九州大学 研究者情報
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恩地 拓己(おんち たくみ) データ更新日:2020.07.01

助教 /  応用力学研究所 附属高温プラズマ理工学研究センター


原著論文
1. Hatem Elserafy, Kazuaki Hanada, Shinichiro Kojima, Takumi Onchi, Ryuya Ikezoe, Kengoh Kuroda, Hiroshi Idei, Makoto Hasegawa, Ryota Yoneda, Masaharu Fukuyama, Arseniy Kuzmin, Aki Higashijima, Takahiro Nagata, Shoji Kawasaki, Shun Shimabukuro, Nicola Bertelli, Masayuki Ono, Electron Bernstein wave conversion of high-field side injected X-modes in QUEST, Plasma Physics and Controlled Fusion, 10.1088/1361-6587/ab6903, 62, 3, 2020.01, [URL], This paper presents a detailed design of the Q-shu University experimental steady state spherical tokamak's (QUEST's) high-field side (HFS) injection system for electron Bernstein wave (EBW) excitation and the results of an experimental comparison of the HFS eXtraordinary X-mode and low-field side (LFS) ordinary O-mode injection of 8.2 GHz radio frequency (RF) power. Waveguides, as an alternative to mirror polarizers for transmitting RF X-mode power from LFS to HFS for EBW conversion, were used instead of the installation of an RF mirror. Testing of LFS-to-HFS RF power transmission at 8.2 GHz, using an RG-50-type vacuum waveguide in a bench-scale device filled with SF6 gas at 0.03 Mpa, revealed that an RF power of 10.8 kW could traverse the fundamental electron cyclotron resonance layer for 60 s without breakdown. The short-length, open-ended waveguide antenna used in the HFS injection-induced wave diffraction reduced the efficiency of power delivery to the upper hybrid resonance (UHR) by approximately 7% at an electron temperature of 50 eV. The HFS injection was able to produce brighter camera images than the standard LFS injection. The location of the UHR, as estimated by measuring the density with an interferometer, agreed with its location as measured by plasma radiation low-field, side-edge positions shown by fast camera imaging. This indicates that the plasma was produced by mode-converted EBW. The HFS injection had an absorption efficiency of 96%, compared to 40% for LFS. A greater fluctuation of floating potential adjustable to the lower hybrid wave (LHW) was observed in the HFS case by installing a Langmuir probe, confirming that EBW conversion efficiency was higher in the HFS case. Moreover, after setting the poloidal field to BPF = 7.6 mT, plasma current (IP ) in the HFS peaked at 1.3 kA, as opposed to 0.3 kA for LFS, despite LFS injection having a total power of 55 kW, compared to 40 kW for HFS. However, as the impurity level was comparatively high, it is believed that this IP is dominated by pressure-drive, which makes it difficult to analyze EBWCD. Finally, the line-integrated density in the HFS injection peaked at 1.6 × 1018 m-2, compared to 8 × 1017 m-2 in the LFS one..
2. H. Idei, T. Onchi, K. Mishra, H. Zushi, T. Kariya, T. Imai, O. Watanabe, R. Ikezoe, K. Hanada, M. Ono, A. Ejiri, J. Qian, K. Nakamura, A. Fujisawa, Y. Nagashima, M. Hasegawa, K. Matsuoka, A. Fukuyama, S. Kubo, M. Yoshikawa, M. Sakamoto, S. Kawasaki, A. Higashijima, S. Ide, Y. Takase, S. Murakami, Electron heating of over-dense plasma with dual-frequency electron cyclotron waves in fully non-inductive plasma ramp-up on the QUEST spherical tokamak, Nuclear Fusion, 10.1088/1741-4326/ab4c12, 60, 1, 2020.01, [URL], A 28 GHz system with a high-power gyrotron tube has been used for the QUEST spherical tokamak to form an over-dense plasma for electron Bernstein wave (EBW) heating and current drive with an 8.2 GHz-wave. Non-inductive high-density plasma ramp-up experiments with dual-frequency (dual-f ) electron cyclotron (EC) (8.2 GHz and 28 GHz) waves were conducted. A spontaneous density jump (SDJ) to an over-dense state was first observed as a bifurcation phenomenon in the dual-f  wave experiment. The over-dense plasma on the 8.2 GHz wave was non-inductively ramped up to 25 kA, and was maintained for 0.4 s under stable plasma equilibrium after two such jumps in one shot. Heating to mildly energetic electrons and bulk electrons was observed even in the over-dense region. The electrostatic EBW heating effect on the mildly energetic electrons in the over-dense region is assessed following a dispersion analysis of the 8.2 GHz wave. The bulk electron heating effect observed is explained as heat exchange from mildly energetic electrons heated by the electrostatic EBW. Remarkably, a high hard-x-ray-radiation temperature of ∼500 keV was also observed in tangential viewing for current-carrying electrons in the over-dense core region. Synergetic heating from the overlap of different 28 GHz EC harmonic resonances as well as higher harmonic heating is discussed for maintaining the highly energetic electrons in the over-dense core region. In addition, the SDJ process and mechanism are considered based on the discussion of the electron heating effects with the 8.2 GHz wave..
3. Takumi Onchi, Hiroshi Idei, K. Nakamura, T. Nagata, S. Kawasaki, R. Ashida, M. Fukuyama, Makoto Hasegawa, Ryuya Ikezoe, A. Higashijima, K. Kuroda, Yoshihiko Nagashima, Kazuaki Hanada, High voltage electrical system of 8.56 GHz CW klystron for electron cyclotron heating on QUEST spherical tokamak, Fusion Engineering and Design, 10.1016/j.fusengdes.2019.04.043, 146, 2567-2570, 2019.09, [URL].
4. K. Nakamura, M. M. Alam, Y. Z. Jiang, O. Mitarai, M. Takechi, M. Hasegawa, K. Tokunaga, K. Hanada, H. Idei, Y. Nagashima, T. Onchi, K. Kuroda, O. Watanabe, A. Higashijima, T. Nagata, S. Shimabukuro, S. Kawasaki, A. Fukuyama, Plasma equilibrium based on EC-driven current profile with toroidal rotation on QUEST, Fusion Engineering and Design, 10.1016/j.fusengdes.2019.04.059, 146, 2628-2631, 2019.09, [URL], 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..
5. M. Fukuyama, Hiroshi Idei, T. I. Tsujimura, S. Kubo, S. Kobayashi, M. Yunoki, Takumi Onchi, Ryuya Ikezoe, Quasi-optical polarizer system for ECHCD experiments in the QUEST, Fusion Engineering and Design, 10.1016/j.fusengdes.2019.02.099, 146, 1437-1440, 2019.09, [URL], 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..
6. Kazuaki Hanada, N. Yoshida, I. Takagi, T. Hirata, A. Hatayama, K. Okamoto, Y. Oya, T. Shikama, Z. Wang, H. Long, C. Huang, M. Oya, Hiroshi Idei, Yoshihiko Nagashima, Takumi Onchi, Makoto Hasegawa, K. Nakamura, H. Zushi, K. Kuroda, S. Kawasaki, A. Higashijima, T. Nagata, S. Shimabukuro, Y. Takase, S. Murakami, X. Gao, H. Liu, J. Qian, R. Raman, M. Ono, Estimation of fuel particle balance in steady state operation with hydrogen barrier model, Nuclear Materials and Energy, 10.1016/j.nme.2019.03.015, 19, 544-549, 2019.05, [URL], 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..
7. K. Hanada, N. Yoshida, M. Hasegawa, A. Hatayama, K. Okamoto, I. Takagi, T. Hirata, Y. Oya, M. Miyamoto, M. Oya, T. Shikama, A. Kuzmin, Z. X. Wang, H. Long, H. Idei, Y. Nagashima, K. Nakamura, O. Watanabe, T. Onchi, H. Watanabe, K. Tokunaga, A. Higashijima, S. Kawasaki, T. Nagata, S. Shimabukuro, Y. Takase, S. Murakami, X. Gao, H. Liu, J. Qian, R. Raman, M. Ono, Particle balance investigation with the combination of the hydrogen barrier model and rate equations of hydrogen state in long duration discharges on an all-metal plasma facing wall in QUEST, Nuclear Fusion, 10.1088/1741-4326/ab1858, 59, 7, 2019.05, [URL], The fuel particle balance during long duration discharges in the Q-shu University Experiment with steady state spherical tokamak (QUEST) was investigated. QUEST has all-metal plasma facing walls (PFWs) that were temperature controlled during the experiments. The presence of a transport barrier for hydrogen (H) at the interface between a plasma-induced deposition layer and metallic substrate was confirmed by nuclear reaction analysis with exposing deuterium plasma. An effective method to evaluate global hydrogen flux to PFWs is proposed, taking advantage of the nature of wall saturation. The outgoing flux of fuel particles from the PFWs just after the plasma termination was proportional to the square of wall-stored H, which indicates that enhanced recombination of solved hydrogen played an essential role in dynamic retention and was in agreement with predictions from the H-barrier model. A simple calculation based on the combination of wall modelling and rate equations of the H states denoted a significant impact of wall modelling on the time response of the plasma density. Hence, a proper wall model including the effects of the deposition layer creating the H barrier is required to be developed, even for all-metal PFW devices..
8. T. Kariya, R. Minami, T. Imai, M. Okada, F. Motoyoshi, T. Numakura, Y. Nakashima, Hiroshi Idei, Takumi Onchi, Kazuaki Hanada, T. Shimozuma, Y. Yoshimura, H. Takahashi, S. Kubo, Y. Oda, R. Ikeda, K. Sakamoto, M. Ono, K. Nagasaki, T. Eguchi, Y. Mitsunaka, Development of high power gyrotrons for advanced fusion devices, Nuclear Fusion, 10.1088/1741-4326/ab0e2c, 59, 6, 2019.04, [URL], 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..
9. Hiroshi Idei, Takumi Onchi, T. Kariya, T. I. Tsujimura, S. Kubo, S. Kobayashi, M. Sakaguchi, T. Imai, Makoto Hasegawa, K. Nakamura, K. Mishra, M. Fukuyama, M. Yunoki, S. Kojima, O. Watanabe, K. Kuroda, Kazuaki Hanada, Yoshihiko Nagashima, A. Ejiri, N. Matsumoto, M. Ono, A. Higashijima, T. Nagata, S. Shimabukoro, Y. Takase, A. Fukuyama, S. Murakami, 28-GHz ECHCD system with beam focusing launcher on the QUEST spherical tokamak, Fusion Engineering and Design, 10.1016/j.fusengdes.2019.02.027, 2019.01, [URL], 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..
10. Hatem Elserafy, Kazuaki Hanada, Kengoh Kuroda, Hiroshi Idei, Ryota Yoneda, Canbin Huang, Shinichiro Kojima, Makoto Hasegawa, Yoshihiko Nagashima, Takumi Onchi, Ryuya Ikezoe, Aki Higashijima, Takahiro Nagata, Shoji Kawasaki, Shun Shimabukuro, Nicola Bertelli, Masayuki Ono, HFS injection of X-mode for EBW conversion in QUEST, Plasma and Fusion Research, 10.1585/pfr.14.1205038, 14, 1, 2019.01, [URL], 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..
11. Yuya Kawamata, Akira Ejiri, Kyohei Matsuzaki, Yuichi Takase, Naoto Tsujii, Takumi Onchi, Yoshihiko Nagashima, Improvement of aperture configuration to reduce the stray light for Thomson scattering measurement using a peripheral beam profile monitor, Plasma and Fusion Research, 10.1585/PFR.14.1402072, 14, 2019.01, [URL], The stray light is a major problem in Thomson scattering (TS) measurements. The main cause of stray light is unnecessary divergence of the incident laser beam and an aperture is a standard component to reduce it. In order to improve the aperture configuration (including size, number and position in the laser injection tube), a peripheral beam profile monitor, consisting of a screen with a through hole for the laser beam and a CMOS camera, was developed. Instead of the actual laser injection tube a mock-up tube was used to measure the peripheral beam profiles under various aperture configurations. The configuration with four 15mm diameter apertures was chosen and installed on the TS system for the TST-2 spherical tokamak. The stray light was reduced to about 4% compared to the smaller diameter injection tube with no apertures. As a result, it became possible to make TS measurements in the electron density range above 1.0 × 1017 m-3..
12. Masaharu Fukuyama, Hiroshi Idei, Miu Yunoki, Ryuichi Ashida, Daichi Ogata, Ryuya Ikezoe, Takumi Onchi, Prototype phased-array patch loop antennae for electron cyclotron emission diagnostics, Plasma and Fusion Research, 10.1585/PFR.14.3402111, 14, 2019.01, [URL], 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..
13. K. Kuroda, R. Raman, Kazuaki Hanada, Makoto Hasegawa, Takumi Onchi, M. Ono, B. A. Nelson, T. R. Jarboe, M. Nagata, O. Mitarai, K. Nakamura, Hiroshi Idei, J. Rogers, S. Kawasaki, T. Nagata, A. Kuzmin, S. Kojima, C. Huang, O. Watanabe, A. Higashijima, Y. Takase, A. Fukuyama, S. Murakami, Initial results from solenoid-free plasma start-up using Transient CHI on QUEST, Plasma Physics and Controlled Fusion, 10.1088/1361-6587/aadcb7, 60, 11, 2018.09, [URL], 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..
14. Santanu Banerjee, H. Zushi, N. Nishino, Kazuaki Hanada, Hiroshi Idei, K. Nakamura, Makoto Hasegawa, akihide fujisawa, Yoshihiko Nagashima, K. Mishra, S. Tashima, Takumi Onchi, A. Kuzmin, K. Matsuoka, Effect of magnetic shear on edge turbulence in SOL-like open field line configuration in QUEST, Plasma Physics and Controlled Fusion, 10.1088/1361-6587/aacb69, 60, 8, 2018.06, [URL], 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..
15. Makoto Hasegawa, Kazuo Nakamura, Kazuaki Hanada, Shoji Kawasaki, Arseniy Kuzmin, Hiroshi Idei, Kazutoshi Tokunaga, Yoshihiko Nagashima, Takumi Onchi, Kengoh Kuroda, Osamu Watanabe, Aki Higashijima, Takahiro Nagata, Modification of plasma control system and hot-wall temperature control system for long-duration plasma sustainment in QUEST, Fusion Engineering and Design, 10.1016/j.fusengdes.2018.02.069, 129, 202-206, 2018.04, [URL], 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..
16. Nao Yoneda, Taiichi Shikama, Hideki Zushi, Kazuaki Hanada, Akinobu Fujikawa, Takumi Onchi, Kengoh Kuroda, Kuniaki Nii, Masahiro Hasuo, Makoto Hasegawa, Hiroshi Idei, Kazuo Nakamura, Yoshihiko Nagashima, Aki Higashijima, Takahiro Nagata, Spectroscopic measurements of impurity ion toroidal and poloidal flow velocities and their dependence on vertical magnetic field in QUEST toroidal ECR plasmas, Plasma and Fusion Research, 10.1585/PFR.13.3402087, 13, 2018.01, [URL], 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)..
17. Hiroshi Idei, T. Kariya, T. Imai, K. Mishra, Takumi Onchi, O. Watanabe, H. Zushi, Kazuaki Hanada, J. Qian, A. Ejiri, M. M. Alam, K. Nakamura, akihide fujisawa, Yoshihiko Nagashima, Makoto Hasegawa, K. Matsuoka, A. Fukuyama, S. Kubo, T. Shimozuma, M. Yoshikawa, M. Sakamoto, S. Kawasaki, H. Nakashima, A. Higashijima, S. Ide, T. Maekawa, Y. Takase, K. Toi, Fully non-inductive second harmonic electron cyclotron plasma ramp-up in the QUEST spherical tokamak, Nuclear Fusion, 10.1088/1741-4326/aa7c20, 57, 12, 2017.10, [URL], Fully non-inductive second (2nd) harmonic electron cyclotron (EC) plasma current ramp-up was demonstrated with a newlly developed 28 GHz system in the QUEST spherical tokamak. A high plasma current of 54 kA was non-inductively ramped up and sustained stably for 0.9 s with a 270 kW 28 GHz wave. A higher plasma current of 66 kA was also non-inductively achieved with a slow ramp-up of the vertical field. We have achieved a significantly higher plasma current than those achieved previously with the 2nd harmonic EC waves. This fully non-inductive 2nd harmonic EC plasma ramp-up method might be useful for future burning plasma devices and fusion reactors, in particular for operations at half magnetic field with the same EC heating equipment..
18. Kazuaki Hanada, N. Yoshida, T. Honda, Z. Wang, A. Kuzmin, I. Takagi, T. Hirata, Y. Oya, M. Miyamoto, H. Zushi, Makoto Hasegawa, K. Nakamura, akihide fujisawa, Hiroshi Idei, Yoshihiko Nagashima, O. Watanabe, Takumi Onchi, K. Kuroda, H. Long, Hideo Watanabe, Kazutoshi Tokunaga, A. Higashijima, S. Kawasaki, T. Nagata, Y. Takase, A. Fukuyama, O. Mitarai, Investigation of hydrogen recycling in long-duration discharges and its modification with a hot wall in the spherical tokamak QUEST, Nuclear Fusion, 10.1088/1741-4326/aa8121, 57, 12, 2017.10, [URL], 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..
19. C. Xiao, A. Rohollahi, Takumi Onchi, M. Dreval, S. Elgriw, A. Hirose, Momentum injection and repetitive CT operation experiments, Radiation Effects and Defects in Solids, 10.1080/10420150.2017.1398244, 172, 9-10, 713-717, 2017.10, [URL].
20. A. Kuzmin, H. Zushi, I. Takagi, S. K. Sharma, M. Kobayashi, Y. Hirooka, Takumi Onchi, Kazuaki Hanada, N. Yoshida, K. Nakamura, akihide fujisawa, Hiroshi Idei, Yoshihiko Nagashima, Makoto Hasegawa, T. Mutoh, K. Mishra, H. Ohwada, Spatial distribution of atomic and ion hydrogen flux and its effect on hydrogen recycling in long duration confined and non-confined plasmas, Nuclear Materials and Energy, 10.1016/j.nme.2017.03.027, 12, 627-632, 2017.08, [URL], 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 Гinc profile behind the plasma facing components (PFCs) is almost constant, ∼2 ×1018 H/s/m2..
21. T. Onchi, A. Fujisawa, A. Sanpei, H. Himura, S. Masamune, and RELAX team, Permutation entropy and statistical complexity in characterising low-aspect-ratio reversed-field pinch plasma, Physica Scripta, https://doi.org/10.1088/1402-4896/aa6689, 92, 5, 055601, 2017.04.
22. O. Mitarai, K. Nakamura, M. Hasegawa, T. Onchi, H. Idei, A. Fujisawa, K. Hanada, H. Zushi, A. Higashijima, H. Nakashima, S. Kawasaki, K. Matsuoka, S. Koike, T. Takahashi, H. Tsutsui, Comparative studies of inner and outer divertor discharges and a fueling study in QUEST, FUSION ENGINEERING AND DESIGN, 10.1016/j.fusengdes.2015.12.022, 109, 1365-1370, 2016.11.
23. T. Onchi, H. Zushi, K. Mishra, Y. Oyama, Y. Nagashima, K. Hanada, H. Idei, M. Hasegawa, A. Kuzmin, H. Miura, K. Nakamura, A. Fujisawa, K. Nagaoka, Response of the far scrape-off layer plasma to strong gas puffing in the high poloidal beta configuration of the QUEST spherical tokamak, PLASMA PHYSICS AND CONTROLLED FUSION, 10.1088/0741-3335/58/11/115004, 58, 11, 2016.11.
24. S. Banerjee, H. Zushi, N. Nishino, K. Mishra, K. Mahira, S. Tashima, A. Ejiri, T. Yamaguchi, T. Onchi, Y. Nagashima, K. Hanada, K. Nakamura, H. Idei, M. Hasegawa, A. Fujisawa, A. Kuzmin, K. Matsuoka, Observation of an edge coherent mode and poloidal flow in the electron cyclotron wave induced high beta(p) plasma in QUEST, PHYSICS OF PLASMAS, 10.1063/1.4960117, 23, 8, 2016.08.
25. A Kuzmin, H Zushi, I Takagi, SK Sharma, Y Hirooka, M Kobayashi, M Sakamoto, K Hanada, T Onchi, Y Oyama, N Youshida, K Nakamura, A Fujisawa, H Idei, Y Nagashima, M Hasegawa, K Mishra, Hydrogen flux measurements with permeation probes in spherical tokamak QUEST, Vacuum, 129, 178, 2016.04.
26. H Idei, K Mishra, MK Yamamoto, M Hamasaki, A Fujisawa, Y Nagashima, Y Hayashi, T Onchi, K Hanada, H Zushi, Adaptive-array Electron Cyclotron Emission diagnostics using data streaming in a Software Defined Radio system, Journal of Instrumentation, 11, 4, C04010, 2016.04.
27. A Fujisawa, Y Nagashima, S Inagaki, T Onchi, S Ohshima, A Shimizu, Tomography as a promising diagnostic tool for plasma turbulence, Plasma Physics and Controlled Fusion, 58, 2, 025005, 2016.02.
28. Takumi Onchi, D. McColl, A. Rohollahi, C. Xiao, A. Hirose, M. Dreval, S. Wolfe, Development Toward a Repetitive Compact Torus Injector, IEEE Transactions on Plasma Science, 44, 2, 195, 2016.02.
29. H Idei, K Mishra, MK Yamamoto, A Fujisawa, Y Nagashima, M Hamasaki, Y Hayashi, T Onchi, K Hanada, H Zushi, Multiple wall-reflection effect in adaptive-array differential-phase reflectometry on QUEST, Journal of Instrumentation, 11, 1, C01014, 2016.01.
30. O Mitarai, K Nakamura, M Hasegawa, T Onchi, H Idei, A Fujisawa, K Hanada, H Zushi, A Higashijima, H Nakashima, S Kawasaki, K Matsuoka, S Koike, T Takahashi, H Tsutsui, Comparative studies of inner and outer divertor discharges and a fueling study in QUEST, Fusion Engineering and Design, 2015.12.
31. A. Kuzmin, H. Zushi, I. Takagi, S.K. Sharma, A. Rusinov, Y. Inoue, Y. Hirooka, H. Zhou, M. Kobayashi, M. Sakamoto, K. Hanada, N. Yoshida, K. Nakamura, A. Fujisawa, K. Matsuoka, H. Idei, Y. Nagashima, M. Hasegawa, T. Onchi, S. Banerjee, K. Mishra, Global gas balance and influence of atomic hydrogen irradiation on the wall inventory in steady-state operation of QUEST tokamak, Journal of Nuclear Materials, 463, 1087, 2015.08.
32. K. Hanada, H. Zushi, N. Yoshida, N. Yugami, T. Honda, M. Hasegawa, K. Mishra, A. Kuzmin, K. Nakamura, A. Fujisawa, H. Idei, Y. Nagashima, O. Watanabe, T. Onchi, H. Watanabe, K. Tokunaga, A. Higashijima, S. Kawasaki, H. Nakashima, Y. Takase, A. Fukuyama, O. Mitarai, Y.K.M. Peng, Particle balance in long duration RF driven plasmas on QUEST, Journal of Nuclear Materials, 463, 1084, 2015.08.
33. T. Onchi, Y. Mahira, K. Nagaoka, S. Tashima, S. Banerjee, K. Mishra, H. Idei, K. Hanada, K. Nakamura, A. Fujisawa, Y. Nagashima, M. Hasegawa, K. Matsuoka, A. Kuzmin, O. Watanabe, S. Kawasaki, H. Nakashima, A. Higashijima, Observation of heat flux and plasma flow in scrape off layer in QUEST, Journal of Nuclear Materials, 463, 428, 2015.08.
34. Kishore Mishra, H Zushi, H Idei, M Hasegawa, T Onchi, S Tashima, S Banerjee, H Hanada, H Togashi, T Yamaguchi, A Ejiri, Y Takase, K Nakamura, A Fujisawa, Y Nagashima, A Kuzmin, Self organization of high βp plasma equilibrium with an inboard poloidal magnetic field null in QUEST, Nuclear Fusion, 55, 8, 083009, 2015.08.
35. T. Onchi, H. Zushi, K. Mishra, Y. Mahira, K. Nagaoka, K. Hanada, H. Idei, M. Hasegawa, K. Nakamura, A. Fujisawa, Y. Nagashima, K. Matsuoka, S. Tashima, S. Banerjee, A. Kuzmin, S. Kawasaki, H. Nakashima, A. Higashijima, O. Watanabe, Heat flux and plasma flow in the far scrape-off layer of the inboard poloidal field null configuration in QUEST, Physics of Plasmas, 22, 8, 082513, 2015.08.
36. S. Banerjee, H. Zushi, N. Nishino, K. Mishra, T. Onchi, A. Kuzmin, Y. Nagashima, K. Hanada, K. Nakamura, H. Idei, M. Hasegawa, A. Fujisawa, Dynamical programming based turbulence velocimetry for fast visible imaging of tokamak plasma, Review of Scientific Instruments, 86, 3, 033505, 2015.03.
37. K. Mishra, H. Idei, H. Zushi, K. Nagata, R. Akimoto, M. K. Yamamoto, K. Hanada, M. Hasegawa, K. Nakamura, A. Fujisawa, Y. Nagashima, S. Banerjee, T. Onchi, A. Kuzmin, Thermal imaging of plasma with a phased array antenna in QUEST, Review of Scientific Instruments, 85, 11, E808, 2014.11.
38. T. Onchi, A. Fujisawa, A. Sanpei, A prototype diagnostics system to detect ultraviolet emission for plasma turbulence, Review of Scientific Instruments, 85, 11, 113502, 2014.11.
39. T. Onchi, D. McColl, M. Dreval, S. Wolfe, C. Xiao, A. Hirose, Design and implementation of fast charging circuit for repetitive compact torus injector, Fusion Engineering and Design, 89, 11, 2559, 2014.11.
40. O. Mitarai, Y. Ding, M. Hubeny, Y. Lu, T. Onchi, D. McColl, C. Xiao, A. Hirose, Plasma current sustainment after iron core saturation in the STOR-M tokamak, Fusion Engineering and Design, 89, 9-10, 2467, 2014.10.
41. T. Onchi, Y. Liu, M. Dreval, D. McColl, S. Elgriw, D. Liu, T. Asai, C. Xiao, A. Hirose, Effects of compact torus injection on toroidal flow in the STOR-M tokamak, Plasma Physics and Controlled Fusion, 55, 3, 035003, 2013.03.
42. M. Dreval, M. Hubeny, Y. Ding, T. Onchi, Y. Liu, K. Hthu, S. Elgriw, C. Xiao, A. Hirose, Plasma confinement modification and convective transport suppression in the scrape-off layer using additional gas puffing in the STOR-M tokamak, Plasma Physics and Controlled Fusion, 55, 3, 035004, 2013.03.
43. R. Ikezoe, S. Masamune, K. Oki, A. Sanpei, H. Himura, T. Onchi, A. Hirose, Asymmetric toroidal flux generation due to phase locking of internally resonant tearing modes in the RELAX reversed-field pinch, Plasma Physics and Controlled Fusion, 55, 1, 015005, 2013.01.
44. R. Ikezoe, S. Masamune, K. Oki, A. Sanpei, H. Himura, T. Onchi, A. Hirose, Phase Locking and Unlocking Associated with Transition to Quasi-Single Helicity State in the RELAX Reversed-Field Pinch, Journal of the Physical Society of Japan, 81, 11, 115001, 2012.10.
45. C. Xiao, S. Elgriw, Y. Liu, T. Onchi, M. Dreval, M. Hubeny, Y. Ding, T. Asai, A. Hirose, Modification of toroidal flow in the STOR-M tokamak, Proc. 24th Int. Conf. on Fusion Energy (San Diego, CA, 2012), 2012.10.
46. A. Sanpei, K. Oki, D. Fukahori, K. Deguchi, S. Nakaki, K. Nishimura, H. Himura, S. Masamune, S. Ohdachi, N. Nishino, T. Onchi, Design and Initial Result of Time-Resolved Vertical Soft X-Ray Imaging System in Low-A RFP, Plasma and Fusion Research, 7, 2402052, 2012.06.
47. C. Xiao, T.Onchi, Y. Liu, S. Elgriw, M.Dreval, D. McColl, T. Asai, and A. Hirose, Recent Experiments on the STOR-M Tokamak, CNS(Canadian Nuclear Society)2012 conference paper, 158, 2012.06.
48. T. Onchi, Y. Liu, M. Dreval, D. McColl, T. Asai, S. Wolfe, C. Xiao and A. Hirose, Fuelling Effect of Tangential Compact toroid injection in STOR-M Tokamak, CNS(Canadian Nuclear Society)2012 conference paper, 137, 2012.06.
49. A. Sanpei, K. Oki, M. Nakamura, A. Higashi, H. Motoi, D. Fukahori, H. Himura, S. Masamune, S. Ohdachi, N. Nishino, T. Onchi, R. Ikezoe, Tangential Image of Helical SXR Emissivity Structure in Low-Aspect-Ratio RFP, Plasma Science, IEEE Transactions on, 39, 11, 2410, 2011.11.
50. T. Onchi, R. Ikezoe, K. Oki, A. Sanpei, H. Himura, S. Masamune, N. Nishino, H. Koguchi, Observation of Helical Structure by Imaging Diagnostics in a Low-Aspect-Ratio Reversed Field Pinch, Journal of the Physical Society of Japan, 80, 11, 114501, 2011.10.
51. A. SANPEI, K. OKI, M. NAKAMURA, A. HIGASHI, H. MOTOI, D. FUKAHORI, H. HIMURA, S. MASAMUNE, S. OHDACHI, N. NISHINO, T. ONCHI, R. IKEZOE, Initial Result of Successive SXR Imaging Measurement in Low-A RFP, Plasma and Fusion Research, 6, 2011.07.
52. R. Ikezoe, K. Oki, T. Onchi, Y. Konishi, M. Sugihara, S. Fujita, A. Sanpei, H. Himura, S. Masamune, Extended operational regimes and MHD behavior in a low-aspect-ratio reversed field pinch in RELAX, Plasma Physics and Controlled Fusion, 53, 2, 025003, 2011.02.
53. M. Sugihara, K. Oki, R. Ikezoe, T. Onchi, A. Sanpei, H. Himura, S. Masamune, T. Akiyama, A. Ejiri, K. Sakamoto, K. Nagasaki, V. Zhuravlev, Density Regimes of Low-Aspect-Ratio RFP Plasmas in RELAX, Plasma and Fusion Research, 5, S2061, 2010.12.
54. A. Sanpei, R. Ikezoe, T. Onchi, K. Oki, M. Sugihara, Y. Konishi, S. Fujita, M. Nakamura, A. Higashi, H. Motoi, H. Himura, S. Masamune, Recent Progress in Low-A RFP Plasma Research in RELAX, Plasma and Fusion Research, 5, S2063, 2010.12.
55. T. Onchi, R. Ikezoe, K. Oki, A. Sanpei, H. Himura, S. Masamune, Tangential soft-x ray imaging for three-dimensional structural studies in a reversed field pinch, Review of Scientific Instruments, 81, 7, 073502, 2010.07.
56. S Masamune, A Sanpei, R Ikezoe, T Onchi, K Oki, T Yamashita, H Shimazu, H Himura, R Paccagnella, MHD Properties of Low-aspect Ratio RFP in RELAX, Journal of fusion energy, 28, 2, 187, 2009.02.
57. Akio Sanpei, Kensuke Oki, Ryuya Ikezoe, Takumi Onchi, Ken-Ichi Murata, Hiroyuki Shimazu, Tetsuo Yamashita, Shinichi Fujita, Haruhiko Himura, Sadao Masamune, Jay K Anderson, Equilibrium Reconstruction and Estimation of Neoclassical Effect in Low-Aspect-Ratio Reversed Field Pinch Experiments on RELAX, Journal of the Physical Society of Japan, 78, 1, 13501, 2009.01.
58. S. Masamune, A. Sanpei, R. Ikezoe, T. Onchi, K. Oki, T. Yamashita, H. Shimazu, K. Murata, H. Himura, N. Nishino, R. Paccagnella, Effects of Loowering the Aspect Ratio on MHD Behaviour in a Reversed Field Pinch, Proc. 22th Int. Conf. on Fusion Energy (Geneva, 2008), EX/7-1Rb, 2008.10.
59. Kensuke Oki, Ryuya Ikezoe, Takumi Onchi, Akio Sanpei, Haruhiko Himura, Sadao Masamune, Roberto Paccagnella, Observation of large-scale profile change of magnetic field in a low-aspect ratio reversed field pinch, Journal of the Physical Society of Japan, 77, 7, 75005, 2008.07.
60. Ryuya Ikezoe, Takumi Onchi, Kensuke Oki, Akio Sanpei, Haruhiko Himura, Sadao Masamune, Quasi-periodic growth of a single helical instability in a low-aspect ratio RFP, Plasma and Fusion Research, 3, 29, 2008.05.
61. Takumi Onchi, Nobuhiro Nishino, Ryuya Ikezoe, Akio Sanpei, Haruhiko Himura, Sadao Masamune, Observation of Simple Helical Structure in Low Aspect Ratio RFP Using Fast Camera, Plasma and Fusion Research, 3, 5, 2008.01.
62. Takumi Onchi, Akio Sanpei, Ryuya Ikezoe, Haruhiko Himura, Sadao Masamune, Application of Soft X-Ray Imaging System to the STE-2 RFP, Plasma and Fusion Research, 2, S1063, 2007.11.
63. Akio Sanpei, Takumi Onchi, Ryuya Ikezoe, Haruhiko Himura, Sadao Masamune, Design of Soft-X Ray Imaging System for Magnetic Islands of RFP Plasmas, Plasma and Fusion Research, 2, S1064, 2007.11.
64. Sadao Masamune, Akio Sanpei, Ryuya Ikezoe, Takumi Onchi, Ken-Ichi Murata, Kensuke Oki, Hiroyuki Shimazu, Tetsuo Yamashita, Haruhiko Himura, Characterization of Initial Low-Aspect Ratio RFP Plasmas in “RELAX”, Journal of the Physical Society of Japan, 76, 12, 123501, 2007.11.
65. S Masamune, A Sanpei, H Himura, R Ikezoe, T Onchi, M Nagata, H Hiramatsu, D Ishijima, Research plans for low-aspect ratio reversed field pinch, Fusion science and technology, 51, 2, 197.
66. Akio Sanpei, Kensuke Oki, Ryuya Ikezoe, Takumi Onchi, Hiroyuki Shimazu, Tetsuo Yamashita, Haruhiko Himura, Sadao Masamune, Characterization of equilibria in a low-aspect-ratio RFP, Journal of Plasma Fusion Research SERIES, 8, 1066.

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