fusengdes.2015.06.054

Language：Japanese   Publishing type：Research paper (bulletin of university, research institution)  

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1016/j.jnucmat.2014.12.092

Language：English   Publishing type：Research paper (scientific journal)  

Successful production of high β_p plasmas (εβ_p 1) fully non-inductively (NI) and their long pulse sustainment with the help of modest power (<100 kW) of electron cyclotron waves is demonstrated. High β_p plasmas are found for the first time to be naturally self organized to form a stable natural inboard poloidal field null (IPN) equilibrium. A critical β_p value is identified, which defines the transition boundary from inboard limiter (IL) to IPN equilibrium. A new feature of plasma self organization is evidenced, which enhances its negative triangular shape to sustain high β_p. These results show a relatively simple method to produce and sustain high β_p plasma close to the equilibrium limit in a stable configuration exploiting its self organization property.

DOI： 10.1088/0029-5515/55/8/083009

Language：English   Publishing type：Research paper (scientific journal)  

Heat flux and plasma flow in the scrape-off layer (SOL) are examined for the inboard poloidal field null (IPN) configuration of the spherical tokamak QUEST. In the plasma current (I_p) ramp-up phase, high heat flux (>1 MW/m²) and supersonic flow (Mach number M > 1) are found to be present simultaneously in the far-SOL. The heat flux is generated by energetic electrons excursed from the last closed flux surface. Supersonic flows in the poloidal and toroidal directions are correlated with each other. In the quasi-steady state, sawtooth-like oscillation of I_p at 20 Hz is observed. Heat flux and subsonic plasma flow in the far-SOL are modified corresponding to the I_p-oscillation. The heat flow caused by motion of energetic electrons and the bulk-particle transport to the far-SOL is enhanced during the low-I_p phase. Modification of plasma flow in the far SOL occurs earlier than the I_p crash. The M-I_p curve has a limit-cycle characteristic with sawtooth-like oscillation. Such a core-SOL relationship indicates that the far-SOL flow plays an important role in sustaining the oscillation of I_p in the IPN configuration.

DOI： 10.1063/1.4928878

Language：English   Publishing type：Research paper (scientific journal)  

Megawatt-class gyrotrons covering a wide frequency range (14 GHz-300 GHz) are in increasing demand for nuclear fusion. Recent electron cyclotron heating and electron cyclotron current drive experiments highlight a requirement of megawatt-scale gyrotrons at a relatively lower frequency (14-35 GHz) range of some plasma devices, like GAMMA 10/PDX of the University of Tsukuba, QUEST of Kyushu University, NSTX-U of Princeton Plasma Physics Laboratory, and Heliotron J of Kyoto University. Collaborative studies for designing a new 28 GHz/35 GHz dual-frequency gyrotron and a 14 GHz gyrotron have commenced. Operation above 1 MW of 28 GHz/35 GHz dual oscillation was demonstrated experimentally. Further in the design of dual-frequency gyrotron, operations with 2 MW 3 s and 0.4 MW CW (continuous wave) at 28 GHz, and power exceeding 1 MW for 3 s at 34.8 GHz have been shown to be feasible. The 14 GHz gyrotron is expected to operate above 1 MW. We are also developing higher frequency gyrotrons (77-300 GHz). The joint program of National Institute for Fusion Science and the University of Tsukuba developed two new 154 GHz gyrotrons for the large helical device after the demonstration of three 77 GHz gyrotrons. The 154 GHz gyrotrons achieved a maximum output power of 1.25 MW and quasi-CW operation of 0.35 MW for 30 min.

DOI： 10.1088/0029-5515/55/9/093009

Language：English   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1016/j.jnucmat.2015.01.013

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1016/j.jnucmat.2014.11.136

Language：English   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (scientific journal)  

At the Plasma Research Center at University of Tsukuba, development of megawatt gyrotrons is being performed as a collaborative electron cyclotron heating (ECH) study with some research organizations. A 28 GHz 1 MW 1 s gyrotron has been developed to upgrade the GAMMA 10/PDX ECH systems. To improve the oscillation efficiency in high current regions, the magnetron injection gun (MIG) of the 28 GHz gyrotron has been modified. Output power of 1.25 MW has been achieved with this gyrotron. For the first step of the collaborative research between Tsukuba University and Kyushu University, the Tsukuba 28 GHz gyrotron was adapted to the Q-shu University Experiment with Steady-State Spherical Tokamak (QUEST) ECH system, and the plasma heating and current drive effect were demonstrated. We obtained successful results, including an electron cyclotron-driven plasma current of 66 kA in the QUEST plasma experiment. For the next step of the collaborative research, the design targets of a 2 MW 3 s and 0.4 MW continuous wave have been achieved in a design study of a new 28 GHz gyrotron.

DOI： 10.13182/FST14-848

Language：English   Publishing type：Research paper (scientific journal)  

A FAst DIrectional Switch(FADIS) performance based on steep slope in a diplexer function was considered for dual-frequency (dual-f) applications to switch the transmission line with small frequency control. A diplexer of FADIS using a Square Corrugated Waveguide (SCW) splitter was designed as one of the most attractive candidates for the dual-f operation. The splitter performance was a key issue for the dual-f operation, and extended operation regions in splitter operations have been considered and surveyed using mode contents analysis based on matching coefficient evaluation. Some operational branches with high matching coefficients (>0.9) were found, and a new operating parameters were proposed for the dual-f operation in the JT-60SA Electron Cyclotron Hearing and Current Drive (ECHCD) system. Radiation pattern distributions from the SCW splitter were defined very well with no serious side lobes in the dual-f operation. In double loop ring resonator system, the diplexer frequency response was sufficiently steep to switch the outputs of the JT-60SA dual-f gyrotron.

DOI： 10.1007/s10762-014-0138-0

Language：English   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (scientific journal)  

Effectiveness of high harmonic electron cyclotron resonance heating (ECRH) was investigated by both experiments and ray-trace analyses. The conditions of both the EC wave injection and the magnetic field configuration were optimized in the large helical device. In the case of the second harmonic ordinary mode injection with a frequency of 77 GHz and with the optimized injection angle, about 30-40% absorption could be kept beyond the cut-off density of the second harmonic extraordinary (X2) mode, which is 3.7 × 10¹⁹ m^-3. In the third harmonic X (X3) mode heating experiment, the dependence of the absorption rate on plasma density and temperature of the target plasma was precisely investigated and compared with the ray-trace (TRAVIS code) calculation. The calculation results of the absorption rate show fairly good agreement with the experimentally obtained ones on the plasma-parameter dependences. The maximum absorption rate in the X3 heating experiment attained approximately 40% around the electron density of 1.5 × 10¹⁹ m^-3 and the electron temperature of 1.2 keV. Superposed stepwise injection from three gyrotrons with a total of 3 MW increased the central electron temperature to about 3.5 times of the initial target plasma temperature of 0.6 keV. This shows that the temperature increase improves the absorption rate of the subsequent injection.

DOI： 10.1088/0029-5515/55/6/063035

Language：English   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (scientific journal)  

A FAst DIrectional Switch (FADIS) of 2-frequency (2-ƒ) gyrotron system for the JT-60SA project is being developed under collaboration between Japan Atomic Energy Agency (JAEA) and Kyushu University. At first, the frequency drift and dip in the gyrotron operation were measured to consider which kind of FADIS is preferred for application in the Electron Cyclotron Heating and Current Drive (ECHCD) system for the JT- 60SA. Various types of the FADIS have been considered. A square corrugated waveguide diplexer system with double resonant rings was considered as one of the most attractive FADIS systems for stable high-power and long-pulse operations in the 2-ƒ JT-60SA ECHCD system.

DOI： http://dx.doi.org/10.1051/epjconf/20158704009

Language：English   Publishing type：Research paper (scientific journal)  

A FAst DIrectional Switch (FADIS) of 2-frequency (2-f{hook}) gyrotron system for the JT-60SA project is being developed under collaboration between Japan Atomic Energy Agency (JAEA) and Kyushu University. At first, the frequency drift and dip in the gyrotron operation were measured to consider which kind of FADIS is preferred for application in the Electron Cyclotron Heating and Current Drive (ECHCD) system for the JT- 60SA. Various types of the FADIS have been considered. A square corrugated waveguide diplexer system with double resonant rings was considered as one of the most attractive FADIS systems for stable high-power and long-pulse operations in the 2-f{hook} JT-60SA ECHCD system.

DOI： 10.1051/epjconf/20158704009

Language：English   Publishing type：Research paper (scientific journal)  

This paper reports the recent status of the ECRH system in the LHD and introduces the experimental results that suggest the reasonable effects of the wave-plasma interaction in the plasma boundary including outside the LCFS. With increasing the available ECRH power, ECRH/ECCD have been performed in higher density target plasmas where the mode coupling between the O-mode and the X-mode caused by the magnetic shear occurs for long distance in the low density region of the plasma boundary including the ergodic layer outside the LCFS. In such a case, the incident polarization should be set by taking into account the mode coupling effect. Moreover, the refraction in the boundary region should be considered for accurate numerical prediction of the wave trajectory and power absorption with use of the ray-tracing calculation.

DOI： 10.1051/epjconf/20158702011

Language：English   Publishing type：Research paper (scientific journal)  

DOI： dx.doi.org/10.1051/epjconf/20158702011

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1051/epjconf/20158702012

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1063/1.4914838

Language：English   Publishing type：Research paper (scientific journal)  

The driving and damping mechanism of plasma flow is an important issue because flow shear has a significant impact on turbulence in a plasma, which determines the transport in the magnetized plasma. Here we report clear evidence of the flow damping due to stochastization of the magnetic field. Abrupt damping of the toroidal flow associated with a transition from a nested magnetic flux surface to a stochastic magnetic field is observed when the magnetic shear at the rational surface decreases to 0.5 in the large helical device. This flow damping and resulting profile flattening are much stronger than expected from the Rechester-Rosenbluth model. The toroidal flow shear shows a linear decay, while the ion temperature gradient shows an exponential decay. This observation suggests that the flow damping is due to the change in the non-diffusive term of momentum transport.

DOI： 10.1038/ncomms6816

Language：English   Publishing type：Research paper (scientific journal)  

Comparison between non-inductive plasma current start-up using electron cyclotron resonance heating (ECRH) with and without fundamental (1st) resonance using 8.2-GHz microwave is experimentally performed on QUEST. Results of soft X-ray measurements in different energy ranges indicate significant difference in the number of current-carrying electrons with and without fundamental resonance. Numerical calculations show effective extendibility of electron energy in ECRH of fundamental resonance.

DOI： 10.1585/pfr.10.3402066

Language：English   Publishing type：Research paper (scientific journal)  

A thermal imaging system to measure plasma Electron Bernstein Emission (EBE) emanating from the mode conversion region in overdense plasma is discussed. Unlike conventional ECE/EBE imaging, this diagnostics does not employ any active mechanical scanning mirrors or focusing optics to scan for the emission cones in plasma. Instead, a standard 3 × 3 waveguide array antenna is used as a passive receiver to collect emission from plasma and imaging reconstruction is done by accurate measurements of phase and intensity of these signals by heterodyne detection technique. A broadband noise source simulating the EBE, is installed near the expected mode conversion region and its position is successfully reconstructed using phase array technique which is done in post processing.

DOI： 10.1063/1.4889903

Language：English   Publishing type：Research paper (international conference proceedings)  

We present a novel method for calculating the linearly polarized mode contents in an overmoded corrugated waveguide. The proposed method is based on calculating the low order irradiance moments of the measured radiated intensity profiles at several distances from the waveguide aperture. The method is validated by the data measured from a 63.5 mm diameter corrugated waveguide at 170 GHz and a 19 mm diameter corrugated waveguide at 250 GHz.

DOI： 10.1109/IRMMW-THz.2014.6956423

Language：English   Publishing type：Research paper (international conference proceedings)  

Language：English   Publishing type：Research paper (international conference proceedings)  

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.3938/jkps.65.1191

Language：English   Publishing type：Research paper (international conference proceedings)  

Progression from low (LR) to high recycling (HR) was observed in full non-inductive long duration discharges up to 12 minutes on QUEST. Repetitive transition between LR and HR was induced by periodic gas puffing and the period to recover to LR, rec, was gradually prolonged with the plasma duration. The period, rec, normalized by gas rate has a linear relation to time-integrated H. This indicates hydrogen recycling induced an intentional gas puffing is increasing with time-integrated H which an indicator of hydrogen (H) fluence to the wall. To understand the dependence, deuterium (D) storing capability of the specimen exposed to QUEST plasmas was investigated by implantation of D2+
ions of 1keV and subsequent thermal desorption spectroscopy (TDS) as a post-mortem analysis. A diffusion-trap model, reconstructing the obtained the results of TDS, shows a clear dependence of recycling ratio on fluence and a time evolution of wall-stored H can be reconstructed. The model calculation indicates surface recombination on the deposition layer plays an essential role in QUEST wall behaviour. Wall temperature dependence on the number of injected H to keep density is experimentally investigated and is agreed with the model calculation. This result is quite helpful to execute hot wall experiments on QUEST.

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1109/TPS.2014.2349797

Language：English   Publishing type：Research paper (international conference proceedings)  

Language：English   Publishing type：Research paper (international conference proceedings)  

Language：English   Publishing type：Research paper (international conference proceedings)  

Language：English   Publishing type：Research paper (international conference proceedings)  

Language：English   Publishing type：Research paper (international conference proceedings)  

Language：English   Publishing type：Research paper (scientific journal)  

In the present RF-driven plasma with a lot of high-energy electrons, there may be anisotropic plasma pressure, which makes the usual equilibrium analysis difficult, but the Cauchy condition surface method can reconstruct the plasma shape precisely regardless of the anisotropy. In addition, the plasma current effect in the open magnetic surfaces outside of the closed magnetic surfaces is considered in the RF-driven divertor plasma. In the reconstruction process, singular value (SV) decomposition is used and optimal criterion function for generalized cross validation is estimated concerning truncation or reduction of the small-SV components.

DOI： 10.1109/TPS.2014.2341616

Language：English   Publishing type：Research paper (scientific journal)  

Formations of a closed flux surface (CFS) on QUEST are achieved by fully non-inductive current start-up driven by RF, which is 8.2GHz in frequency and more than 40kW in power. It found that appropriate magnetic configuration with positive n-index and reduction of particle recycling was crucial to achieve the non-inductive plasma current start-up (PCS) successfully. Especially the controllability of particle recycling should be improved by wall conditioning based on successive plasma production and wall cleaning with electron cyclotron resonance heating (ECR) plasmas induced by RF in frequency of 2.45GHz.

DOI： 10.1088/1742-6596/511/1/012041

Language：English   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (scientific journal)  

Statistical features of fluctuations are investigated using the fast camera imaging technique in the scrape of layer (SOL) of electron cyclotron resonance heated Ohmic plasma. Fluctuations in the SOL towards low field side are dominated by coherent convective structures (blobs). Two dimensional structures of the higher order moments (skewness s and kurtosis k) representing the shape of probability density function (PDF) are studied. s and k are seen to be functions of the magnetic field lines. s and k are consistently higher towards the bottom half of the vessel in the SOL showing the blob trajectory along the field lines from the top towards bottom of the vessel. Parabolic relation (k = As-2 + C) is observed between s and k near the plasma boundary, featuring steep density gradient region and at the far SOL. The coefficient A, obtained experimentally, indicates a shift of prominence from pure drift-wave instabilities towards fully developed turbulence. Numerical coefficients characterizing the Pearson system are derived which demonstrates the progressive deviation of the PDF from Gaussian towards gamma from the density gradient region, towards the far SOL. Based on a simple stochastic differential equation, a direct correspondence between the multiplicative noise amplitude, increased intermittency, and hence change in PDF is discussed. (C) 2014 AIP Publishing LLC.

DOI： 10.1063/1.4890359

Language：English   Publishing type：Research paper (scientific journal)  

High βp (εβp ∼ 1) equilibria obtained in a ECW heated Ohmic plasma is investigated using a simple analytic solution of Grad-Shafranov equation. The formation of a natural inboard poloidal null associated with high βp is explained consistently by high diamagnetism and negative triangularity. As βp is increased, the poloidal null point penetrates further into the vacuum vessel, which is qualitatively explained by the analytic model. Transition from inboard (high field side) limiter bound to the natural divertor configuration is associated with a reduction of the edge safety factor without appreciable enhancement of MHD activities. Such a scenario is also addressed successfully with the model.

DOI： 10.1585/pfr.9.3402093

Language：English   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (scientific journal)  

A Phased Array Antenna (PAA) was considered as launching and receiving antennae in reflectometry to attain good directivity in its applied microwave range. A well-focused beam was obtained in a launching antenna application, and differential-phase evolution was properly measured by using a metal reflector plate in the proof-of-principle experiment at low power test facilities. Differential-phase evolution was also evaluated by using the PAA in the Q-shu University Experiment with Steady State Spherical Tokamak (QUEST). A beam-forming technique was applied in receiving phased-array antenna measurements. In the QUEST device that should be considered as a large oversized cavity, standing wave effect was significantly observed with perturbed phase evolution. A new approach using derivative of measured field on propagating wavenumber was proposed to eliminate the standing wave effect.

DOI： 10.1063/1.4891710

Language：English   Publishing type：Research paper (international conference proceedings)  

DOI： dx.doi.org/10.7566/JPSCP.1.015031

Language：English   Publishing type：Research paper (international conference proceedings)  

Language：English   Publishing type：Research paper (scientific journal)  

A scenario for non-inductive current ramp-up has been demonstrated using electron cyclotron waves in the spherical tokamak QUEST. The configuration was characterized by a high toroidal magnetic mirror ratio of 2 and a steady vertical magnetic field of more than 10% of the toroidal magnetic field. The generation and confinement of energetic electrons having energy greater than 10 keV were studied using hard x-rays. Because of the energetic electron pressure, a natural divertor formed with an inboard poloidal field null at the high poloidal beta (approximately 3-4).

DOI： 10.1088/0029-5515/54/2/023010

Language：English   Publishing type：Research paper (other academic)  

In the large helical device (LHD), the theoretically predicted width of the ordinary-extraordinary-electron Bernstein wave (O-X-B) mode conversion (MC) window is comparable to the beam width and the power deposition is located in the off-axis region if the 77GHz fundamental electron cyclotron (EC) wave of is launched from an existing horizontal port antenna. In the experiment, the actual MC window location was looked for with changing the aiming. The effective aiming with that the increase of the stored energy was observed was two degrees apart from the location of the theoretical MC window at a maximum. Measurement of the waves originated from the thermally emitted EBW and radiated via the B-X-O mode conversion process is effective to improve the accuracy of the theoretical prediction with comparison between the theoretical and the experimental results. The theoretical prediction suggests that the width of the MC window of the fundamental 77GHz EC wave can be expanded if the lower port antenna is used. On the other hand, the MC window of the second harmonic 154GHz EC wave is blocked by horizontal port wall if another horizontal port antenna is used. It is required to move the final mirror of the quasi-optical antenna toward the plasma surface. Focusing of the beam at the plasma cutoff is (PC) also necessary for the effective mode conversion.

DOI： 10.1063/1.4864595

Language：English   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (other academic)  

In the present RF-driven plasma with a lot of high-energy electrons, there may be anisotropic plasma pressure, which makes difficult a usual equilibrium analysis, but the CCS method can reconstruct the plasma shape precisely regardless of the anisotropy. And the plasma current effect in the open magnetic surfaces outside of the closed magnetic surfaces is considered in the RF-driven plasma. Further high-energy particle guiding center orbits are calculated aiming at estimation of the open-magnetic-surface current distribution.

DOI： 10.1109/SOFE.2013.6635507

Language：English   Publishing type：Research paper (international conference proceedings)  

A method for the in-situ characterization of spurious modes in high power microwave transmission lines is presented. It uses hole couplers, which are integrated into a mitre-bend. The signals are proportional to the field strength at the mirror and allow to partly reconstruct the mode spectrum in the waveguide. The theoretical background is presented as well as results from measurements on prototypes.

DOI： 10.1109/IRMMW-THz.2013.6665625

Language：English   Publishing type：Research paper (scientific journal)  

The progress of physical understanding as well as parameter improvement of net-current-free helical plasma is reported for the Large Helical Device since the last Fusion Energy Conference in Daejeon in 2010. The second low-energy neutral beam line was installed, and the central ion temperature has exceeded 7 keV, which was obtained by carbon pellet injection. Transport analysis of the high-T-i plasmas shows that the ion-thermal conductivity and viscosity decreased after the pellet injection although the improvement does not last long. The effort has been focused on the optimization of plasma edge conditions to extend the operation regime towards higher ion temperature and more stable high density and high beta. For this purpose a portion of the open helical divertors are being modified to the baffle-structured closed ones aimed at active control of the edge plasma. It is compared with the open case that the neutral pressure in the closed helical divertor increased by ten times as predicted by modelling. Studies of physics in a three-dimensional geometry are highlighted in the topics related to the response to a resonant magnetic perturbation at the plasma periphery such as edge-localized-mode mitigation and divertor detachment. Novel approaches of non-local and non-diffusive transport have also been advanced.

DOI： 10.1088/0029-5515/53/10/104015

Language：English   Publishing type：Research paper (scientific journal)  

A plasma control system to sustain divertor configurations is developed on QUEST (Q-shu university experiment with steady-state spherical tokamak). Magnetic fluxes are numerically integrated at 100 kHz using FPGA (Field-Programmable Gate Array) modules and transferred to a main calculation loop at 4 kHz. With these signals, plasma shapes are identified in real time at 2 kHz under the assumption that the plasma current can be represented as one filament current. This calculation is done in another calculation loop in parallel by taking advantage of a multi-core processor of the plasma control system. The inside and outside plasma edge positions are controlled to their target positions using PID (proportional-integral-derivative) control loops. Whereas the outside edge position can not be controlled by the outer PF coil current, the inside edge position can be controlled by the inner PF coil current. (C) 2013 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.fusengdes.2013.03.035

Language：English   Publishing type：Research paper (international conference proceedings)  

A method for the in-situ characterization of spurious modes in high power microwave transmission lines is presented. It uses hole couplers, which are integrated into a mitre-bend. The signals are proportional to the field strength at the mirror and allow to partly reconstruct the mode spectrum in the waveguide. The theoretical background is presented as well as results from measurements on prototypes.

Language：English   Publishing type：Research paper (scientific journal)  

A particle image velocimetry technique based on orthogonal dynamic programming is developed to measure the time resolved flow field of the fluctuating structures at the plasma edge and scrape off layer. This non-intrusive technique can provide two dimensional velocity fields at high spatial and temporal resolution from a fast framing image sequence and hence can provide better insights in plasma flow as compared to conventional probe measurements. Applicability of the technique is tested with simulated image pairs. Finally, it is applied to tangential fast visible images of QUEST as a test case to estimate the scrape off layer flow in Ohmic and ECRH driven plasma discharges.

Language：English   Publishing type：Research paper (scientific journal)  

Blob-filaments have been observed by combined measurement with a fast camera and a movable Langmuir probe in an open magnetic field line configuration of electron cyclotron resonance (ECR) heating plasma in QUEST. Blob-filaments extended along field lines do correspond to over-dense plasma structures and propagated across the field lines to the outer wall. The radial velocity of the blob structure, Vb, was obtained by three methods and was dominantly driven by the E x B force. The radial velocity, size of the blob showed good agreements with the results obtained by sheath-connected interchange theoretical model. Vb corresponds to roughly 0.02-0.07 of the local sound speed (Cs) in QUEST. The higher moments (skewness S and kurtosis K) representing the shape of PDF of density fluctuation are studied. Their least squares fitting with quadratic polynomial is K = (1.60 +/- 0.27) S-2 - (0.46 +/- 0.20). The larger blob structures, occurring only 10% of the time, can carry more than 60% loss of the entire radial particle flux. (C) 2013 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.jnucmat.2013.01.106

Language：English   Publishing type：Research paper (scientific journal)  

High harmonic electron cyclotron resonance heating (ECH) can extend the plasma heating region to higher density and higher β compared to the normal heating scenario. In this study, the heating characteristics of the second-harmonic ordinary (O2) and third-harmonic extraordinary (X3) modes and the possible extension of heating regime are experimentally confirmed. At the same time, a comparative study using ray-tracing calculation was performed in the realistic three-dimensional configuration of the Large Helical Device. The O2 mode heating showed a 40% absorption rate even above the X2 mode cut-off density. The X3 mode heating using powerful 77 GHz gyrotrons demonstrated an increase of about 40% in the central electron temperature in the plasmas at β-value of about 1%. These results were quantitatively explained to some extent by ray-tracing calculations.

Language：English   Publishing type：Research paper (scientific journal)  

The steady-state operation of high-performance plasmas in the Large Helical Device (LHD) has progressed since the 2010 IAEA Conference in Korea by means of a newly installed ion cyclotron heating (ICH) antenna (HAS antenna) and an improved electron cyclotron heating (ECH) system. The HAS antenna can control the launched parallel wave number and heat the core plasma efficiently in the case of dipole mode operation. Understanding of the physics and technology of wave heating, particle and heat flow balances, and plasma-wall interactions in LHD has also improved. The heating power of steady-state ICH and ECH exceeded 1MWand 500kW, respectively, and a higher density helium plasma with minority hydrogen ions was maintained using the HAS antenna and new 77 GHz gyrotrons. As a result, plasma performance improved, e. g. electron temperature of more than 2 keV at a density of more than 2 x 10(19) m(-3) became possible for more than 1 min. Heat flow balance and particle flux balance of steady-state operation were evaluated. Particle balance analysis indicated that externally fed helium and hydrogen particles were mainly absorbed by the chamber wall and divertor plates, even after the 54 min operation.

DOI： 10.1088/0029-5515/53/6/063017

Language：English   Publishing type：Research paper (scientific journal)  

In the Large Helical Device (LHD), evident electron Bernstein wave (EBW) heating was successfully performed. The experiment was carried out using the electron cyclotron heating (ECH) system that was upgraded by installation of high-power, long-pulse 77 GHz gyrotrons. The EBW heating was achieved by a mode conversion from injected EC wave to EBW, by the so-called slow-XB technique where an X-mode wave is injected to the plasma from the high magnetic field side. The specific magnetic configuration of LHD provides a good opportunity to realize the slow-XB technique, which is generally difficult for tokamaks. With the slow-XB technique, increases in kinetically evaluated electron energyWpe and electron temperature Te were observed in overdense plasmas. An electron heating in the so-called super dense core plasma in LHD, which is characterized with an internal diffusion barrier and a steep density gradient at the plasma core, was successfully demonstrated in the plasma core region where the central electron density n(e0) of 17 x 10(19) m(-3) was about 1.2 times higher, at the beginning of the EC-wave injection, than the left-hand cut-off density of applied 77 GHz EC waves.

DOI： 10.1088/0029-5515/53/6/063004

Language：English   Publishing type：Research paper (scientific journal)  

The latest development achievements in the University of Tsukuba of over-1MW power level gyrotrons required in present-day fusion devices, GAMMA 10, Large Helical Device (LHD), QUEST, Heliotron J and NSTX, are presented. The obtained maximum outputs are 1.9MW for 0.1 s on the 77 GHz LHD tube and 1.2MW for 1ms on the 28 GHz GAMMA 10 one, which are new records in these frequency ranges. In long-pulse operation, 0.3 MW for 40 min at 77 GHz and 0.54 MW for 2 s at 28 GHz are achieved. A new programme of 154 GHz 1MW development has started for high-density plasma heating in LHD. On the first 154 GHz tube, 1.0MW for 1 s is achieved. As a next activity of the 28 GHz gyrotron, an over-1.5MW gyrotron is designed and fabricated to study the multi-MW oscillation. The possibility of 0.4MW continuous wave and 2MW level output in operations of a few seconds, after the improvements of output window and mode converter, is shown. Moreover, a new design study of dual-frequency gyrotron at 28 and 35 GHz has started, which indicates the practicability of the multi-purpose gyrotron.

DOI： 10.1088/0029-5515/53/6/063003

Language：English   Publishing type：Research paper (international conference proceedings)  

Language：English   Publishing type：Research paper (scientific journal)  

QUEST has a divertor configuration with a high and a negative n-index, and the problem of plasma vertical position instability control in QUEST is still under extensive study for achieving high efficiency plasma. The instability we considered is that the toroidal plasma moves either up or down in the vacuum chamber until it meets the vessel wall and is extinguished. The actively controlled coils (HCU and HCL) outside the vacuum vessel are serially connected in feedback with a measurement of the plasma vertical position to provide stabilizing control. In this work, a robust controller is employed by using the loop synthesis method, and provides robust stability over a wide range of n-index. Moreover, the gain of the robust controller is lower than that of a typical proportional derivative (PD) controller in the operational frequency range; it indicates that the robust controller needs less power consumption than the PD controller does.

Language：English   Publishing type：Research paper (scientific journal)  

The corrugated horn antenna is widely used for the evaluation of transmission lines in electron cyclotron resonant heating systems, but little is known about the intensity and phase of its radiation profile over a wide frequency range. This paper is concerned with the development of a full D-band (110 - 170 GHz) corrugated horn antenna. The antenna was designed based on electromagnetic simulation codes for the finite element method and the method of moment. To verify these numerical simulations, a low-power test system was established, and its dynamic power range was defined for the precise measurement of antenna radiation. The fabricated antenna was measured and analyzed with basic Gaussian optics for a number of frequencies throughout the D-band. The measured radiation profiles are Gaussian-like and agree well with the numerical simulations. The radiated intensity pattern is slightly elliptical in the lower frequency range, as unwanted higher-order modes arose at the corrugated mode conversion section and generated the elliptical radiation pattern. The evolutions of the antenna radiation do not correspond with those expected by basic Gaussian optics.

DOI： 10.1585/pfr.8.1405163

Language：English   Publishing type：Research paper (scientific journal)  

A particle image velocimetry technique based on orthogonal dynamic programming is developed to measure the time resolved flow field of the fluctuating structures at the plasma edge and scrape off layer. This nonintrusive technique can provide two dimensional velocity fields at high spatial and temporal resolution from a fast framing image sequence and hence can provide better insights in plasma flow as compared to conventional probe measurements. Applicability of the technique is tested with simulated image pairs. Finally, it is applied to tangential fast visible images of QUEST as a test case to estimate the scrape off layer flow in Ohmic and ECRH driven plasma discharges.

DOI： 10.1585/pfr.8.2402098

Language：English   Publishing type：Research paper (scientific journal)  

To investigate the effects of confinement of fast electrons on poloidal beta equilibrium, experiments under different toroidal magnetic mirror ratios, MOMFC, were performed in electron cyclotron heated plasma in the spherical tokamak QUEST. The proportional constants of the relation between plasma current and vertical magnetic fields decreased with increase in MOMFC. The poloidal field null point appeared in the chamber for MOMFC > 1.8 at a plasma current value of ~15 kA. Poloidal beta and temperature of hard X-rays increased with MOMFC. This suggests that high poloidal beta equilibrium plasma is produced by better confinement of energetic trapped particles.

DOI： 10.1585/pfr.8.2402118

Language：English   Publishing type：Research paper (scientific journal)  

High harmonic electron cyclotron resonance heating (ECH) can extend the plasma heating region to higher density and higher β compared to the normal heating scenario. In this study, the heating characteristics of the second-harmonic ordinary (O2) and third-harmonic extraordinary (X3) modes and the possible extension of heating regime are experimentally confirmed. At the same time, a comparative study using ray-tracing calculation was performed in the realistic three-dimensional configuration of the Large Helical Device. The O2 mode heating showed a 40% absorption rate even above the X2 mode cut-off density. The X3 mode heating using powerful 77 GHz gyrotrons demonstrated an increase of about 40% in the central electron temperature in the plasmas at β-value of about 1%. These results were quantitatively explained to some extent by ray-tracing calculations.

DOI： 10.1585/pfr.8.2402073

Language：English   Publishing type：Research paper (scientific journal)  

QUEST has a divertor configuration with a high and a negative n-index, and the problem of plasma vertical position instability control in QUEST is still under extensive study for achieving high efficiency plasma. The instability we considered is that the toroidal plasma moves either up or down in the vacuum chamber until it meets the vessel wall and is extinguished. The actively controlled coils (HCU and HCL) outside the vacuum vessel are serially connected in feedback with a measurement of the plasma vertical position to provide stabilizing control. In this work, a robust controller is employed by using the loop synthesis method, and provides robust stability over a wide range of n-index. Moreover, the gain of the robust controller is lower than that of a typical proportional derivative (PD) controller in the operational frequency range; it indicates that the robust controller needs less power consumption than the PD controller does.

DOI： 10.1088/1009-0630/15/3/21

Language：English   Publishing type：Research paper (scientific journal)  

Fluctuations in slab plasma produced by electron cyclotron waves in an open magnetic configuration with vertical (B-z) and toroidal (B-t) fields are investigated as a function of B-z/B-t (field pitch) using a high speed visible camera in the spherical tokamak QUEST. Higher order moments (skewness s and kurtosis k) of spatio- temporal fluctuations are investigated. Background fluctuations, generation of coherent convective structures (blobs) and their propagation are analysed at the intensity gradient (R-im) and the source- free (essentially vacuum on the low- field side) regions, respectively. Parabolic relation (k = As-2 + C) is observed between s and k on the entire R- Z plane for a wide range of B-z/B-t. Progressive shift from the Gaussian statistics beyond R-im is observed with increasing B-z/B-t. From the moments of the probability density functions, possible location for blob generation is inferred to be R-im. Direct correspondence is observed with the prominence of the blobs and increase in stochastic forces with the increase in field pitch. Distribution of waiting time suggests that the blob generation may be a Poisson process. Accelerated cross field transport is observed for reasonably large-sized blobs.

DOI： 10.1088/0029-5515/52/12/123016

Language：English   Publishing type：Research paper (other academic)  

Language：English   Publishing type：Research paper (scientific journal)  

Oversized corrugated waveguide transmissions are used to transmit high power millimeter wave for electron cyclotron heating in ITER. The HE11 mode purity of the waveguide components is a critical issue not only in the high power operation but also in the low power operation to evaluate the components. Conventional single Gaussian beam system causes edge diffraction and excites higher order modes at the waveguide aperture. A proposed HE₁₁-mode exciter uses beam interference between two Gaussian beams interference and serves the high purity HE11 mode without the edge diffraction. The HE11-mode exciter consists of a scalar feed horn antenna, quasi-optical mirrors and a beam splitter (combiner). Phase matched mirrors correct the Gaussian-like beam excited from the horn antenna to pure Gaussian beam. Authors report the experimental study of the exciter.

DOI： 10.1585/pfr.7.2405037

Language：English   Publishing type：Research paper (scientific journal)  

In the large helical device (LHD), fundamental electron cyclotron resonance heating (ECRH) by the electron Bernstein wave (EBW) excited via the ordinary-extraordinary-EBW (O-X-B) mode conversion process was performed with high power (∼1MW) launching. Profiles of increase of the electron temperature (T_e) and the soft X-ray signals during the power injection suggest power absorption in the core region. Effects of the local modification of the rotational transform ι/2π(=1/q) by electron cyclotron current drive (ECCD) on the formation and sustainment of the electron internal transport barrier (e-ITB) was investigated for the first time. Co ECCD raised ι/2π close to 0.5 in the core region and caused the flattening of the T_e profile. Additional ECRH power is required to form the e-ITB. On the contrary, counter (cntr.) ECCD separates ι/2π from 0.5 in the core region and avoids the flattening of the T_e profile. The e-ITB can be formed and sustained without additional ECRH. Analysis of the heat pulse transport with use of the modulation ECRH (MECH) shows the good confinement region extends to the ι/2π =0.5 rational surface in the case of cntr. ECCD.

DOI： 10.1051/epjconf/20123202006

Language：English   Publishing type：Research paper (scientific journal)  

Real-time in-situ measurement of spurious modes in HE₁₁ transmission lines is becoming an important topic for the design of next-generation ECRH installations (e.g. ITER), because the acceptable tolerances for the alignment of the waveguides and coupling optics are small for oversized waveguides. Also, the effects of spurious modes (ohmic heating, wrong beam parameters at the launcher) become increasingly critical. We present a method for in-situ characterization of 4 dominant spurious modes by using a 5-port coupler, which is integrated into a miter bend. The coupler signals can be directly transformed into the mode spectrum by a matrix multiplication. A general formalism for obtaining the coefficients of the transformation matrix is presented along with a method for optimizing the coupler positions in order to obtain the maximum dynamic range for the diagnostics.

DOI： 10.1051/epjconf/20123204010

Language：English   Publishing type：Research paper (scientific journal)  

Electron cyclotron heating (ECH) using electron Bernstein waves (EBWs) was studied in the large helical device (LHD). Oblique launching of the slow extraordinary (SX-) mode from the high field side and oblique launching of the ordinary (O-) mode from the low field side were adopted to excite EBWs in the LHD by using electron cyclotron (EC) wave antennas installed apart from the plasma surface. Increases in the stored energy and electron temperature were observed for both cases of launching. These launching methods for ECH using EBWs (EBWH) is promising for high-density operation in future helical fusion devices instead of conventional ECH by normal electromagnetic modes.

DOI： 10.1585/pfr.7.2402110

Language：English   Publishing type：Research paper (scientific journal)  

A CW phased-array antenna system for electron cyclotron/Bernstein wave heating and current drive (ECWH/CD, EBWH/CD) experiments was developed in the QUEST. The antenna was designed to excite an elliptically polarized pure O-mode wave in oblique injection for the O-X-B mode conversion scenario, and its good performance was confirmed at a high power level. Long pulse discharges with a plasma current of 10 kA and 15 kA were non-inductively attained for 37 s and 20 s, respectively, with only radio frequency (RF) power. Divertor configurationswere also obtained in the RF-sustained plasmas. A new operational window for sustained plasma current was observed in the high-density plasma with a higher RF incident power. Two new heating and current drive systems with an 8.56GHz klystron and a 28GHz gyrotron are being prepared to conduct CW EBWH/CD experiments in the high-density plasma.

DOI： 10.1585/pfr.7.2402112

Language：English   Publishing type：Research paper (international conference proceedings)  

Language：English   Publishing type：Research paper (scientific journal)  

Ambient fluctuations and the generation of coherent convective structures are investigated as a function of vertical to toroidal magnetic field ratio (B _z/B _t) in the spherical tokamak QUEST [K. Hanada et al., Plasma Fusion Research 5, S1007]. Fluctuation signals are obtained at a high spatial and temporal resolution using the fast visible camera. The coherent structures (blobs) were seen to be apparent only beyond a critical Bz/Bt ratio (∼ 10 ^-2). This clearly indicates a shift from the drift to the interchange mode with the increase in vertical magnetic field. The blobs are propagated radially at a high speed ∼ 10 ³ m/s which is ∼ 1/10 ^th of the ion acoustic speed in QUEST. Lifetime of the blobs is about 700 μs. Second part of the study comprises of the investigation of the finer structures in the current ramp-up phase which is characterized by lower levels of fluctuations. Singular value decomposition (SVD) and principal component analysis (PCA) techniques are applied on the fast camera images to reveal the structures which are otherwise embedded in the overall intensity image. Electron banana orbits were seen to form from the slab annular plasma and grow wider to eventually shape the plasma current front.

DOI： 10.1541/ieejfms.132.545

Language：English   Publishing type：Research paper (scientific journal)  

Recent progress towards SSO of a spherical tokamak (ST) on QUEST is introduced. Zero-dimensional calculations of particle balance for QUEST experiment, refresh wall, and hot wall were performed showing that a SSO time of more than 1000s is required to obtain reliable results in regard to particle balance. Basic features of blobs were investigated in electron cyclotron resonance (ECR) heated plasmas. Formation and maintenance of a single-null divertor configuration was successfully obtained. Plasma duration in the divertor configuration was limited by out-gassing from a non-water-cooled first wall. Design studies for a hot wall are introduced.

DOI： 10.1541/ieejfms.132.490

Language：English   Publishing type：Research paper (scientific journal)  

The power deposition profiles were analyzed with a multiple ray tracing code for the Electron Bernstein Wave Heating and Current Drive (EBWH/CD) experiments in QUEST. In the EBWH/CD experiments in QUEST, the OXB mode conversion scenario was selected for the plasma current sustainment in the rather low-density case. The algorithm for the wave penetration through evanescent layer beyond a O-mode cutoff position was developed for the multiple-ray analysis. The launching antenna positions were considered to obtain the significant wave absorption in the specific propagating direction for current drive experiments using the developed ray-tracing code.

DOI： 10.1541/ieejfms.132.505

Language：English   Publishing type：Research paper (scientific journal)  

Electron Bernstein Wave Heating and Current Drive (EBWH/CD) experiments have been conducted to sustain a spherical tokamak configuration in a steady state in QUEST. In the EBWH/CD experiments on QUEST, the O-X-B mode conversion scenario was selected for plasma current sustainment in rather lowdensity plasmas. A new phased-array antenna system was proposed to launch a pure elliptically-polarized O-mode in the oblique injection. The prototype antenna system was designed and its performance was checked at low power test facilities. Good focusing and steering properties were confirmed in the low power test.

DOI： 10.1541/ieejfms.132.511

Language：English   Publishing type：Research paper (scientific journal)  

In order to identify plasma shape, there is a way to represent the plasma current profile with several parameters, and adjust these parameters with least-square technique in order for calculated magnetic values to accord with measured ones. Here, the plasma shape parameters such as minor radius, elongation, and triangularity are chosen as the fitting parameters to represent plasma shape more directly, and the applicability to the control of the plasma shape are described by evaluating its calculation time. In order to find minimum of an objective function with least-square technique, two methods are compared, namely a linear approximation method and a downhill simplex method. While high accuracies of the measured magnetic signals are required, the good reproducibility is obtained, and the plasma shape identification can be done within several milliseconds in both methods.

DOI： 10.1541/ieejfms.132.477

Language：English   Publishing type：Research paper (scientific journal)  

A fast visible imaging system is installed on the spherical tokamak QUEST to study edge turbulence. The camera uses a complementary metal-oxide semiconductor detector with a maximum resolution of 1024 x 1024 at 7000 frames/s (fps) and can achieve 775 kfps at a resolution of 128 x 24. In this paper, we present the salient features of the system and its application to study edge turbulence in 8.2 GHz ECRH driven slab plasma, without plasma current. Vertical magnetic field (B-z) topology is varied with three sets of poloidal field (PF) coils and the variation in the edge turbulence is investigated as a function of the B-z strength and curvature. Fluctuation amplitude was highest for the shallow PF well. Cross-correlation coefficient shows distinct coherent mode along z direction at the steep density gradient region and it grows with the PF mirror ratio. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4739080]

DOI： 10.1063/1.4739080

Language：English   Publishing type：Research paper (international conference proceedings)  

Language：English   Publishing type：Research paper (international conference proceedings)  

Language：English   Publishing type：Research paper (scientific journal)  

A CW phased-array antenna system for electron cyclotron/Bernstein wave heating and current drive (ECWH/CD, EBWH/CD) experiments was developed in the QUEST. The antenna was designed to excite an elliptically polarized pure O-mode wave in oblique injection for the O-X-B mode conversion scenario, and its good performance was confirmed at a high power level. Long pulse discharges with a plasma current of 10 kA and 15 kA were non-inductively attained for 37 s and 20 s, respectively, with only radio frequency (RF) power. Divertor configurations were also obtained in the RF-sustained plasmas. A new operational window for sustained plasma current was observed in the high-density plasma with a higher RF incident power. Two new heating and current drive systems with an 8.56 GHz klystron and a 28 GHz gyrotron are being prepared to conduct CW EBWH/CD experiments in the high-density plasma.

Language：English   Publishing type：Research paper (international conference proceedings)  

Language：English   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (international conference proceedings)  

Language：English   Publishing type：Research paper (scientific journal)  

Electron Bernstein Wave Heating and Current Drive (EBWH/CD) experiments have been conducted to sustain a spherical tokamak configuration in a steady state in QUEST. In the EBWH/CD experiments on QUEST, the O-X-B mode conversion scenario was selected for plasma current sustainment in rather low-density plasmas. A new phased-array antenna system was proposed to launch a pure elliptically-polarized O-mode in the oblique injection. The prototype antenna system was designed and its performance was checked at low power test facilities. Good focusing and steering properties were confirmed in the low power test.

Language：English   Publishing type：Research paper (scientific journal)  

In order to identify plasma shape, there is a way to represent the plasma current profile with several parameters, and adjust these parameters with least-square technique in order for calculated magnetic values to accord with measured ones. Here, the plasma shape parameters such as minor radius, elongation, and triangularity are chosen as the fitting parameters to represent plasma shape more directly, and the applicability to the control of the plasma shape are described by evaluating its calculation time. In order to find minimum of an objective function with least-square technique, two methods are compared, namely a linear approximation method and a downhill simplex method. While high accuracies of the measured magnetic signals are required, the good reproducibility is obtained, and the plasma shape identification can be done within several milliseconds in both methods.

Language：English   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (scientific journal)  

QUEST計画の大きな目標の一つは、高温プラズマの定常化にかかわる研究である。本論文は、論文は論文は発表時点での長時間維持についての報告（国際球状トカマクワークショップ：平成23年11月於核融合科学研究所）をまとめたものである。特に長時間運転での粒子バランスについてのポイントモデルを用いて、QUESTのパラメータを用いると粒子バランスの定常化に必要な時間が1018秒程度になることを示した。

Language：English   Publishing type：Research paper (international conference proceedings)  

Real-time in-situ measurement of spurious modes in HE11 transmission lines is becoming an important topic for the design of next-generation ECRH installations (e.g. ITER), because the acceptable tolerances for the alignment of the waveguides and coupling optics are small for oversized waveguides. Also, the effects of spurious modes (ohmic heating, wrong beam parameters at the launcher) become increasingly critical. We present a method for in-situ characterization of 4 dominant spurious modes by using a 5-port coupler, which is integrated into a miter bend. The coupler signals can be directly transformed into the mode spectrum by a matrix multiplication. A general formalism for obtaining the coefficients of the transformation matrix is presented along with a method for optimizing the coupler positions in order to obtain the maximum dynamic range for the diagnostics.

Language：English   Publishing type：Research paper (scientific journal)  

After intensive discharge cleaning of the chamber wall, non-inductive current start-up experiments have been successfully performed in QUEST in moderate vertical fields of about 1.0-1.5mT with positive n-index. Simultaneously, with increasing plasma current, an asymmetric toroidal flow of energetic electrons was observed and direct measurements of current driven by this asymmetric flow were taken with a newly developed Langmuir probe technique. A numerical study of the energetic electron orbits indicates that the total current is enough to play a dominant role in the formation of a closed flux surface in QUEST. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4731700]

DOI： 10.1063/1.4731700

Language：English   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (scientific journal)  

A directional coupler array with 5 ports integrated into a mitre bend for corrugated waveguide transmission lines at 140 GHz has been manufactured. The design is reviewed, and calculations on the performance for in-situ power measurement and mode discrimination are shown. Emphasis is given on detection of errors in alignment of the transmission system. Experimental tests are performed to benchmark the calculations. The results confirm the predictions and show that such a coupler is a viable tool for power measurement and basic mode analysis in high-power transmission systems.

DOI： 10.1007/s10762-012-9883-0

Language：English   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (scientific journal)  

In order to investigate the overall atomic hydrogen background and the dynamic characteristics of wall pumping/fuelling phenomenon, a permeation probe system has been developed and applied in the spherical tokamak QUEST. Reliability of measurements, within +/- 3% accuracy and a positive correlation with the hydrogen line emission over three orders of magnitude have been demonstrated for more than 3000 various plasma discharges. By comparison of the experimental permeation (flux) curves with the numerically simulated curves, the net incident atomic hydrogen flux is evaluated in the range of 1 x 10(19) H m(-2) s(-1) 4 x 10(20) H m(-2) s(-1). The atomic flux has been investigated as a function of various plasma operation parameters like RF power, gas pressure and magnetic configuration. Using the static particle balance and permeation measurements, the progress in wall conditioning has been investigated. An inverse correlation between the atomic hydrogen flux and improvement in wall pumping has been observed over the two campaigns. (C) 2011 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.jnucmat.2011.09.026

Language：English   Publishing type：Research paper (scientific journal)  

After several non-inductive current startup experimental campaigns in the spherical tokamak QUEST, its metallic first walls have revealed various kinds of damages as a signature of strong plasma wall interaction (PWI). Several types of footprint traces, namely colored regions formed due to material erosion/redeposition, melting of plasma facing components (PFCs) and numerous arc tracks on the chamber walls are recognized. Analysis of the re-deposited materials on collector probes is carried out using Xray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and energy dispersive X-ray spectrometry (EDS). Redeposition of several impurity materials such as carbon, oxygen and tungsten is identified. The footprint traces are majorly formed on the lower side PFCs, showing a large up/down asymmetry. Both toroidally symmetric and asymmetric footprint traces are formed on the bottoms side divertor plate and the lower part of the outboard side walls, respectively. Localized melting occurred on the outboard side limiters is attributed to the loss of energetic electrons produced via electron cyclotron resonance (ECR) heating. The observed damages are discussed in view of localized PWI, loss of energetic electrons, particle drifts, sputtering, arcing and redeposition of eroded materials. Material analysis and numerically calculated guiding center orbits of the charge particles are used to discuss these damages. (C) 2011 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.fusengdes.2011.10.001

Language：English   Publishing type：Research paper (other academic)  

The 16-element phased-array antenna has been developed for the EBWH/CD experiments. The 2-D k-spectra could be controlled. The heat load and stress were analyzed to be moderate in the 200kW CW operation. The symmetrically 2-D narrow-field distributions were excited by the developed antenna, indicating its good applicability to the EBWH/CD experiments.

DOI： 10.1109/irmmw-THz.2011.6105070

Language：English   Publishing type：Research paper (other academic)  

Language：English   Publishing type：Research paper (scientific journal)  

Because QUEST tokamak has a divertor configuration with a higher κ and a negative n-index, a precise power supply with a rapid response is needed to control the vertical position of the plasma. A matrix converter is a direct power conversion device that uses an array of controlled bidirectional switches as the main power elements for creating a variable-output current system. This paper presents a novel three-phase to two-phase topological matrix converter as a proposed power supply that stabilizes the plasma vertical position and achieves unity input power factor. An indirect control strategy in which the matrix converter is split into a virtual rectifier stage and a virtual inverter stage is adopted. In the virtual rectifier stage, the instantaneous active power and reactive power are decoupled on the basis of system equations derived from the DQ transformation; hence, unity power factor is achieved. Space vector pulse width modulation is adopted to determine the switching time of each switch in the virtual rectifier; the output voltage of the virtual rectifier is adjusted by the virtual inverter stage to obtain the desired load current. Theoretical analyses and simulation results are provided to verify its feasibility.

DOI： 10.1585/pfr.6.2405137

Language：English   Publishing type：Research paper (scientific journal)  

Ohmic plasma currents of up to 17 kA with a discharge duration of 0.32 s have been obtained in the Kyushu University Experiment with Steady-State Spherical Tokamak (QUEST) with the help of electron cyclotron wave (ECW) and cancellation coils (CCs). The CCs, originally installed to create a field null in the plasma breakdown phase, are essential for producing plasma current in QUEST. Although the ohmic coil current is initially biased and then reduced completely to zero to induce the plasma current in 15-20 ms, we demonstrate that the flat top of the plasma current exceeding 20 ms is maintained by the vertical field after the ohmic current is switched off. This type of operation is quite favorable for extending pulsed operation to the steady state by electron Bernstein wave current drive (EBCD).

DOI： 10.1585/pfr.6.1402003

Language：English   Publishing type：Research paper (other academic)  

The phased-array antenna system for Electron Cyclotron/Bernstein Wave Heating and Current Drive experiments has been developed in the QUEST. The antenna was designed to excite a pure O-mode wave in the oblique injection for the O-X-B mode conversion experiments, and its good performances were confirmed at a low power level. The plasma current (<∼15kA) with an aspect ratio of 1.5 was started up and sustained by only RF injection in the low-density operations. The long pulse discharge of 10 kA was also attained for 37 s. The new density window to sustain the plasma current was observed in the high-density plasmas. The single-null divertor configuration with the high plasma current (<∼25kA) was attained in the 17 s plasma sustainment.

DOI： 10.1063/1.3665017

Language：English   Publishing type：Research paper (scientific journal)  

In magnetized plasmas, the presence of a significant number of energetic electrons has been observed but quantitative characteristics of these electrons are proving difficult to investigate. A Langmuir probe offers a means to provide quantitative measurement of these energetic electrons that takes into account electron emissions (secondary electron emission and electron reflection) from the probe tips and sheath expansion around the probe tips caused by a considerable negative potential. In this paper, these effects are experimentally confirmed and an analytical means to measure energetic electron characteristics are proposed. An analysis of plasmas produced by a high frequency wave is then applied leading to the successful detection of an asymmetric flow of energetic electrons. The estimated electron temperature and current density were approximately 4-5 keV and 2-3 kA/m(2). (C) 2011 American Institute of Physics. [doi:10.1063/1.3662204]

DOI： 10.1063/1.3662204

Language：English   Publishing type：Research paper (scientific journal)  

A permeation measuring system with a nickel membrane of 30 pm thickness was installed near the mid plane of the spherical tokamak, QUEST. Hydrogen permeation through the membrane heated at fix temperatures (422 -506 K) was measured during short pulse (< 1 s) and long pulse (1 hour) plasma discharges. After the membrane was heated to a required temperature, hydrogen plasma was discharged using a 2.45 GHz or 8.2 GHzRF system. Significant plasma-driven permeation was observed even for very short plasma discharges (e.g. 0.1s). Numerical calculations with the use of diffusion equation under recombination boundary conditions were conducted to simulate the transient permeation behavior. The numerical calculations were also used to estimate diffusion coefficient and recombination coefficients of membrane material. Temperature dependence of both the coefficients was explained by the Arrhenius law. A one hour long permeation curve was also numerically reproduced using the same set of parameters except an increasing recombination coefficient on the plasma side of the membrane.

DOI： 10.13182/FST11-A12719

Language：English   Publishing type：Research paper (scientific journal)  

Edge turbulence has been imaged on QUEST at high temporal resolution (50 kHz) using a fast framing visible camera. Statistical features of fluctuations including blobs are investigated in both open and closed magnetic configurations. Higher order moments of fluctuations including blobs and their mutual relationships are investigated. Close resemblance of the density pdf with Gaussian or beta distributions has earlier been predicted. Hence, an attempt has been made to characterize the density pdf with respect to the Pearson system to ascertain the implications of the stochastic forces.

DOI： 10.1109/TPS.2011.2163423

Language：English   Publishing type：Research paper (scientific journal)  

A permeation measuring system with a nickel membrane of 30 ion thickness was installed near the mid plane of the spherical tokamak, QUEST Hydrogen permeation through the membrane heated at fix temperatures (422 - 506 K) was measured during short pulse (< 1 s) and long pulse (I hour) plasma discharges. After the membrane was heated to a required temperature, hydrogen plasma was discharged using a 2.45 GHz or 8.2 GHz RF system. Significant plasma-driven permeation was observed even for very short plasma discharges (e.g. 0.1s). Numerical calculations with the use of diffusion equation under recombination boundary conditions were conducted to simulate the transient permeation behavior. The numerical calculations were also used to estimate diffusion coefficient and recombination coefficients of membrane material. Temperature dependence of both the coefficients was explained by the Arrhenius law. A one hour long permeation curve was also numerically reproduced using the same set of parameters except an increasing recombination coefficient on the plasma side of the membrane.

Language：English   Publishing type：Research paper (scientific journal)  

CCS (Cauchy Condition Surface) method is a numerical approach to reproduce plasma shape, which has good precision in conventional tokamaks. In order to apply it in plasma shape reproduction of ST (Spherical Tokamak). the calculation precision of the CCS method in a spherical tokamak CPD (Compact PWI experimental Device)(B(t) = 0.25 T, R = 0.3 m, a = 0.2 m) has been analyzed. The precision was confirmed also in ST and decided to be applied to a spherical tokamak QUEST (B(t), = 0.25 T, R = 0.68 m, a = 0.40 m).. In present stage from the magnetic measurement, it is known that the eddy current effect is large in QUEST experiment, and there are no special magnetic measurements for eddy current now, so some proper model should be selected to evaluate the eddy current effect. The eddy current density by not only CS (Center Solenoid) coil but also plasma current is calculated using EDDYCAL (JAEA). The eddy current magnitudes are taken as unknown variables and solved together with plasma shape reconstruction in ohmic discharge and ECCD (Electron Cyclotron Current Drive) discharge. (C) 2011 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.fusengdes.2011.04.045

Language：English   Publishing type：Research paper (international conference proceedings)  

The 16-element phased-array antenna has been developed for the EBWH/CD experiments. The 2-D k-spectra could be controlled. The heat load and stress were analyzed to be moderate in the 200kW CW operation. The symmetrically 2-D narrow-field distributions were excited by the developed antenna, indicating its good applicability to the EBWH/CD experiments.

Language：English   Publishing type：Research paper (scientific journal)  

A study of radial propagation and electric fields induced by charge separation in blob-like structures has been performed in a non-confined cylindrical electron cyclotron resonance heating plasma on Q-shu University Experiment with a Steady-State Spherical Tokamak using a fast-speed camera and a Langmuir probe. The radial propagation of the blob-like structures is found to be driven by E x B drift. Moreover, these blob-like structures were found to have been accelerated, and the property of the measured radial velocities agrees with the previously proposed model [C. Theiler et al., Phys. Rev. Lett. 103, 065001 (2009)]. Although the dependence of the radial velocity on the connection length of the magnetic field appeared to be different, a plausible explanation based on enhanced short-circuiting of the current path can be proposed. (C) 2011 American Institute of Physics. [doi:10.1063/1.3640494]

DOI： 10.1063/1.3640494

Language：English   Publishing type：Research paper (scientific journal)  

Possibility of the measurement of radiated waves derived from the thermally emitted electron Bernstein wave (EBW) is numerically investigated based on the assumption of the super dense core (SDC) plasma generated in LHD. EBW that is thermally emitted in the electron cyclotron resonance (ECR) layer may couple with the electromagnetic wave and be emitted to the vacuum via the EBW-extraordinary-ordinary (B-X-O) mode conversion process. We consider the use of one of the transmission lines for electron cyclotron heating (ECH) in LHD as a receiving system of the emission. It is derived that the waves in the fundamental cyclotron frequency range are emitted as the EBW near their upper hybrid resonance (UHR) layer outside the last close flux surface (LCFS). On the other hand, waves in the second harmonics cyclotron frequency range are emitted in the core region. It means that successful measurement of waves of the second harmonic frequency range emitted from extremely high dense core plasma with setting an aim angle for receiving indicates a possibility of the second harmonic ECH by EBW in the core region with setting the same aim angle and the same polarization for launching.

DOI： 10.1088/1009-0630/13/4/04

Language：English   Publishing type：Research paper (scientific journal)  

Blob-like structures are observed in an open magnetic field line configuration of electron cyclotron resonance (ECR) heating plasma in QUEST. The combined measurement with a fast camera and a movable Langmuir probe show that visible filamentary structures extended along field lines do correspond to over-dense plasma structures. Ion saturation current time trace shows a front-like asymmetric burst when a visible structure reaches the probe during its outward propagation. The typical radial velocity of the blob-like structure, V-b, was measured as similar to 1 km/s with the fast camera at the intermediate area. V-b increased as the blob-like structure moved along major radius, R (outward). The average velocity of E x B drift derived from the probe measurement, slightly larger than V-b, was also increased as the blob-like structure propagated to larger R. (C) 2010 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.jnucmat.2010.12.225

Language：English   Publishing type：Research paper (scientific journal)  

Metallic dust behavior was observed in full lower hybrid current drive plasmas on TRIAM-1M, which had all-metal plasma facing components. TRIAM-1M was shut-down on December 2005 and then in-vessel dusts were collected with a suction aspirator through a membrane filter of 100 nm meshes. The toroidal distribution of the total mass of collected in-vessel dusts was measured and the largest amount was found to be near the vertical movable limiter (VML). The typical size of in-vessel dusts was 1-10 mu m and the impact on augmenting the surface area was not significant. Dusts were observed to be either flaky or spherical in form and the majority of the flakes had accumulated around the VML Eruption detritus were observed on the surface of large flakes and this suggests that the burst of blistering are one of the origins of in-vessel dusts. (C) 2010 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.jnucmat.2010.11.017

Language：English  

Language：English   Publishing type：Research paper (international conference proceedings)  

Language：English   Publishing type：Research paper (scientific journal)  

Both start-up and sustainment of plasma were successfully achieved by fully non-inductive current drive using microwave with a frequency of 8.2 GHz. Plasmas current of 15 kA was implemented for 1 s. Magnetic surface reconstruction exhibited a plasma shape with an aspect ratio of below 1.5. The plasma current was dependent significantly on the launched microwave power and vertical magnetic field, while not affected by the mode of launched wave and the toroidal refractive index. Hard X-ray (HXR) emitted from energetic electrons accelerated by the microwave was observed, and the discharge with a plasma current over 4 kA followed the same trend as the number of photons of 10 keV to 12 keV. This suggests that the plasma current may be driven by energetic electrons. Based on the experimental conditions, alternative explanations of how the plasma current could be driven are discussed.

DOI： 10.1088/1009-0630/13/3/08

Language：English   Publishing type：Research paper (scientific journal)  

Ohmic plasma currents of up to 17 kA with a discharge duration of 0.32 s have been obtained in the Kyushu University Experiment with Steady-State Spherical Tokamak (QUEST) with the help of electron cyclotron wave (ECW) and cancellation coils (CCs). The CCs, originally installed to create a field null in the plasma breakdown phase, are essential for producing plasma current in QUEST. Although the ohmic coil current is initially biased and then reduced completely to zero to induce the plasma current in 15-20 ms, we demonstrate that the flat top of the plasma current exceeding 20 ms is maintained by the vertical field after the ohmic current is switched off. This type of operation is quite favorable for extending pulsed operation to the steady state by electron Bernstein wave current drive (EBCD).

Language：English   Publishing type：Research paper (international conference proceedings)  

Language：English   Publishing type：Research paper (scientific journal)  

This paper presents a scheme for combining a Langmuir probe with a fast camera for measurement of the behavior of blob-like structures in the boundary region of an electron cyclotron resonance (ECR) heating plasma in Q-shu University Experiment with a Steady-State Spherical Tokamak (QUEST, major radius, R = 0.68 m, minor radius, a = 0.40 m, and toroidal magnetic field, Bt = 0.25 T at R = 0.64 m). The frame rate of the camera was typically set to 40,000 frames per second (FPS) with 192 × 144 pixels per frame. Radial motion of blob-like structures was observed in the half of the plasma space where the probe head was located. A radially movable and rotatable probe system was used to measure the floating potential from single unbiased tips, the potential of the positively biased tip, and the ion saturation current in two orthogonal directions in the outboard midplane region. Time series of the ion saturation current measured by the Langmuir probe and of pixels in a 40,000 FPS movie were compared and cross-correlated. The results of the two diagnostics agreed well, and the spatial scale was found to be of the same as the size of the probe head. The ion saturation current was asymmetric in terms of the time the blob-like structure was passing; fast camera imaging also clearly demonstrated the blobs' filamentary structures and radial motion at the edge of QUEST. This means that plasma in the blob-like structure hunches over, like blobs in other devices. The typical radial velocity of the structures is ∼1 km/s, and the structures were accelerated along their path of radial motion from the inner to the outer parts of the vacuum vessel.

Language：English   Publishing type：Research paper (other academic)  

The role of energetic electrons during the non-inductive current ramp-up is investigated in electron cyclotron heated plasma (≤ 60kW) in the spherical tokamak QUEST. The pulse height analysis of the hard X-ray (HX) is carried out with a fast time resolution of ≥1 ms, and acceleration of the electrons are studied by measuring the evolution of spectrum. It is found that wall recycling condition affects the formation of the tokamak configuration from an initial open magnetic field configuration. The vertical magnetic field Bz dependence of driven current Ip (~ 11 kA) is compared with those of HXs for studying contribution of fast electron. For high recycling case, Ip and HXs show peaks at an optimal low value of B_z/B_t = 7 ×10^-3, however, for low recycling case, both increase monotonically with B _z/B_t = 7 ×10^-3 - 21×10 ^-3.

Language：English   Publishing type：Research paper (scientific journal)  

It is important to determine the best method for reconstructing the magnetic flux when eddy currents are significantly induced during magnetic measurement in spherical tokamaks (STs). Four methods for this reconstruction are investigated, and the calculated magnetic fluxes are compared to those measured in the cavity of a vacuum vessel. The results show that the best method is the one that uses currents from virtual coils for reconstruction. In this method, the placement of the virtual coils is optimized with numerical simulations using the Akaike information criterion (AIC), which indicates the goodness of fit of models used to fit measured data. The virtual coils are set on a line 15 cm outside the vacuum vessel.

Language：English   Publishing type：Research paper (scientific journal)  

Particle retention and recycling in plasma fusion devices are generally associated with the diffusion of atomic hydrogen into the materials. The resulted permeation of atomic hydrogen is known as plasma driven permeation (POP). This permeation may also be significant, even in the walls, which are not directly exposed to the plasma. Under similar conditions, the permeation flux (Gamma(perm)) of hydrogen through a 30 mu m thick Ni membrane heated at 412-575K has been measured in the spherical tokamak QUEST. Gamma(perm) is being measured during the scans of different operating parameters like RF power (P(RF)), chamber pressure (P(chamber)), discharge widths (tau(dis)) and vertical magnetic field (B(Z)). Simultaneously edge plasma density and spectral intensities of atomic (Balmer) lines and molecular (Fulcher) bands have been compared with the permeation measurements. A linear relationship has been established between the time integrated Gamma(perm) i.e. permeation fluence (Q(perm)) and the time integrated H(alpha) intensity i.e. H(alpha) fluence (Q(alpha)). Q(perm) also shows a strong relationship with the edge plasma density and various spectral fluences. The obtained results are discussed for exploring the applicability of the permeation probes in measuring the atomic flux near the first walls. (C) 2010 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.fusengdes.2010.08.032

Language：English   Publishing type：Research paper (other academic)  

The Quasi-Optical (QO) HE11-mode exciter has been considered for mode coupling into the circular corrugated waveguides. It contained a feed antenna and QO mirror system. A fields radiated from the feed antenna were measured, and the mode contents was analyzed with irradiant waveguide modes.

DOI： 10.1109/ICIMW.2010.5612688

Language：English   Publishing type：Research paper (other academic)  

In LHD, an ECH system for fusion-relevant plasma heating has been greatly progressed with the development of megawatt gyrotrons. Net power of 3.7 MW could be injected into LHD. For stable operation, a new position and profile monitor of the mmw-beams in evacuated transmission lines is developed.

DOI： 10.1109/ICIMW.2010.5613060

Language：English   Publishing type：Research paper (other academic)  

The square waveguide phased-array antenna system has been developed for Electron Bernstein Emission diagnostics. The broadband orthomode transducer was prepared to measure the wave polarization. The measured fields were in good agreements with those evaluated from the Kirchhoff integral.

DOI： 10.1109/ICIMW.2010.5612710

Language：English   Publishing type：Research paper (other academic)  

The differential-phase reflectometry has been developed to measure the density profile, concerning electron Bernstein Wave (EBW) heating and current drive in the QUEST. The Phased Array Antenna (PAA) system was proposed for the reflectometry to improve the ratio of signal to noise. The PAA was confirmed to be effective, as a launching antenna to receive the larger reflected-wave signal.

DOI： 10.1109/ICIMW.2010.5612434

Language：English   Publishing type：Research paper (other academic)  

The CW phased-array antenna for the EBWH/CD has been developed in QUEST. The measured fields were in excellent agreement with those evaluated by the Kirchhoff integral. The thermal load and stress were analyzed. The phase array has been fast scanned to control the incident polarization and angle.

DOI： 10.1109/ICIMW.2010.5612352

Language：English   Publishing type：Research paper (international conference proceedings)  

Language：English   Publishing type：Research paper (international conference proceedings)  

Language：English   Publishing type：Research paper (international conference proceedings)  

The square waveguide phased-array antenna system has been developed for Electron Bernstein Emission diagnostics. The broadband orthomode transducer was prepared to measure the wave polarization. The measured fields were in good agreements with those evaluated from the Kirchhoff integral.

Language：English   Publishing type：Research paper (international conference proceedings)  

The differential-phase reflectometry has been developed to measure the density profile, concerning electron Bernstein Wave (EBW) heating and current drive in the QUEST. The Phased Array Antenna (PAA) system was proposed for the reflectometry to improve the ratio of signal to noise. The PAA was confirmed to be effective, as a launching antenna to receive the larger reflected-wave signal.

Language：English   Publishing type：Research paper (international conference proceedings)  

The CW phased-array antenna for the EBWH/CD has been developed in QUEST. The measured fields were in excellent agreement with those evaluated by the Kirchhoff integral. The thermal load and stress were analyzed. The phase array has been fast scanned to control the incident polarization and angle.

Language：English   Publishing type：Research paper (international conference proceedings)  

Language：English   Publishing type：Research paper (scientific journal)  

The differential-phase reflectometry has been developed to measure the density profile evolution, concering electron Bernstein Wave (EBW) heating and current drive in the QUEST. The proper phase dependence was confirmed in developed single-side-band heterodyne differential-phase reflectometry at RIAM microwave/mm-wave test facilities. The phase time evolutions in the QUEST were also successfully measured, but the weak reflected-wave caused the small ratio of signal to noise in the measurements. The Phased Array Antenna (PAA) system was proposed for the reflectometry to obtain the large reflected-wave. The oblique viewing EBW emission can be measured by the precise phasearray measurements between the PAA waveguide elements. The PAA was confirmed to be effective, as a launching antenna for the reflectometry, and as a receiving antenna for the EBW radiometry. Keywords: AM reflectometry, Electron Bernstein wave, phased-array antenna, QUEST

Language：English  

Language：English  

Language：English   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (scientific journal)  

The electron cyclotron resonance heating (ECRH) system on the Large Helical Device (LHD) has been in stable operation for similar to 11 yr in numerous plasma experiments. During this time, many upgrades to the system have been made, such as reinforcement of the gyrotron tubes, modification of the power supply depending on gyrotron type, and increase in the number of transmission lines and antennas. These efforts allow the stable injection of millimeter-wave power in excess of 2 MW. In parallel, various transmission components were evaluated, and antenna performance was confirmed at a high power level. The coupling efficiency of the millimeter wave from the gyrotron to the transmission line and the transmission efficiency through the waveguide were further improved in recent years. The feedback control of the wave polarization has also been tried to maximize the efficiency of wave absorption. The gyrotron oscillation frequency was reconsidered in order to extend the flexibility of the magnetic configuration in plasma experiments. The development of 77-GHz gyrotrons with the output of 1 MW per few seconds in a single tube is currently taking place in collaboration with the University of Tsukuba. Two such gyrotron tubes already have been installed and were used for plasma experiments recently. An ECRH system with a capability of the steady operation is required, because the LHD can continuously generate confinement magnetic fields using superconducting magnets. Not only the gyrotron but also the transmission system and components must withstand continuous power operation. Further acceleration of both the power reinforcement and a steady-state capability will allow the sustainment of high-performance plasmas.

Language：English   Publishing type：Research paper (scientific journal)  

For expanding applicable parameter ranges of electron cyclotron resonance heating (ECRH), various methods of ECRH have been studied with use of millimeter-wave sources of 77-, 82.7-, 84-, and 168-GHz gyrotrons in the Large Helical Device (LHD). The fundamental ordinary (O-) mode and the second-harmonic extraordinary (X-) mode are mainly used for starting up, sustaining, and controlling the plasma. Heating efficiencies of ECRH by launching of these modes have been investigated experimentally for wide range of the central electron density and compared with power absorption rates obtained by ray-tracing calculation. ECRH by the third-harmonic X-mode has been performed in each magnetic configuration B(ax) = 1 and 2 T with launching of 84-GHz range and 168-GHz millimeter waves. Increases of the electron temperature and the stored energy were observed in both cases. ECRH by the electrostatic electron Bernstein wave (EBW) has been expected to be a promising substitute in parameter ranges where the conventional methods of ECRH by the electromagnetic modes are not available. To perform ECRH by the EBW in LHD, extraordinary-EBW (X-B) and ordinary-extraordinary-EBW (O-X-B) mode conversion processes, the propagation of the wave, and the absorption have been investigated experimentally and theoretically.

Language：English   Publishing type：Research paper (scientific journal)  

Trials of steady-state operation (SSO) in the Large Helical Device (LHD) were started when a continuous wave (cw) gyrotron with the output power up to 0.2 MW was introduced to the electron cyclotron heating (ECH) system on LHD in 2003. During the first trial of SSO in the seventh LHD experimental campaign in 2004, severe temperature increase on the waveguide transmission line and, as a result, intense pressure increase in the evacuated wave guide occurred, which terminated the operation at 756 s. Additional pumping sections and cooling structures on the transmission line worked well, allowing a 3900-s sustainment of plasma with n(e) = 0.15 x 10(19) m(-3) and T(e0) = 1.7 keV by 0.1 MW injection power in 2005. The improvement of the ECH system by introducing cw gyrotrons with higher power for further improvement of plasma performance in SSO is in progress.Investigations on electron cyclotron current drive (ECCD) physics have been advanced a few years after the proof of ECCD in LHD. By obliquely injecting second-harmonic X-mode EC waves in toroidal direction, stable current up to 5.5 kA is driven, which was evaluated as a difference in plasma currents of the co- and counter-ECCD discharges with 0.1-MW EC wave power. It takes a few seconds for the driven current to saturate. Change in profile of rotational transform by ECCD and profile of driven current density are investigated by use of motional Stark effect measurement. Peaked and localized driven current profile at the plasma core region was confirmed for on-axis second-harmonic ECCD discharges.

Language：English   Publishing type：Research paper (scientific journal)  

Core electron-root confinement (CERC), observed in the Large Helical Device as well as in other helical devices, is an improved electron energy confinement mode. It is characterized by a highly peaked electron temperature profile in the core region and appears when the centrally focused electron cyclotron resonance heating power exceeds a certain threshold value. This threshold value has been clarified to associate with the transition of the radial electric field (E(r)) from the ion root (small negative value) to the electron root (large positive value greater than a few kV/m), based on the bifurcation nature of Er due to the ambipolarity condition of neoclassical transport fluxes that is specific in nonaxisymmetric configurations. It has been experimentally recognized that a steeper T(e) gradient is realized with a clear transition (power threshold nature) in target plasmas with counter neutral beam injection (NBI) than ones with codirectional NBI. It has been interpreted, based on the heat pulse propagation experiment, to be related to the rational sutface or the island induced by the NBI-driven current. Transport analyses have shown that the incremental thermal diffusivity of electron heat transport becomes lower, and the standard thermal diffusivity decreases with the increase of heating power in CERC plasmas.

Language：English   Publishing type：Research paper (other academic)  

Language：English   Publishing type：Research paper (international conference proceedings)  

Language：English   Publishing type：Research paper (international conference proceedings)  

Language：English   Publishing type：Research paper (international conference proceedings)  

Language：English   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (international conference proceedings)  

Language：English   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (scientific journal)  

After the non-inductive current startup experimental campaign on the spherical tokamak compact plasma wall interaction device (CPD), various localized damage tracks or footprint traces have been observed on plasma facing components (PFCs), such as the chamber walls and the 'stiffeners' that support them. Although the magnetic field configuration is mainly open, the footprint traces are classified as (1) radially distributed traces, (2) toroidal imperfect circular traces with small gaps, (3) arc-shaped traces and (4) vertically distributed traces. The surface analysis of the samples attached near the traces has been carried out by scanning electron microscopy and x-ray photoelectron spectroscopy. They suggest thin deposition of impurity materials (C, Cu, Ti, Fe and their oxides) over the traces. These footprint traces are analysed in view of the localized plasma wall interaction and the loss of energetic electrons using orbit calculations. Radially distributed traces correspond to the loss of co- and countermoving passing electrons mainly escaping along the magnetic field lines. The imperfect circular traces are found corresponding to lost orbits of the energetic trapped electrons largely crossing the magnetic field lines. Other traces are also discussed from a viewpoint of loss along the magnetic field lines and impurity deposition.

DOI： 10.1088/0029-5515/50/2/025017

Language：English   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (scientific journal)  

QUEST focuses on the steady state operation of the spherical tokamak by controlled PWI and electron Bernstein wave current drive. One of the main purposes of QUEST is an achievement of long duration discharge with MW-class injected power. As the result, QUEST should be operated in the challenging region on heat and particle handling. To do the particle handling, high temperature all metal wall up to 623K and closed divertors are planned, which is to realize the steady-state operation under recycling ratio, R = 1. This is a dispensable check to DEMO, because wall pumping should be avoided as possible in the view of tritium retention. The QUEST project will be developed in increment step such as, I. low β steady state operation in limiter configuration, II. low β steady state operation in divertor configuration, III. relatively high β steady state operation in closed divertor configuration. Phase I in the project corresponds to these two years, and final goal of phase I is to make full current drive plasma up to 20 kA. Closed divertor will be designed and tested in the Phase II. QUEST is running from Oct., 2008 and the first results are introduced.

DOI： 10.1585/pfr.5.S1007

Language：English   Publishing type：Research paper (scientific journal)  

For expanding applicable parameter ranges of electron cyclotron resonance heating (ECRH), various methods of ECRH have been studied with use of millimeter-wave sources of 77-, 82.7-, 84-, and 168-GHz gyrotrons in the Large Helical Device (LHD). The fundamental ordinary (0-) mode and the secondharmonic extraordinary (X-) mode are mainly used for starting up, sustaining, and controlling the plasma. Heating efficiencies of ECRH by launching of these modes have been investigated experimentally for wide range of the central electron density and compared with power absorption rates obtained by ray-tracing calculation. ECRH by the third-harmonic X-mode has been performed in each magnetic configuration B_ax = 1 and 2 T with launching of 84-GHz range and 168-GHz millimeter waves. Increases of the electron temperature and the stored energy were observed in both cases. ECRH by the electrostatic electron Bernstein wave (EBW) has been expected to be a promising substitute in parameter ranges where the conventional methods of ECRH by the electromagnetic modes are not available. To perform ECRH by the EBW in LHD, extraordinary-EBW (X-B) and ordinary-extraordinary-EBW (O-X-B) mode conversion processes, the propagation of the wave, and the absorption have been investigated experimentally and theoretically.

DOI： 10.13182/FST10-A10841

Language：English   Publishing type：Research paper (scientific journal)  

It is important to determine the best method for reconstructing the magnetic flux when eddy currents are significantly induced during magnetic measurement in spherical tokamaks (STs). Four methods for this reconstruction are investigated, and the calculated magnetic fluxes are compared to those measured in the cavity of a vacuum vessel. The results show that the best method is the one that uses currents from virtual coils for reconstruction. In this method, the placement of the virtual coils is optimized with numerical simulations using the Akaike information criterion (AIC), which indicates the goodness of fit of models used to fit measured data. The virtual coils are set on a line 15 cm outside the vacuum vessel.

DOI： 10.1585/pfr.5.S2083

Language：English   Publishing type：Research paper (scientific journal)  

Trials of steady-state operation (SSO) in the Large Helical Device (LHD) were started when a continuous wave (cw) gyrotron with the output power up to 0.2 MW was introduced to the electron cyclotron heating (ECH) system on LHD in 2003. During the first trial of SSO in the seventh LHD experimental campaign in 2004, severe temperature increase on the waveguide transmission line and, as a result, intense pressure increase in the evacuated waveguide occurred, which terminated the operation at 756 s. Additional pumping sections and cooling structures on the transmission line worked well, allowing a 3900-s sustainment of plasma with n_e=0.15 ×10¹⁹ m^-3 and T_e0 = 1.7 keV by 0.1 MW injection power in 2005. The improvement of the ECH system by introducing cw gyrotrons with higher power for further improvement of plasma performance in SSO is in progress. Investigations on electron cyclotron current drive (ECCD) physics have been advanced a few years after the proof of ECCD in LHD. By obliquely injecting secondharmonic X-mode EC waves in toroidal direction, stable current up to 5.5 kA is driven, which was evaluated as a difference in plasma currents of the co-and counter-ECCD discharges with 0.1-MW EC wave power. It takes a few seconds for the driven current to saturate. Change in profile of rotational transform by ECCD and profile of driven current density are investigated by use of motional Stark effect measurement. Peaked and localized driven current profile at the plasma core region was confirmed for on-axis second-harmonic ECCD discharges.

DOI： 10.13182/FST58-551

Language：English   Publishing type：Research paper (scientific journal)  

Various phenomena of plasma-wall interactions during long duration discharges in TRIAM-1M are investigated from macroscopic and microscopic viewpoints. It is found that the density dependence of the hydrogen neutral flux decay length is not very sensitive to the density (i.e. n_e^-0.2 ). Neutral transport through the scrape-off layer is important for structural formation of hydrogen recycling. The hydrogen retention in the co-deposited layer, which was obtained using a material probe during long duration discharges, is consistent with the global wall pumping rate estimated from particle balance analysis. The wall temperature and co-deposition play important roles on hydrogen re-emission and hydrogen absorption, respectively. Oxygen impurities should affect the erosion and deposition properties of the plasma-facing wall as well as the hydrogen retention property of the co-deposited layer. The co-deposition of hydrogen with molybdenum and the oxygen impurities both play important roles in the hydrogen recycling.

DOI： 10.1585/pfr.5.S2009

Language：English   Publishing type：Research paper (scientific journal)  

A new method is proposed to analyze the mode content of high power electromagnetic waves that are propagating through corrugated waveguides in the electron cyclotron resonance heating (ECH) systems for nuclear fusion devices. The method was applied to the 168GHz transmission line of the ECH system in the Large Helical Device (LHD) to evaluate the waveguide alignment. The mode content of the propagating waves could be accurately analyzed using this method. Furthermore, the wave field in the waveguide was reconstructed using the mode content information obtained for each mode.

DOI： 10.1585/pfr.5.S1029

Language：English   Publishing type：Research paper (scientific journal)  

This paper presents a scheme for combining a Langmuir probe with a fast camera for measurement of the behavior of blob-like structures in the boundary region of an electron cyclotron resonance (ECR) heating plasma in Q-shu University Experiment with a Steady-State Spherical Tokamak (QUEST, major radius, R = 0.68 m, minor radius, a = 0.40 m, and toroidal magnetic field, B_t = 0.25T at R = 0.64 m). The frame rate of the camera was typically set to 40,000 frames per second (FPS) with 192 ×144 pixels per frame. Radial motion of blob-like structures was observed in the half of the plasma space where the probe head was located. A radially movable and rotatable probe system was used to measure the floating potential from single unbiased tips, the potential of the positively biased tip, and the ion saturation current in two orthogonal directions in the outboard midplane region. Time series of the ion saturation current measured by the Langmuir probe and of pixels in a 40,000 FPS movie were compared and cross-correlated. The results of the two diagnostics agreed well, and the spatial scale was found to be of the same as the size of the probe head. The ion saturation current was asymmetric in terms of the time the blob-like structure was passing; fast camera imaging also clearly demonstrated the blobs' filamentary structures and radial motion at the edge of QUEST. This means that plasma in the blob-like structure hunches over, like blobs in other devices. The typical radial velocity of the structures is ~1km/s, and the structures were accelerated along their path of radial motion from the inner to the outer parts of the vacuum vessel.

DOI： 10.1585/pfr.5.S2077

Language：English   Publishing type：Research paper (scientific journal)  

Core electron-root confinement (CERC), observed in the Large Helical Device as well as in other helical devices, is an improved electron energy confinement mode. It is characterized by a highly peaked electron temperature profile in the core region and appears when the centrally focused electron cyclotron resonance heating power exceeds a certain threshold value. This threshold value has been clarified to associate with the transition of the radial electric field (E_r) from the ion root (small negative value) to the electron root (large positive value greater than a few kV/m), based on the bifurcation nature of E_r due to the ambipolarity condition of neoclassical transport fluxes that is specific in nonaxisymmetric configurations. It has been experimentally recognized that a steeper T_e gradient is realized with a clear transition (power threshold nature) in target plasmas with counter neutral beam injection (NBI) than ones with codirectional NBI. It has been interpreted, based on the heat pulse propagation experiment, to be related to the rational surface or the island induced by the NBI-driven current. Transport analyses have shown that the incremental thermal diffusivity of electron heat transport becomes lower, and the standard thermal diffusivity decreases with the increase of heating power in CERC plasmas.

DOI： 10.13182/FST10-A10791

Language：English   Publishing type：Research paper (scientific journal)  

The electron cyclotron resonance heating (ECRH) system on the Large Helical Device (LHD) has been in stable operation for ∼11 yr in numerous plasma experiments. During this time, many upgrades to the system have been made, such as reinforcement of the gyrotron tubes, modification of the power supply depending on gyrotron type, and increase in the number of transmission lines and antennas. These efforts allow the stable injection of millimeter-wave power in excess of 2 MW. In parallel, various transmission components were evaluated, and antenna performance was confirmed at a high power level. The coupling efficiency of the millimeter wave from the gyrotron to the transmission line and the transmission efficiency through the waveguide were further improved in recent years. The feedback control of the wave polarization has also been tried to maximize the efficiency of wave absorption. The gyrotron oscillation frequency was reconsidered in order to extend the flexibility of the magnetic configuration in plasma experiments. The development of 77-GHz gyrotrons with the output of 1 MW per few seconds in a single tube is currently taking place in collaboration with the University of Tsukuba. Two such gyrotron tubes already have been installed and were used for plasma experiments recently. An ECRH system with a capability of the steady operation is required, because the LHD can continuously generate confinement magnetic fields using superconducting magnets. Not only the gyrotron but also the transmission system and components must withstand continuous power operation. Further acceleration of both the power reinforcement and a steady-state capability will allow the sustainment of high-performance plasmas.

DOI： 10.13182/FST58-530

Language：English   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (other academic)  

Electron Cyclotron Resonance Heating (ECRH) has contributed to the achievement of high performance plasma production, high electron temperature plasmas and sustainment of steady-state plasmas in the Large Helical Device (LHD). Our immediate targets of upgrading the ECRH system are 5 MW several seconds and 1 MW longer than one hour power injection into LHD. The improvement will greatly extend the plasma parameter regime. For that purpose, we have been promoting the development and installation of 77 GHz/1-1.5 MW/several seconds and 0.3 MW/CW gyrotrons in collaboration with University of Tsukuba. The transmission lines are re-examined and improved for high and CW power transmission. In the recent experimental campaign, two 77 GHz gyrotrons were operated. One more gyrotron, which was designed for 1.5 MW/2 s output, was constructed and is tested. We have been promoting to improve total ECRH efficiency for efficient gyrotron-power use and efficient plasma heating, e.g. a new waveguide alignment method and mode-content analysis and the feedback control of the injection polarization. In the last experimental campaign, the 77 GHz gyrotrons were used in combination with the existing 84 GHz range and 168 GHz gyrotrons. Multi-frequency ECRH system is more flexible in plasma heating experiments and diagnostics. A lot of experiments have been performed in relation to high electron temperature plasmas by realization of the core electron-root confinement (CERC), electron cyclotron current drive (ECCD), Electron Bernstein Wave heating, and steady-state plasma sustainment. Some of the experimental results are briefly described.

DOI： 10.1063/1.3273796

Language：English   Publishing type：Research paper (scientific journal)  

Two-dimensional structure of density fluctuations is examined during the current jump phase, indicating a change from the open magnetic fields to the closed ones. During the smooth current ramp-up phase the two-dimensional contour of the LiI intensity shows vertically alignment, consistent with the magnetic surfaces. At the inflection point in I_p ramp-up the LiI intensity contour becomes flat in the observation regime and then suddenly a steep gradient and higher intensity regime are formed in the vertical direction. This higher intensity corresponds to a burst of LiI waveform. According to these changes in the contour, it is found that, within 1 ms around the burst of LiI, a low frequency coherent wave with a long wavelength rapidly grows. The relations with other signals (magnetic flux and microwave stray power) are discussed with respect to the topological change in the magnetic configuration and mode conversion of the incident electromagnetic waves.

DOI： 10.1088/1009-0630/11/4/06

Language：English   Publishing type：Research paper (other academic)  

In a spherical tokamak (ST), an operating frequency of the reflectometry is low due to the low magnetic field of the device. In the large ST, the reflected wave signal level is weak in the large propagating length, and a direct coupling between the launching and receiving antennae may become comparable level to the reflected signal due to the diffraction effect in the low operating frequency at the ST. New single-side-band heterodyne differential-phase reflectometry is proposed on a large ST device of the QUEST, to remove the direct coupling effect in an amplitude modulation reflectometry.

DOI： 10.1109/ICIMW.2009.5324785

Language：English   Publishing type：Research paper (other academic)  

The phased-array antenna has been designed for steady-state electron Bernstein wave heating and current drive at 8.2 GHz in the QUEST tokamak. The prototype antenna had rather large side-lobe components. In the new designed antenna the side-lobe components were reduced by modification of the antenna structure. For the steady-state operation, forced water-cooling and thermal stress were analyzed for the antenna component using the HFSS/ePhysics/ANSYS codes.

DOI： 10.1109/ICIMW.2009.5324704

Language：English   Publishing type：Research paper (other academic)  

The fields radiated from the waveguide were used to analyze propagating mode content in an oversized circular corrugated waveguide. The radiated fields were calculated by the Kirchhoff integral in the paraxial approximation. The irradiant waveguide modes of the radiated field are orthogonal within the paraxial approximation. The complex fractions in the irradiant mode expansion were deduced from an iteration process in which the reconstructed amplitude profile from the complex fractions coincided with the measured amplitude profile.

DOI： 10.1109/ICIMW.2009.5324607

Language：English   Publishing type：Research paper (scientific journal)  

Millimeter-wave components were re-examined for high power (Mega-Watt) and steady-state (greater than one hour) operation. Some millimeter-wave components, including waveguide joints, vacuum pumping sections, power monitors, sliding waveguides, and injection windows, have been improved for high power CW (Continuous Waves) transmission. To improve transmission efficiency, information about the wave phase and mode content of high power millimeter-waves propagating in corrugated waveguides, which are difficult to measure directly, were obtained by a newly developed method based on retrieved phase information. To optimize the plasma heating efficiency, a proof-of-principle study of the injection polarization feedback control was performed in the low power test stand.

Language：English   Publishing type：Research paper (other academic)  

A numerical code has been developed to treat the mode conversion process between electromagnetic and electron Bernstein waves for arbitrary propagation angle and polarization considering real experimental configurations on various devices. Robustness of any heating/detection condition with change of the plasma parameters or the frequency can be also estimated with this numerical code in a short time.

DOI： 10.1109/ICIMW.2009.5325602

Language：English   Publishing type：Research paper (scientific journal)  

Remarkable progress in the physical parameters of net-current free plasmas has been made in the Large Helical Device (LHD) since the last Fusion Energy Conference in Chengdu, 2006 (Motojima et al 2007 Nucl. Fusion 47 S668). The beta value reached 5% and a high-beta state beyond 4.5% from the diamagnetic measurement has been maintained for longer than 100 times the energy confinement time. The density and temperature regimes have also been extended. The central density has exceeded 1 x 10(21) m(-3) due to the formation of an internal diffusion barrier. The ion temperature has reached 5.2 keV at the density of 1.6 x 10(19) m(-3), which is associated with the suppression of ion heat conduction loss. Although these parameters have been obtained in separated discharges, each fusion-reactor relevant parameter has elucidated the potential of net-current free heliotron plasmas. Diversified studies in recent LHD experiments are reviewed in this paper.

DOI： 10.1088/0029-5515/49/10/104015

Language：English   Publishing type：Research paper (scientific journal)  

The QUEST (Kyushu, pronounced as "Q-shu" University Experiment with Steady-state spherical Tokamak) project is promoted from 2005 in the frame work of bi-directional collaboration organized by N1FS (National Institute for Fusion Science) to study plasma wall interaction on high temperature all-metal wall in long duration plasma on a spherical tokamak configuration. Present status and future plan of QUEST are presented.

DOI： 10.1541/ieejfms.129.589

Language：English   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (scientific journal)  

Language：Japanese   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (international conference proceedings)  

Language：English   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (international conference proceedings)  

A numerical code has been developed to treat the mode conversion process between electromagnetic and electron Bernstein waves for arbitrary propagation angle and polarization considering real experimental configurations on various devices. Robustness of any heating/detection condition with change of the plasma parameters or the frequency can be also estimated with this numerical code in a short time.

Language：English   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (scientific journal)  

Two-dimensional structure of density fluctuations is examined during the current jump phase, indicating a change from the open magnetic fields to the closed ones. During the smooth current ramp-up phase the two-dimensional contour of the LiI intensity shows vertically alignment, consistent with the magnetic surfaces. At the inflection point in I(p) ramp-up the LiI intensity contour becomes flat in the observation regime and then suddenly a steep gradient and higher intensity regime are formed in the vertical direction. This higher intensity corresponds to a burst of LiI wave form. According to these changes in the contour, it is found that, within +/-1 ms around the burst of LiI, a low frequency coherent wave with a long wavelength rapidly grows. The relations with other signals (magnetic flux and microwave stray power) are discussed with respect to the topological change in the magnetic configuration and mode conversion of the incident electromagnetic waves.

Language：English   Publishing type：Research paper (scientific journal)  

Active particle control capabilities of a lithium-gettered rotating drum poloidal limiter have been demonstrated during 50 kW RF-current drive discharges in a compact spherical tokamak with the major and minor radii of 30 and 20 cm, respectively. The pulse length is typically similar to 300 ms with a flat-top of similar to 250 ms. The rotating limiter is in the shape of cylinder with the diameter and axial length of 15 and 12 cm, respectively. It has reproducibly been observed that, as soon as the rotating drum is gettered with lithium, hydrogen recycling measured with H(proportional to) spectroscopy decreases by a factor of similar to 3 not only near the limiter but also in the center stack region. Also, the oxygen impurity level measured with O-II spectroscopy is reduced by a factor of similar to 3. Meanwhile, the core electron temperature increases from around 7 eV to 20 eV along which the flat-top toroidal plasma current is found to nearly double even at the same vertical magnetic field. Comprehensive surface analysis has been conducted to investigate hydrogen and lithium distributions over the rotating drum after plasma exposure. (C) 2009 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.jnucmat.2009.01.060

Language：English   Publishing type：Research paper (international conference proceedings)  

Electron Cyclotron Resonance Heating (ECRH) has contributed to the achievement of high performance plasma production, high electron temperature plasmas and sustainment of steady‐state plasmas in the Large Helical Device (LHD). Our immediate targets of upgrading the ECRH system are 5 MW several seconds and 1 MW longer than one hour power injection into LHD. The improvement will greatly extend the plasma parameter regime. For that purpose, we have been promoting the development and installation of 77 GHz/1–1.5 MW/several seconds and 0.3 MW/CW gyrotrons in collaboration with University of Tsukuba. The transmission lines are re‐examined and improved for high and CW power transmission. In the recent experimental campaign, two 77 GHz gyrotrons were operated. One more gyrotron, which was designed for 1.5 MW/2 s output, was constructed and is tested. We have been promoting to improve total ECRH efficiency for efficient gyrotron‐power use and efficient plasma heating, e.g. a new waveguide alignment method and mode‐content analysis and the feedback control of the injection polarization. In the last experimental campaign, the 77 GHz gyrotrons were used in combination with the existing 84 GHz range and 168 GHz gyrotrons. Multi‐frequency ECRH system is more flexible in plasma heating experiments and diagnostics. A lot of experiments have been performed in relation to high electron temperature plasmas by realization of the core electron‐root confinement (CERC), electron cyclotron current drive (ECCD), Electron Bernstein Wave heating, and steady‐state plasma sustainment. Some of the experimental results are briefly described.

Language：English   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (scientific journal)  

Two approaches associated with wall recycling have been performed in a small spherical tokamak device CPD (compact plasma wall interaction experimental device), that is, (1) demonstration of active particle recycling control, namely, 'active wall pumping' using a rotating poloidal limiter whose surface is continuously gettered by lithium and (2) a basic study of the key parameters which discriminates between 'wall pumping and fuelling'. For the former, active control of 'wall pumping' has been demonstrated during 50kW RF current drive discharges whose pulse length is typically similar to 300 ms. Although the rotating limiter is located at the outer board, as soon as the rotating drum is gettered with lithium, hydrogen recycling measured with H a spectroscopy decreases by about a factor of 3 not only near the limiter but also in the centre stack region. Also, the oxygen impurity level measured with O II spectroscopy is reduced by about a factor of 3. As a consequence of the reduced recycling and impurity level, RF driven current has nearly doubled at the same vertical magnetic field. For the latter, global plasma wall interaction with plasma facing components in the vessel is studied in a simple torus produced by electron cyclotron waves with I(p) < 1 kA. A static gas balance (pressure measurement) without external pumping systems has been performed to investigate the role of particle flux on a transition of 'wall fuelling' to 'wall pumping'. It is found that a critical particle flux exists to discriminate between them. Beyond the critical value, a large fraction (similar to 80%) of pressure drop ('wall pumping') is found, suggesting that almost all injected particles are retained in the wall. Below it, a significant pressure rise ('wall fuelling') is found, which indicates that particles are fuelled from the wall during/just after the discharge. Shot history effects (integrated particle recycling behaviour from the plasma facing surfaces) are seen on that the critical particle flux is reducing from 0.8 x 10(-4) to similar to 0.1 x 10(-4) Torr during the experimental campaign (similar to 3000 shots). In the wall pumping pressure range the wall pumping fraction is reduced with increasing surface temperature up to 150 degrees C.

DOI： 10.1088/0029-5515/49/5/055020

Language：English   Publishing type：Research paper (international conference proceedings)  

The fields radiated from the waveguide were used to analyze propagating mode content in an oversized circular corrugated waveguide. The radiated fields were calculated by the Kirchhoff integral in the paraxial approximation. The irradiant waveguide modes of the radiated field are orthogonal within the paraxial approximation. The complex fractions in the irradiant mode expansion were deduced from an iteration process in which the reconstructed amplitude profile from the complex fractions coincided with the measured amplitude profile.

Language：English   Publishing type：Research paper (international conference proceedings)  

In a spherical tokamak (ST), an operating frequency of the reflectometry is low due to the low magnetic field of the device. In the large ST, the reflected wave signal level is weak in the large propagating length, and a direct coupling between the launching and receiving antennae may become comparable level to the reflected signal due to the diffraction effect in the low operating frequency at the ST. New single-side-band heterodyne differential-phase reflectometry is proposed on a large ST device of the QUEST, to remove the direct coupling effect in an amplitude modulation reflectometry.

Language：English   Publishing type：Research paper (international conference proceedings)  

The phased-array antenna has been designed for steady-state electron Bernstein wave heating and current drive at 8.2 GHz in the QUEST tokamak. The prototype antenna had rather large side-lobe components. In the new designed antenna the side-lobe components were reduced by modification of the antenna structure. For the steady-state operation, forced water-cooling and thermal stress were analyzed for the antenna component using the HFSS/ePhysics/ANSYS codes.

Language：English   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (scientific journal)  

Experiments to study wall recycling behaviour have been performed in the small spherical tokamak compact plasma-wall interaction experimental device (CPD) from the viewpoint of global as well as local plasma wall interaction condition. Electron cyclotron resonance (ECR) plasma of typically similar to 50 to 400 ms duration is produced using similar to 40 to 80 kW RF power. In order to study the global wall recycling behaviour, pressure measurements are carried out just before and after the ECR plasma in the absence of any external pumping. The recycling behaviour is found to change from release to pumping beyond a certain level of pressure value which is again found to be a function of shot history. The real-time local wall behaviour is studied in similar RF plasma using a rotating tungsten limiter, actively coated with lithium. Measurement of H. light intensity in front of the rotating surface has indicated a clear reduction (similar to 10%) in the steady-state hydrogen recycling with continuous Li gettering of several minutes. (C) 2008 Elsevier BY. All rights reserved.

DOI： 10.1016/j.fusengdes.2008.05.009

Language：English   Publishing type：Research paper (other academic)  

A new method is proposed to analyze mode contents of high power electromagnetic waves that are propagating through corrugated waveguides in the electron cyclotron resonance heating (ECH) system for nuclear fusion devices. The method was applied to a 168GHz transmission line of the ECH system in Large Helical Device (LHD) to evaluate the waveguide misalignment. Mode contents of the propagating wave could be well obtained, and the wave field in the waveguide was reconstructed by using the information of each mode.

DOI： 10.1109/ICIMW.2008.4665825

Language：English   Publishing type：Research paper (international conference proceedings)  

Language：English   Publishing type：Research paper (international conference proceedings)  

Language：English   Publishing type：Research paper (scientific journal)  

Compact toroid (CT) injection experiments have recently been carried out in a compact plasma wall interaction experimental device (CPD) in the presence of toroidal as well as poloidal magnetic field and low density RF target plasma. Spectroscopic studies have been carried out using an IR camera, an IR spectrometer and an H(alpha) monitor to estimate the heat load deposition on the target and also to study the magnetic field as well as RF target plasma effects on CT plasma penetration behaviour. Using IR camera imaging of the target plate, heat load deposition on it has been estimated from the observed temperature rise of the plate and has been compared with the energy content of CT plasma. The estimation suggests that nearly 700 J of energy is uniformly deposited on the target plate under vacuum condition without any magnetic field. On the other hand, post exposure analysis of target plate using scanning electron microscopy has indicated local melting of the target plate. It has been observed that radial penetration of CT towards the target plate is reduced with the increase in the external magnetic field, both toroidal and poloidal. Moreover, the presence of RF target plasma in external magnetic fields seems to change the CT penetration behaviour significantly.

DOI： 10.1088/0029-5515/48/10/105001

Language：English   Publishing type：Research paper (international conference proceedings)  

Language：English   Publishing type：Research paper (international conference proceedings)  

Language：English   Publishing type：Research paper (international conference proceedings)  

Language：English   Publishing type：Research paper (international conference proceedings)  

QUEST (R=0.68m, a=0.4m) focuses on the steady state operation of the spherical tokamak (ST) by controlled PWI and electron Bernstain wave (EBW) current drive (CD). The QUEST project will be developed along two phases, phase I: steady state operation with plasma current, Ip=20-30kA on open divertor configuration and phase II: steady state operation with Ip=100kA and β of 10% in short pulse on closed divertor configuration. Feasibility of the missions on QUEST was investigated and the suitable machine size of QUEST was decided based on the physical view of plasma parameters. Electron Bernstein wave (EBW) current drive are planned to establish the maintenance of plasma current in steady state. Mode conversion efficiency to EBW was calculated and the conversion of 95% will be expected. A new type antenna for QUEST has been fabricated to excite EBW effectively. The situation of heat and particle handling is challenging, and W and high temperature wall is adopted. The start-up scenario of plasma current was investigated based on the driven current by energetic electron and the most favorable magnetic configuration for start-up is proposed.

Language：English   Publishing type：Research paper (scientific journal)  

An active current drive is a promising technique for improving plasma performances by controlling rotational transform and/or magnetic shear profiles in helical devices. A current drive based on electron cyclotron resonance heating is the most appropriate scheme for this purpose in terms of locality of driven current. Optimum conditions for an efficient electron cyclotron current drive (ECCD) in Large Helical Device (LHD) are being investigated using three-dimensional ray-tracing code, which can simulate propagation and power dissipation of electron cyclotron waves with large parallel refractive index. In the present experiment, inversion of directions of driven plasma current corresponding to injected ECCD modes was demonstrated successfully, and the results could be elucidated by the Fisch-Boozer theory. In addition, clear shifts of rotational transform were observed by motional Stark effect polarimetry. Our findings verified that the ECCD can be used as an effective actuator for controlling the rotational transform and magnetic shear profile in LHD.

Language：English   Publishing type：Research paper (scientific journal)  

In TRIAM-1M, plasma discharge can be sustained for over five hours [H. Zushi, et al., Steady-state tokamak operation, ITB transition and sustainment and ECCD experiments in TRIAM-1M, Nucl. Fusion 45 (2005) S142-S156]. In order to avoid sitting in front of one console for the purpose of monitoring the plasma discharge, it is recommended that the experimental information be accessible from any location at any time. In addition, simple services to access experimental information are required in order to promote the participation of multiple researchers in the TRIAM-1M experiment. Thus, A WEB-based integrated data processing system that provides management for experiment planning, an experimental log, numerical data, and plasma supervision has been installed in the TRIAM-1M. These services are composed primarily of an Apache WEB server, a Tomcat JSP/Servlet container, and a MySQL relational database. This system is constructed using the object-oriented Java language, which is easy to maintain and develop because of the intrinsic characteristics of the Java language. When participating in experiments, researchers are required only to prepare a WEB browser on any platform and are no longer required to memorize complex operations because all services are provided with a uniform user interface through a WEB browser. Furthermore, with the integration of these services, the required information and numerical data can be provided promptly by tracing HTML links that are created dynamically by server applications. (C) 2007 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.fusengdes.2007.12.012

Language：English   Publishing type：Research paper (scientific journal)  

Superconducting tokamak TRIAM-1M has just ceased its operation in December 2005. However, in order to develop the systematic study on plasma-wall interaction in long duration discharges in the TRIAM-1M tokamak, spherical tokamak QUEST is under construction. To fulfill the mission of QUEST under a small group of the university in collaboration with other group, control and interlock system must be designed appropriately. Hence, a spherical tokamak CPD was constructed and the experiment is carried out for preparation of QUEST. The control and interlock system for QUEST is tested and developed during the experiment in the CPD. (c) 2008 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.fusengdes.2008.01.004

Language：English   Publishing type：Research paper (scientific journal)  

Fully non-inductive lower hybrid current drive (LHCD) plasmas have been achieved in TRIAM-1M and HT-7 superconducting tokamaks, but the current control in toroidal plasma was performed in different way during their steady-state operation (SSO). In HT-7, the plasma current was controlled mainly by the ohmic field, and partially by LHCD through ohmic transformer magnetic flux feedback control. In TRIAM-1M, the plasma current was controlled by lower hybrid wave (LHW) power directly according to a simple simulation model. (c) 2008 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.fusengdes.2008.01.018

Language：English   Publishing type：Research paper (scientific journal)  

On CPD [1] (Compact PWI experimental Device) experiment, information of electronic logbook and sequence status are distributed by Web services to prepare future experimental environment such as steady state operation and remote participation. Hence, all the researchers can acquire information with a Web browser installed on a personal computer if they are connected to the Internet. However, to carry a notebook computer all the time is a burden to researchers. Furthermore, the researchers may not be always connected to the Internet. Mobile phones are superior in portability compared to notebook computers, and are easy to connect with Internet through the wireless network of the telecom carriers. Moreover, since recent mobile phones have full browsing function, their affinities to the Web services are becoming high. On this account, Web services for mobile phones are developed to access experimental information. For sequence monitoring, a mobile application MIDlet that utilizes special functions of mobile phone such as sound and vibration is also developed to draw attentions of researchers to sequence status. (C) 2007 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.fusengdes.2007.10.014

Language：English   Publishing type：Research paper (scientific journal)  

Two dimensional electron density profile measurement has been performed in the spherical tokamak CPD (Compact PWI experimental Device) using Li sheet beam imaging technique. The topological change from the open magnetic field line configuration to the closed one is visualized by this technique. The plasma current can be driven by RF waves itself in a weak mirror configuration and a clear change is observed in plasma boundary as well as magnetic field topology associated with the transition of the current from low (∼1 kA) to high (∼3 kA) value.

Language：English   Publishing type：Research paper (scientific journal)  

Two-dimensional lithium beam imaging technique has been applied in the spherical tokamak CPD (compact plasma wall interaction experimental device) to study the effects of magnetic field configurations on rf plasma boundary in the absence of any plasma current, and also for the measurement of a two-dimensional edge electron density profile. With the present working condition of the diagnostics, the minimum measured electron density can be similar to 1.0 X 10 16 m(-3); this is considered to be the definition for the plasma boundary. The performance of the lithium sheet beam is absolutely calibrated using a quartz crystal monitor. Experimental results reveal that magnetic field configuration, either mirror or so-called null, critically affects the rf plasma boundary. A sharp lower boundary is found to exist in magnetic null configuration, which is quite different from that in the weak mirror configuration. Theoretical calculations of particle drift orbit and. magnetic connection length (wall-to-wall) suggest that only mirror trapped particles are confined within a region where the magnetic connection length is similar to 4.0 in or more. A two-dimensional edge electron density profile is obtained from the observed Li I intensity profile. Overdense plasma formation is discussed from the viewpoint of mode conversion of rf wave into electron Bernstein wave and its dependence on the electron density profile. (c) 2008 American Institute of Physics.

DOI： 10.1063/1.2839034

Language：English   Publishing type：Research paper (international conference proceedings)  

Language：English   Publishing type：Research paper (scientific journal)  

Two dimensional electron density profile measurement has been performed in the spherical tokamak CPD (Compact PWI experimental Device) using Li sheet beam imaging technique. The topological change from the open magnetic field line configuration to the closed one is visualized by this technique. The plasma current can be driven by RF waves itself in a weak mirror configuration and a clear change is observed in plasma boundary as well as magnetic field topology associated with the transition of the current from low (~1 kA) to high (~3kA) value.

DOI： 10.1585/pfr.3.010

Language：English   Publishing type：Research paper (scientific journal)  

An active current drive is a promising technique for improving plasma performances by controlling rotational transform and/or magnetic shear profiles in helical devices. A current drive based on electron cyclotron resonance heating is the most appropriate scheme for this purpose in terms of locality of driven current. Optimum conditions for an efficient electron cyclotron current drive (ECCD) in Large Helical Device (LHD) are being investigated using three-dimensional ray-tracing code, which can simulate propagation and power dissipation of electron cyclotron waves with large parallel refractive index. In the present experiment, inversion of directions of driven plasma current corresponding to injected ECCD modes was demonstrated successfully, and the results could be elucidated by the Fisch-Boozer theory. In addition, clear shifts of rotational transform were observed by motional Stark effect polarimetry. Our findings verified that the ECCD can be used as an effective actuator for controlling the rotational transform and magnetic shear profile in LHD.

DOI： 10.1585/pfr.3.S1077

Language：English   Publishing type：Research paper (scientific journal)  

On tokamak devices, an induction electric field induced by poloidal field (PF) coils plays a role to produce initial plasma. On a DIII-D tokamak, the required induction electric field for plasma breakdown agrees well with theoretical predictions based on the Townsend avalanche theory. According to the Townsend avalanche theory, the minimum induction electric field for plasma breakdown depends on neutral gas pressure and connection length. For stable plasma breakdown, a sufficiently large induction electric field is required. However, in the case of spherical tokamaks without electric insulation in the toroidal direction, the effect of eddy currents flowing in the toroidal direction should be considered in evaluating a feasible induction electric field because this effect suppresses the generation of an induction electric field. On a QUEST spherical tokamak, the possibility of Townsend avalanche breakdown is studied by evaluating the connection length and achievable induction electric field. The connection length is greater than 100 in in the case where a null point is set to be R = 0.56 in with a CS coil current of 2.0 kA and a PF26 coil current of 0.4 kA. Moreover, the induction electric field is about 1.5 V at this point including the effect of eddy currents. With these values, the initial plasma production by the induction electric field is sufficiently possible on QUEST.

DOI： 10.1143/JJAP.47.287

Language：English   Publishing type：Research paper (international conference proceedings)  

Language：English   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (other academic)  

The broadband fast frequency-switching heterodyne radiometers have been developed to measure simultaneously three-band components of electron cyclotron emission with high time and frequency resolutions in the large helical device (LHD). The fast frequency-switching system was prepared as a local oscillator part of the heterodyne radiometers. The frequency switching time was 10 μs. The fundamental electron cyclotron emission spectrum was successfully measured using the radiometer in the LHD experiment.

Language：English   Publishing type：Research paper (other academic)  

The fields radiated from the waveguide were used to analyze propagating mode content in an oversized circular corrugated waveguide. The radiated fields were calculated by the Kirchhoff Integral in the paraxial approximation. The irradiant waveguide modes of the radiated field are orthogonal within the paraxial approximation. The waveguide mode content was evaluated from the irradiant waveguide modes. The radiated intensity and phase profiles were directly measured with a vector network analyzer system at 84GHz for the analysis. The analyzed waveguide mode content was consistent with that excited in the waveguide, predicted by the coupling theory.

Language：English   Publishing type：Research paper (other academic)  

The amplitude modulation reflectometer system has been developed to measure the density profile in detail on the Compact PWI ( plasma-wall interaction ) Device (CPD) experiments. The probing frequency range was 6-18 GHz, and its frequency-switching time was 10 μs in an arbitrary step. Using the broadband fast-scanning reflectometer, the wide-range density profile can be measured with high time resolution. In X-mode reflectometory, the developed system successfully measured the phase delay of propagating waves at 6.6, 8.8 and 13.2 GHz, reflected at the vessel wall of the CPD and the R-cutoff layers in the CPD plasma.

Language：English   Publishing type：Research paper (other academic)  

The phased array antenna system has been designed for electron Bernstein wave heating and current drive in the QUEST tokamak at 8.2 GHz. In order to control the launching polarization state, an orthomode transducer was designed to mix two orthogonal electric field components. The polarization state was arbitrarily controlled using phase difference and intensity ratio between the two field components. The fields radiated from the phased array antenna were evaluated using the Kirchhoff integral and HFSS codes. New designed phased array antenna steered a radiated beam to the desired launching-direction by the phased array.

Language：English   Publishing type：Research paper (scientific journal)  

A new technique to measure the current profile in plasmas with asymmetric distribution function such as lower hybrid current drive (LHCD) by using hard X-ray (HXR) energy spectrum measurement along the top and bottom identical line of sights (ILOS) is proposed and is applied to the full and partially LHCD plasmas on the TRIAM-1M tokamak at the first time in the world. The pitch angles were measured at R - R0 = ±2.5 cm, where R, R0 mean the major radii of the ILOS and the magnetic axis, respectively. The measured pitch angle of the magnetic field inverted at the magnetic axis estimated magnetic measurement in partially LHCD plasmas. This indicates that the difference of the measured pitch angles is caused by the plasma current in the plasma and this new method is available in detecting the current profile in tokamaks. In full LHCD plasma, no difference between the HXR signals along the top and bottom ILOS appear. This indicates that the current density around the magnetic axis was reduced compared with that in partially LHCD plasmas. This observation is no contradiction with power deposition of LHCD.

Language：English   Publishing type：Research paper (scientific journal)  

Two dimensional lithium beam imaging technique has been applied in the spherical tokamak, CPD (Compact Plasma wall interaction experimental Device), to study the effects of various magnetic field configurations on RF plasma confinement topology. The performance of the lithium sheet beam is absolutely calibrated by a quartz crystal monitor. Experimental results show that plasma initiation takes place at the electron cyclotron resonance layer in a simple torus configuration and then it expands quickly to the low magnetic field side. Different magnetic field configurations critically affect the RF plasma confinement topology. A sharp lower boundary exists for the RF plasma in magnetic null configuration. Magnetic connection length plays the key role in defining plasma boundary and the critical value of connection length for plasma to exist in CPD is found to be ∼ 5-6 meter for a given pressure condition.

Language：English   Publishing type：Research paper (international conference proceedings)  

Language：English   Publishing type：Research paper (scientific journal)  

In order to investigate the surface temperature effects on plasma fuel recycling and impurity release from the plasma facing components, plasma discharges have been performed under selected plasma-wall interaction (PWI) conditions in the high-field superconducting tokamak, TRIAM-1M. By moving a water-cooled molybdenum movable limiter (ML) beyond the last closed flux surface, as defined by poloidal limiters, the surface temperature profile on it is varied. Hot spots have been observed on the ML surface in such conditions. The release behaviour of fuel as well as impurity particles from the ML surface has been studied as a function of hot spot temperature (T-hot) by means of wide range spectroscopy (200-1600 nm). A critical Thot is found to be similar to 2100 K above which the emission of both hydrogen and impurity particles enhances significantly. This is indicative of some thermally activated process playing an important role in PWIs between the limiter and the edge plasma. With the rise in hot spot temperature localized PWI at the ML is found to dominate the global recycling even when external fuelling is stopped.

DOI： 10.1088/0029-5515/47/8/018

Language：English   Publishing type：Research paper (scientific journal)  

For initial plasma production, an induction electric field generated by applying voltage to a poloidal field (PF) coil system is used to produce a Townsend avalanche breakdown. When the avalanche margins are small, as for the International Thermonuclear Experimental Reactor (ITER) in which the induction electric field is about 0.3 V/m, the assistance of radio-frequency waves (RF) is provided to reduce the induction electric field required for reliable breakdown. However, the conditions of RF-assisted breakdown are not clear. Here, the effects of both RF and induction electric field on the RF-assisted breakdown are evaluated considering the electron loss. When traveling loss is the dominant loss, a simple model of an extended Townsend avalanche is proposed. In this model, the induction electric field required for RF-assisted breakdown can be decreased to half that required for induction breakdown.

DOI： 10.1143/JPSJ.76.084501

Language：English   Publishing type：Research paper (scientific journal)  

Combined experiments with lower hybrid (LH) waves (LHWs) and electron cyclotron (EC) waves (ECWs) have been performed to study counter (ctr) current drive (CD) (ctr-CD) in LHCD plasma. Although there exists a large spectrum gap under the condition of the forward (FW) LHCD at high ratio (> 13) of the thermal velocity and the phase velocity, the relativistic Doppler resonance for backward (BW) ECWs coupled with the energetic electrons streaming along the current direction is used to confirm that the gap in the opposite velocity region is also filled by BW LHWs and the counter current tends to be driven. Three experimental scenarios have been studied for ctr-CD by the following experiments: (a) BW-LHW injection into FW LHW, (b) BW-ECW injection into FW LHW, and (c) BW-ECW injection into bidirectional BW-LHCD and FW-LHCD plasma. A transition depending on the power ratio is observed in case (a). The Ohkawa current is discussed for the co-driven current observation for case (b). The role of the amount of the resonance electrons is understood as a function of the power ratio of BW LHW to FW LHW for case (c).

Language：English   Publishing type：Research paper (scientific journal)  

Calorimetric measurements using the temperature increment of cooling-water were carried out to estimate the heat load distribution on the plasma facing components (PFCs) in the limiter discharges on TRIAM-1M. Line averaged electron density, n(e), and LH power, P-LH, dependences of the heat load on PFCs were measured. The heat load on the limiters was proportional to n(e)(1.5) in the range of n(e) = 0.2-1.0 x 10(19) m(-3) and P-LH(I) in the range of P-LH = 0.005-0.09 MW. For P-LH > 0.1 MW, the plasma transition to an enhanced current drive (ECD) mode appeared and the n(e) dependences on the heat load on the limiter moderated. This indicates that the heat flux to scrape-off layer (SOL) region was reduced due to the improvement of the plasma confinement. The up-down asymmetry of the heat load on the vacuum vessel was enhanced in the ECD mode, which may be caused by the increasing of the direct loss of energetic electrons. (c) 2007 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.jnucmat.2007.01.255

Language：English   Publishing type：Research paper (scientific journal)  

A sheet thermal Li beam to study the density contour near X-point and divertor strike points has been developed for the spherical tokamak CPD. The CCD camera with a time resolution of 1 ms and spatial resolution of similar to 1 mm can record two-dimensional images of LiI resonance line (300 mm x 500 mm). The performance of the sheet beam is absolutely calibrated by a quartz crystal monitor. The Li flux of 10(18)-10(19) m(-2) s(-1) is achieved near the X-point. The sheet characteristics of beam width and uniformity are also confirmed. This technique is applied during plasma initiation and electron cyclotron heating phase of plasma in a simple torus with a small vertical field. The formation of vertically stretched plasma at the position of the cyclotron resonance is clearly found. The expansion process towards the low field side is observed and an over dense plasma (> 1 x 10(18) m(-3)) is obtained. A model calculation of LiI emission with density range of 1-5 x 10(18) m(-3) agrees fairly well with the observations. Over dense plasma formation is also discussed. (c) 2007 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.jnucmat.2007.01.259

Language：English   Publishing type：Research paper (scientific journal)  

The observation of dusts in plasmas was carried out by high speed camera in full non-inductive lower hybrid current drive (LHCD) plasma on TRIAM-1M. The velocities of dusts were 10-50 m/s. The number of dust generated per second increased with the discharge duration in the range of 20-57 s. This suggests that a part of dusts were generated from a movable limiter whose the surface temperature increases with the discharge duration. Dusts were coming from various directions even close to the movable limiter. Dusts were collected in the vacuum vessel by use of a kind of cleaner and the composition and the size of dust were examined by Scanning Electron Microscope (SEM). Dust size was several mu m and composition was Molybdenum mixed with small amount of elements of stainless metal. The poloidal distribution of the collected mass of dusts was almost uniform. (c) 2007 Published by Elsevier B.V.

DOI： 10.1016/j.jnucmat.2007.01.199

Language：English   Publishing type：Research paper (scientific journal)  

Tokamak operation has been performed under the localized plasma wall interaction conditions in TRIAM-1M, using a water cooled rail movable limiter (ML) made of Molybdenum. The limiter surface temperature dependence on hydrogen recycling and neutral molybdenum impurity production behaviour has been studied using visible as well as infrared spectroscopy during low power long pulse and high power short pulse discharges. In these experiments it is considered that escaped energetic electrons are lost mainly on ML and localised heat deposition makes a 'hot spot' on ML. It has been observed that H-2, H-alpha and MoI intensities critically depend on the hot spot temperature (T-hot) and enhanced recycling due to localized PWI on ML can fuel the discharge without any external fueling source. (c) 2007 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.jnucmat.2007.01.117

Language：English   Publishing type：Research paper (scientific journal)  

A diagnostic technique to study the global structure of the recycling and impurity (molybdenum) influx from the plasma facing components is proposed and is tested using the three kinds of perturbations (gas puffing, transport oscillation, and localized heat deposition). Balmer line intensities I-H alpha, charge exchange flux Gamma(CX) and neutral molybdenum line I-Mol are measured and their correlations are analyzed with response to the perturbation parameters (I-H alpha at the gas port, density n(e), and hot spot temperature T-hot). A simple model calculation is done to understand these correlations. It is found that the phase reversal of Gamma(CX) with respect to n(e) modulation is well reproduced and a critical density exists for the phase reversal of I-Mol. The evaporation Mo flux evaluated with measured T-hot is also compared with enhanced I-Mol for the heat load perturbation and it contribution to the total content of Mo ions is evaluated by similar to 30% increment. (c) 2007 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.jnucmat.2007.01.201

Language：English   Publishing type：Research paper (scientific journal)  

An in situ and real time measurement system of erosion and deposition has been developed, which is based on interference of light on a thin semi-transparent layer of redeposited material on substrate. It has been applied to long duration discharges in TRIAM-1M. The sapphire window is used as substrate. The deposition pattern on the window indicates up down and toroidal asymmetry. In the 5 h 16 min discharge, the thickness of the deposited layer increased monotonically with time and its deposition rate is similar to 1.5 x 10(16) Mo m(-2) S-1. In the low density and long duration discharge, the Mo deposition rate on the window depends on the ratio of Mo flux to hydrogen flux. (c) 2007 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.jnucmat.2007.01.055

Language：English   Publishing type：Research paper (scientific journal)  

Oxygen impurity behaviors of one long duration discharge and repetition of short duration discharges have been investigated in TRIAM-1M. In the former case, the OII line intensity divided by the line averaged electron density, which is considered as a monitor of oxygen concentration on the plasma facing surface (PFS), decreased with the time constant, tau(d), of 30-50 s during the discharge due to the hydrogen flux to PFS. In the latter case, tau(d) is in the range of 70-600 s. There exists a big difference of global behavior of oxygen impurity between both cases. The difference seems to result from the absence or presence of the interval time between the discharges. The oxygen concentration on PFS increases during the interval time due to adsorption of H2O. The time constant of the increase in the oxygen concentration is evaluated to be about 5500 s from Langmuir adsorption isotherms analysis. (c) 2007 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.jnucmat.2007.01.238

Language：English   Publishing type：Research paper (international conference proceedings)  

The phased array antenna system has been designed for electron Bernstein wave heating and current drive in the QUEST tokamak at 8.2 GHz. In order to control the launching polarization state, an orthomode transducer was designed to mix two orthogonal electric field components. The polarization state was arbitrarily controlled using phase difference and intensity ratio between the two field components. The fields radiated from the phased array antenna were evaluated using the Kirchhoff integral and HFSS codes. New designed phased array antenna steered a radiated beam to the desired launching-direction by the phased array.

Language：English   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (scientific journal)  

A new technique to measure the current profile in plasmas with asymmetric distribution function such as lower hybrid current drive (LHCD) by using hard X-ray (HXR) energy spectrum measurement along the top and bottom identical line of sights (ILOS) is proposed and is applied to the full and partially LHCD plasmas on the TRIAM-1M tokamak at the first time in the world. The pitch angles were measured at R - R₀ = ±2.5cm, where R, R₀ mean the major radii of the ILOS and the magnetic axis, respectively. The measured pitch angle of the magnetic field inverted at the magnetic axis estimated magnetic measurement in partially LHCD plasmas. This indicates that the difference of the measured pitch angles is caused by the plasma current in the plasma and this new method is available in detecting the current profile in tokamaks. In full LHCD plasma, no difference between the HXR signals along the top and bottomILOS appear. This indicates that the current density around the magnetic axis was reduced compared with that in partially LHCD plasmas. This observation is no contradiction with power deposition of LHCD.

DOI： 10.1585/pfr.2.S1071

Language：English   Publishing type：Research paper (scientific journal)  

Two dimensional lithium beam imaging technique has been applied in the spherical tokamak, CPD (Compact Plasma wall interaction experimental Device), to study the effects of various magnetic field configurations on RF plasma confinement topology. The performance of the lithium sheet beam is absolutely calibrated by a quartz crystal monitor. Experimental results show that plasma initiation takes place at the electron cyclotron resonance layer in a simple torus configuration and then it expands quickly to the lowmagnetic field side. Different magnetic field configurations critically affect the RF plasma confinement topology. A sharp lower boundary exists for the RF plasma in magnetic null configuration. Magnetic connection length plays the key role in defining plasma boundary and the critical value of connection length for plasma to exist in CPD is found to be ~ 5-6 meter for a given pressure condition.

DOI： 10.1585/pfr.2.S1103

Language：English   Publishing type：Research paper (scientific journal)  

A simple model of radio frequency wave (RF) assisted breakdown based on Townsend avalanche theory is proposed for the purpose of evaluating the effect of RF on Townsend avalanche breakdown. According to this model, the required minimum electric field for RF-assisted breakdown can be decreased down to half of that for breakdown by the induction electric field alone. The electric field of RF reaches a minimum when the frequency of the RF is equal to the electron cyclotron frequency.

DOI： 10.1585/pfr.2.007

Language：English   Publishing type：Research paper (scientific journal)  

Plasma shape reconstruction is important for plasma control of tokamak. Cauchy-Condition Surface (CCS) method is a numerical approach to reproduce plasma shape which has good precision in conventional tokamak. In order to apply it in the plasma shape reproduction of Compact PWI experimental Device (CPD) which is a new spherical tokamak in Kyushu University, the calculation precision of CCS method in CPD is analyzed in the paper.

DOI： 10.1585/pfr.2.S1095

Language：English   Publishing type：Research paper (scientific journal)  

Combined experiments with lower hybrid (LH) waves (LHWs) and electron cyclotron (EC) waves (ECWs) have been performed to study counter (ctr) current drive (CD) (ctr-CD) in LHCD plasma. Although there exists a large spectrum gap under the condition of the forward (FW) LHCD at high ratio (>13) of the thermal velocity and the phase velocity, the relativistic Doppler resonance for backward (BW) ECWs coupled with the energetic electrons streaming along the current direction is used to confirm that the gap in the opposite velocity region is also filled by BW LHWs and the counter current tends to be driven. Three experimental scenarios have been studied for ctr-CD by the following experiments: (a) BW-LHW injection into FW LHW, (b) BW-ECW injection into FW LHW, and (c) BW-ECW injection into bidirectional BW-LHCD and FW-LHCD plasma. A transition depending on the power ratio is observed in case (a). The Ohkawa current is discussed for the co-driven current observation for case (b). The role of the amount of the resonance electrons is understood as a function of the power ratio of BW LHW to FW LHW for case (c).

DOI： 10.13182/FST07-A1503

Language：English   Publishing type：Research paper (other academic)  

A new technique to measure the current profile in lower hybrid current drive (LHCD) plasmas by using hard X-ray (HXR) energy spectrum measurement along the top and bottom identical line of sight (ILOS) is proposed. An angle between ILOS and the magnetic filed line is out of alignment from the perpendicular by the poloidal magnetic field. As the emission cross-section of HXR strongly depends on the angle to the drift direction of energetic electrons, that is the magnetic filed line, this disagreement of the angles makes significant difference between the signals of detectors located on the top and bottom. As the result, the poloidal magnetic field is able to be derived from the difference of the HXR emission along the top and bottom ILOS. The method was applied to the full LHCD plasmas on the TRIMA-1M tokamak and the plasma current around the magnetic axis was estimated.

Language：English   Publishing type：Research paper (other academic)  

DOI： 10.1109/ICIMW.2006.368709

Language：English   Publishing type：Research paper (other academic)  

The limiter surface temperature dependence on hydrogen recycling and molybdenum impurity production behavior was studied with optical spectroscopy during long pulse plasma discharges in TRIAM-1M. It has been observed that the H _α intensity critically depends on the limiter surface temperature. The Mo-I intensity from the hot spot has shown a negative temperature dependence. This is believed to be due to the reduction of heat load and enhanced CX flux due to ion temperature rise.

DOI： 10.1109/FUSION.2005.252980

Language：English   Publishing type：Research paper (other academic)  

Internal Reconnection Event (IRE) is a characteristic relaxation phenomenon in STs (Sperical Tokamaks). To investigate the deformation of the plasma shape during IRE, four PIN diode arrays of 20 channels were installed on the TST-2@K (TST-2 at Kyushu University). Precursor of IRE was observed for several milli-seconds. The fluctuation was composed of two dominant components in frequency of 10kHz and 4kHz. The mode structure of 10kHz component is n/m=1/1 helical structure and 4kHz is n/m= 3/4. The overlap of modes (10kHz and 4kHz) was considered to be cause of IRE in TST-2 from the position and the growth of the modes.

Language：English   Publishing type：Research paper (other academic)  

A new diagnostic technique to study the global structure of the recycling among the plasma facing components PFCs has been proposed and tested using the three kinds of perturbations (gas puffing, transport oscillation, and localized heat deposition) at the frequency less than a few Hz . H_α intensities are measured at various toroidal and poloidal positions and FFT analysis shows that the propagation velocity of the neutral hydrogen in the toroidal direction is < 25 m/s for the gas puff modulation. For toroidally uniform transport oscillations, on the other hand, in-out asymmetry of recycling structure is found. The localized heat deposition causes the localized particle source distribution in the torus due to enhanced efflux of the hydrogen particles only fueled from the hot area.

Language：English   Publishing type：Research paper (international conference proceedings)  

Language：English   Publishing type：Research paper (international conference proceedings)  

In order to investigate the spectrum gap problem in LHCD and controllability for the current profile by means of counter current drive in full current drive plasma, experiments by combination of backward (BW) propagating LHW and/or ECW have been performed in a plasma sustained by forward (FW) propagating LHW. Three ctr-CD aspects have been investigated for relativistic resonance with respect to the power ratio of backward (PBW) to forward (PFW) LHWs; 1) a clear reduction of co-ICD for PBW/PFW < 0.8; 2) a rapid positive change in co-ICD and broadening in j(r) for PBW/PFW > 0.8 [H. Zushi et al., Nucl. Fusion 41 (2001) 1483]; and 3) a large positive change in co-ICD by ctr ECW into FW LHCD plasma and further changes to negative value in co-ΔICD depending on PBW/PFW of LHW.

Language：English   Publishing type：Research paper (international conference proceedings)  

Language：English   Publishing type：Research paper (international conference proceedings)  

Language：English   Publishing type：Research paper (international conference proceedings)  

Language：English   Publishing type：Research paper (scientific journal)  

A discharge longer than 5h was successfully achieved on TRIAM-1M by fully non-inductive lower hybrid current drive (LHCD). The heat load distribution into the plasma facing components (PFCs) during the 5 h discharge was investigated using calorimetric measurements, which estimated that the injected radio frequency (RF) power coincided with the total heat load amount to the PFCs. The power balance, including the portion of direct loss power of the fast electrons and the heat flux due to the charge exchange (CX) process, was also investigated. (c) 2006 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.fusengdes.2006.03.004

Language：English   Publishing type：Research paper (scientific journal)  

Several techniques for initiating the plasma current without the use of the central solenoid are being developed in TST-2. While TST-2 was temporarily located at Kyushu University, two types of start-up scenarios were demonstrated. (1) A plasma current of 4 kA was generated and sustained for 0.28 s by either electron cyclotron wave or electron Bernstein wave, without induction. (2) A plasma current of 10 kA was obtained transiently by induction using only outboard poloidal field coils. In the second scenario, it is important to supply sufficient power for ionization (100 kW of EC power was sufficient in this case), since the vertical field during start-up is not adequate to maintain plasma equilibrium. In addition, electron heating experiments using the X-B mode conversion scenario were performed, and a heating efficiency of 60% was observed at a 100 kW RF power level. TST-2 is now located at the Kashiwa Campus of the University of Tokyo. Significant upgrades were made in both magnetic coil power supplies and RF systems, and plasma experiments have restarted. RF power of up to 400 kW is available in the high-harmonic fast wave frequency range around 20 MHz. Four 200 MHz transmitters are now being prepared for plasma current start-up experiments using RF power in the lower-hybrid frequency range. Preparations are in progress for a new plasma merging experiment (UTST) aimed at the formation and sustainment of ultra-high beta ST plasmas.

DOI： 10.1088/0029-5515/46/8/S05

Language：English   Publishing type：Research paper (international conference proceedings)  

Language：English   Publishing type：Research paper (scientific journal)  

Plasma start-up and sustainment without an inductive field have been studied in the TST-2@K spherical tokamak using high power RF sources (8.2 GHz/up to 170 kW). Steady state discharges with a plasma current of 4 kA were achieved. The line integrated density was about 3 x 10(17) m(-2) and the electron temperature was 160 eV. A truncated equilibrium was introduced to reproduce magnetic measurements. It was found that a positive Pfirsch-Schluter current in the open field line region at the outboard boundary makes a significant contribution to the current. Insensitivity of the current to variations in the vertical field and RF power variation was also found.

DOI： 10.1088/0029-5515/46/7/003

Language：English   Publishing type：Research paper (international conference proceedings)  

Language：English   Publishing type：Research paper (international conference proceedings)  

Language：English   Publishing type：Research paper (international conference proceedings)  

Language：English   Publishing type：Research paper (scientific journal)  

The first successful high power heating of a high dielectric constant spherical tokamak plasma by an electron Bernstein wave (EBW) is reported. An EBW was excited by mode conversion (MC) of an X mode cyclotron wave injected from the low magnetic field side of the TST-2 spherical tokamak. Evidence of electron heating was observed as increases in the stored energy and soft x-ray emission. The increased emission was concentrated in the plasma core region. A heating efficiency of over 50% was achieved, when the density gradient in the MC region was sufficiently steep.

DOI： 10.1103/PhysRevLett.96.185003

Language：English   Publishing type：Research paper (international conference proceedings)  

As for hot gas flow according to fluid equation with interaction with electrical arc and solid chamber, fluid equation must be solved with moving boundary of the chamber and with interaction with hot arc and impurity intrusion. Here we propose application of CIP (Cubic Interpolated Propagation) scheme, which is convenient and good at efficient advection and boundary motion. The CIP code was tested in simulation of normal shock tube, which has an analytical solution according to Rankine-Hugoniot relation, and applied in radial shock tube. And it was tested also in simulation of shock tube with solid piston between two fluids, using density function and functional transformation.

Language：English   Publishing type：Research paper (international conference proceedings)  

The limiter surface temperature dependence on hydrogen recycling and molybdenum impurity production behavior was studied with optical spectroscopy during long pulse plasma discharges in TRIAM-1M. It has been observed that the H-alpha intensity critically depends on the limiter surface temperature. The Mo-I intensity from the hot spot has shown a negative temperature dependence. This is believed to be due to the reduction of heat load and enhanced CX flux due to ion temperature rise.

Language：English   Publishing type：Research paper (international conference proceedings)  

Intensity and phase profiles of the output beams from a remote steering antenna were measured in the anti-symmetric and symmetric directions. The beam properties are discussed with the imaging property calculations.

Language：English   Publishing type：Research paper (scientific journal)  

Conventional data acquisition systems cannot deal with data acquisition for a long-time discharge of a nuclear fusion reactor. Thus, continuous data acquisition with a real-time data presentation during discharge must be developed. Two data acquisition systems, which include alternating CAMAC data acquisition and long-time PCI data acquisition, are designed for the longtime operation of HT-7 tokamak. Since an effective alternating mode is adopted, the alternating CAMAC data acquisition can accurately and continuously acquire data at a rate of 10 kHz. The acquired data is immediately transmitted to a data server and real-time results can be presented during the plasma discharge. As for the long-time PCI data acquisition, a special kind of PCI A/D card, which has a hard disk on board, is designed to collect data at a max speed of 200 kHz. Thus, the total sampling duration is only related to the capacity of the hard disk on board. These two types of data acquisitions were applied to HT-7 tokamak and a 250 s discharge was acquired. These data acquisition systems were also successfully demonstrated on a 2500 s plasma discharge on TRIAM-1M. This paper describes the two data acquisitions in detail. (c) 2005 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.fusengdes.2005.09.055

Language：English   Publishing type：Research paper (scientific journal)  

Magnetic sensorless control experiments of the plasma horizontal position have been carried out in the superconducting tokamak HT-7. Previously the horizontal position was calculated from the vertical field coil current and voltage without using signals of magnetic sensors like magnetic coils and flux loops placed near the plasma. The calculations are made focusing on the ripple frequency component of the power supply with thyristor and directly from them without time integration. There is no drift problem of integrator of magnetic sensors. Two kinds of experiments were carried out, to keep the position constant and swing the position in a triangular waveform. (c) 2005 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.fusengdes.2005.08.094

Language：English   Publishing type：Research paper (scientific journal)  

Magnetic sensorless sensing and control experiments with the plasma horizontal position have been carried out in the superconducting tokamak HT-7. The sensing is made to focus on the ripple frequency component of the power supply with thyristor and directly from them without time integration. There is no drift problem with the integrator of magnetic sensors. Two kinds of control experiments have been carried out: to keep the position constant and swing the position in a triangular waveform. And magnetic sensorless sensing of plasma shape is discussed.

DOI： 10.1088/1009-0630/8/1/18

Language：English   Publishing type：Research paper (other academic)  

As for hot gas flow according to fluid equation with interaction with electrical arc and solid chamber, fluid equation must be solved with moving boundary of the chamber and with interaction with hot arc and impurity intrusion. Here we propose application of CIP (Cubic Interpolated Propagation) scheme, which is convenient and good at efficient advection and boundary motion. The CIP code was tested in simulation of normal shock tube, which has an analytical solution according to Rankine-Hugoniot relation, and applied in radial shock tube. And it was tested also in simulation of shock tube with solid piston between two fluids, using density function and functional transformation.

DOI： 10.1109/ICPST.2006.321521

Language：English   Publishing type：Research paper (other academic)  

In TRIAM-1M, the influence of the local structure of the first wall on the global wall recycling has been studied using the ML with good cooling capability. No wall saturation was observed in the 5 h 16 min discharge with the ML, since particle release was suppressed by less temperature rise of the main chamber. The global wall pumping rate without the ML, ~8.6 x 10
¹⁶
atoms m
^-2
s
^-1
, is 3.6 times higher than that of the long duration discharge without the ML. The temperature rise in the wall is attributed to the transition of the wall role from the particle sink to the source. The co-deposition of hydrogen with Mo is a key for the wall role of the particle sink. No clear difference of the growth rate of Mo deposits on the viewing window located from the limiter surface between with and without the ML. The growth rate is from ~2 x 10
^-4
nm/s to ~8 x 10
^-4
nm/s. The toroidal structure of hydrogen recycling changes significantly near at the ML. However, integration of H intensity with and without the ML over the whole toroidal length is about the same within 3% , which is less than the error, i.e. 15%.

Language：English   Publishing type：Research paper (scientific journal)  

Two types of improved core confinement were observed during centrally focused electron cyclotron heating (ECH) into plasmas sustained by counter (CNTR) and Co neutral beam injections (NBI) in the Large Helical Device. The CNTR NBI plasma displayed transition phenomena to the high-electron-temperature state and had a clear electron internal transport barrier, while the Co NBI plasma did not show a clear transition or an ECH power threshold but showed broad high temperature profiles with moderate temperature gradient. This indicated that the Co NBI plasma with additional ECH also had an improved core confinement. The electron heat transport characteristics of these plasmas were directly investigated using heat pulse propagation excited by modulated ECH. These effects appear to be related to the m/n = 2/1 rational surface or the island induced by NBI beam-driven current.

DOI： 10.1088/0029-5515/45/11/021

Language：English   Publishing type：Research paper (scientific journal)  

Experiments aiming at 'day long operation at high performance' have been carried out in TRIAM-1M. The record value of the discharge duration was updated to 5 h and 16 min. Steady-state tokamak operation is studied under the localized plasma wall interaction conditions. The distributions of the heat load, the particle recycling flux and impurity source are investigated to understand the co-deposition and wall pumping. The formation and sustainment of an internal transport barrier (ITB) in enhanced current drive mode has been investigated by controlling the lower hybrid driven current profile by changing the phase spectrum. An ITER relevant remote steering antenna for electron cyclotron wave injection was installed and a relativistic Doppler resonance of the oblique propagating extraordinary wave with energetic electrons driven by lower hybrid waves was studied.

DOI： 10.1088/0029-5515/45/10/S12

Language：English   Publishing type：Research paper (international conference proceedings)  

Language：English   Publishing type：Research paper (international conference proceedings)  

Language：English   Publishing type：Research paper (scientific journal)  

One of the main objectives of LHD is to extend the plasma confinement database for helical systems and to demonstrate such extended plasma confinement properties to be sustained in the steady state. Among the various plasma parameter regimes, the study of confinement properties in the collisionless regime is of particular importance. Electron cyclotron resonance heating (ECRH) has been extensively used for these confinement studies of LHD plasma from the initial operation. The system optimizations including the modification of the transmission and antenna system are performed with special emphasis on the local heating properties. As a result, a central electron temperature of more than 10 keV with an electron density of 0.6 x 10(19) m(-3) is achieved near the magnetic axis. The electron temperature profile is characterized by a steep gradient similar to those of an internal transport barrier observed in tokamaks and stellarators. The 168 GHz ECRH system demonstrated efficient heating at densities more than 1.0 x 10(20) m(-3). The continuous wave ECRH system is successfully operated to sustain a 756 s discharge.

DOI： 10.1088/0741-3335/47/5A/008

Language：English   Publishing type：Research paper (scientific journal)  

Low-power tests are carried out to evaluate the performance of the newly designed MOU by reversely injecting a Gaussian beam from the output port of the MOU. The output beam from the input port of the MOU is measured with the analyzer. In this inverse process, the: injected Gaussian beam should be converted to the real beam pattern radiated from the gyrotron. Using correlation analysis, the measured- and calculated-electric fields are compared in their amplitude- and phase-profiles. Good agreement between the measurement and the calculation is obtained. Therefore, it is judged that the phase-correcting mirrors in the MOU will work correctly in upcoming high-power tests. &COPY; 2005 Elsevier B.V. All rights reserved.A low-power test system using a vector network analyzer is constructed to evaluate the performance of phase correcting mirrors in a matching optics unit (MOU). The MOU is used in order to facilitate the good coupling of a millimeter wave beam from a gyrotron into corrugated waveguides. The newly developed diagnostic system allows us to make measurements not only of intensity-, but also of phase-profiles of millimeter wave beams and opens a new range of possibilities for designing and testing high-precision quasi-optical systems.

DOI： 10.1016/j.fusengdes.2004.12.002

Language：English   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (scientific journal)  

This paper reports on the results of an experimental investigation into electron cyclotron resonance heating (ECRH) using the fundamental O-mode in the large helical device (LHD). The aim of the experiment is to understand the mechanism of the selective excitation of the O-mode EC wave and, thereby, optimize the performance of the ECRH. The polarization angle and the ellipticity of the electric field of the incident high power millimetre waves were varied in the experiment using a pair of grating polarizers. The responses of the plasma stored energy and electron temperature were measured to evaluate the effects. The optimum values of the polarization angle and the ellipticity for the ECRH were identified experimentally. A simple model of power branching between the O- and X-modes is introduced in order to interpret the observed experimental results. Specific to the ECRH in the LHD configuration, the effect of magnetic shear in the plasma peripheral region is considered.

DOI： 10.1088/0741-3335/47/3/009

Language：English   Publishing type：Research paper (scientific journal)  

The polarization state of a wave is an important factor in electron cyclotron resonance heating (ECRH) and current drive (ECCD), for it strongly affects the propagation and absorption of the wave in the plasma. A real-time monitor of the polarization of the EC beam has been developed for use in ECRH/ECCD experiments in large helical device (LHD). Two orthogonal components of the wave field are measured in one of the miter-bends by use of a specially designed coupler and a waveguide circuit with a 0degrees-90degrees phase switch to deduce the polarization parameters: the polarization angle alpha and the ellipticity beta. Since fast-response pin diodes are used for the switches, the polarization is determined every 3 ms, facilitating real time acquisition of the polarization. This article reports on the design and the principle of this monitor as well as on the algorithm used to calculate alpha and beta. This article also reports on the method of calibration, for the accuracy of this measurement depends on it. Finally, a comparison is made between polarization parameters measured by a monitor actually installed on one of the ECRH transmission lines on LHD and the value set by the polarizer in ECRH experiments. (C) 2005 American Institute of Physics.

DOI： 10.1063/1.1848657

Language：English   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (international conference proceedings)  

Language：English   Publishing type：Research paper (international conference proceedings)  

Language：English   Publishing type：Research paper (other academic)  

A remote steering antenna designed as a symmetric direction antenna is prepared for Electron Cyclotron Heating (ECH) and Current Drive (ECCD) experiments on the TRIAM-1M tokamak. The steering angles of 8-19 degrees are available in the design, in addition to the angle near 0 degrees. The antenna is tested at both of high and low power levels. The fraction of radiated power to injected power at the antenna is roughly evaluated at the low power test. The angle radiated from the antenna is checked from both of the intensity and phase measurements at the low power test. Several percentage losses at the antenna are evaluated at the high power test. The antenna system is evaluated to work correctly.

DOI： 10.1109/ICIMW.2004.1422297

Language：English   Publishing type：Research paper (international conference proceedings)  

Language：English   Publishing type：Research paper (international conference proceedings)  

Language：English   Publishing type：Research paper (international conference proceedings)  

Language：English   Publishing type：Research paper (international conference proceedings)  

Language：English   Publishing type：Research paper (scientific journal)  

Internal transport barrier (ITB) has been obtained in full lower hybrid current driven (LHCD) plasmas on a superconducting tokamak, TRIMA-1M (R=0.84m, a x b=0.12mx0.18m, BT<8T). The formation of ITB depends on the current density profile, j(r), varied by the power deposition of the lower hybrid (LH). The plasma with ITB can be maintained by the LH power deposited around the foot point of ITB up to 25 sec, which corresponds to more than 100 times of current diffusion time, τL/R. ITB is terminated by the reduction of current drive efficiency caused by metal impurities accumulation. In some condition, self-organized slow sawtooth oscillations (SSSO) of plasma current, density, temperature, and so on with the period comparable to the current diffusion time have been also observed during ITB discharge. The oscillation has the capability of particle exhaust, as the result, it may play an role in the avoidance of the impurity accumulation and the dilution in the future steady state fusion plasma with ITB, as the edge-localized mode in H-mode.

Language：English   Publishing type：Research paper (scientific journal)  

Characteristics of ion acceleration in the internal magnetic reconnection events (IRE) have been studied by means of a neutral particle energy analyzer (NPA) in Tokyo Spherical Tokamak (TST-2). The major and minor radii are 0.38 m and 0.25m, respectively. The magnetic field strength is 0.3T and the maximum plasma current is up to 140 kA. The electron and ion temperatures are 0.4-0.5 keV and 0.1 keV, respectively and the electron density is ~1x1019 m-3. The NPA can be scanned toroidally from q = 74&deg; (cw) to q = 114&deg; (ccw), where q = 90&deg; corresponds to the perpendicular sightline. The direction of the plasma current is cw. The NPA signals are digitized at every 50 ms. The NPA is calibrated in the energy range of 0.1 keV < E < 8.4 keV. When the IRE occurs, it is observed that the plasma current increases by ~ 20% and the loop voltage drops from 0.6 V to-5 V for ~ 0.1 ms. The enhanced charge exchange flux is observed by more than one order of magnitude at ~ 1 keV for this reconnection phase. The ion temperature increases by 80 eV at IREs. The angle q dependence of increment of Ti shows that DTi (q = 74&deg;) is higher than that for q = 114&deg;. This observation suggests that an ion is accelerated initially in the direction of magnetic field lines. The time evolution of the ion distribution function is simulated with a Fokker-Planck code taking into account the electric field effects.

Language：English   Publishing type：Research paper (international conference proceedings)  

Language：English   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (scientific journal)  

Recent progress in the Large Helical Device (LHD) experiment during the last 2 yr is reviewed. The LHD has been extending its operational regime toward fusion-relevant conditions while taking advantage of the net-current-free heliotron concept employing a superconducting coil system. Heating capability has exceeded 10 MW, and the central ion and electron temperatures have reached 7 and 10 keV respectively. The maximum values of beta and pulse length have been extended to 3.2% and 150 s, respectively. Several encouraging physics observations have been obtained, i.e., simultaneous achievement of the mitigation of the magnetohydro-dynamic instability criteria and good confinement, and formation of an internal transport barrier. The initial results have been obtained using a local island divertor, which shows the possibility of particle control at the plasma edge.

Language：English   Publishing type：Research paper (international conference proceedings)  

Language：English   Publishing type：Research paper (scientific journal)  

The characteristics of transport in electron internal transport barriers (ITB) and in the vicinity of a rational surface with a magnetic island were analyzed using trasient transport analysis and steady state transport analysis. An electron ITB appeared in the large helical device (LHD), when the heating power of the electron cyclotron heating exceeded a power threshold. It was shown by the transport analysis that both the standard electron thermal diffusivity and the incremental electron thermal diffusivity are reduced significantly in the ITB. The results show that the radial heat flux near the rational surface is focused at the X-point region, and that may be the mechanism to induce an ITB near a rational surface.

DOI： 10.1063/1.1688787

Language：English   Publishing type：Research paper (scientific journal)  

The formation of electron internal transport barriers (ITBs) was studied using electron cyclotron (EC) heating in JT-60U positive shear (PS) and reversed shear (RS) plasmas with scan of neutral beam (NB) power. With no or low values of NB power and with a small radial electric field (Er) gradient, a strong, box-type electron ITB was formed in RS plasmas while a peaked profile with no strong electron ITBs was observed in PS plasmas within the available EC power. When the NB power and the Er gradient were increased, the electron transport in strong electron ITBs with EC heating in RS plasmas was not affected, while electron thermal diffusivity was reduced in conjunction with the reduction of ion thermal diffusivity, and strong electron and ion ITBs were formed in PS plasmas.

DOI： 10.1088/0741-3335/46/5A/003

Language：English   Publishing type：Research paper (scientific journal)  

Plasmas with an electron internal transport barrier (ITB), which is characterized by peaked electron temperature profiles, are obtained in the JT-60U tokamak and in the large helical device (LHD) when the electron cyclotron heating (ECH) is focused on the magnetic axis. The maximum values of R/L-Te, where R is the major radius and LT. is the scale length of the electron temperature gradient, are similar for the LHD and JT-60U ITB plasmas. However, there is a clear jump of R/L-Te observed in LHD but not in JT-60U in the ECH power scan. This result is consistent with the fact that the trigger mechanism of the electron ITB is the fast transition of the radial electric field from a small negative E, to a large positive E, in LHD and a change of the magnetic shear from positive to negative is required for the formation of the electron ITB in JT-60U. There are also differences in the electron temperature profiles inside the ITB. The flattening of the electron temperature profile inside the strong ITB could be explained by the sharp increase of q values observed in JT-60U, while no flattening of the electron temperature profile is observed in LHD, where the central q values stay low.

DOI： 10.1088/0741-3335/46/5A/004

Language：English   Publishing type：Research paper (scientific journal)  

Characteristics of transport in electron internal transport barriers (ITB) and in the vicinity of a rational surface with a magnetic island are studied with transient transport analysis as well as with steady state transport analysis. Associated with the transition of the radial electric field from a small negative value (ion-root) to a large positive value (electron-root), an electron ITB appears in the Large Helical Device [M. Fujiwara , Nucl. Fusion 41, 1355 (2001)], when the heating power of the electron cyclotron heating exceeds a power threshold. Transport analysis shows that both the standard electron thermal diffusivity, chi(e), and the incremental electron thermal diffusivity, chi(e)(inc) (the derivative of normalized heat flux to temperature gradient, equivalent to heat pulse chi(e)), are reduced significantly (a factor 5-10) in the ITB. The chi(e)(inc) is much lower than the chi(e) by a factor of 3 just after the transition, while chi(e)(inc) is comparable to or even higher than chi(e) before the transition, which results in the improvement of electron transport with increasing power in the ITB, in contrast to its degradation outside the ITB. In other experiments without an ITB, a significant reduction (by one order of magnitude) of chi(e)(inc) is observed at the O-point of the magnetic island produced near the plasma edge using error field coils. This observation gives significant insight into the mechanism of transport improvement near the rational surface and implies that the magnetic island serves as a poloidally asymmetric transport barrier. Therefore the radial heat flux near the rational surface is focused at the X-point region, and that may be the mechanism to induce an ITB near a rational surface. (C) 2004 American Institute of Physics.

DOI： 10.1063/1.1668787

Language：English   Publishing type：Research paper (international conference proceedings)  

A remote steering antenna designed as a symmetric direction antenna is prepared for Electron Cyclotron Heating (ECH) and Current Drive (ECCD) experiments on the TRIAM-IM tokamak. The steering angles of 8-19 degrees are available in the design, in addition to the angle near 0 degrees. The antenna is tested at both of high and low power levels. The fraction of radiated power to injected power at the antenna is roughly evaluated at the low power test. The angle radiated from the antenna is checked from both of the intensity and phase measurements at the low power test. Several percentage losses at the antenna are evaluated at the high power test. The antenna system is evaluated to work correctly.