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

Electron parallel force balance for drift-wave fluctuation is evaluated using fluctuation data obtained by a single probe fast voltage sweeping method in a linear plasma. Waveforms of the floating potential obtained by the method were validated by conventional high-impedance floating potential measurement. It is found that consistency in amplitude and phase relationships in the force balance is high when the electron temperature fluctuation is taken into account.

DOI： 10.7566/JPSJ.84.063501

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

Non-Gaussian statistical properties of the azimuthally averaged momentum and particle fluxes driven by turbulence have been simultaneously observed in inhomogeneous magnetized plasmas for the first time. We identified the stretched Gaussian distribution of the both fluxes and the transition from the point-wise distribution to averaged ones was confirmed. The change of the particle flux precedes that of the momentum flux, demonstrating that the momentum flux is induced by the relaxation of density gradient. (C) 2011 American Institute of Physics. [doi:10.1063/1.3601767]

DOI： 10.1063/1.3601767

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

Edge electron temperature fluctuation is evaluated by the use of fast voltage sweeping technique on TST-2. The validity of obtained current-voltage characteristic curve was checked by comparing the time evolutions of floating potential between that obtained from the fast voltage sweeping technique and that measured with floating probe method. Good agreement between them was confirmed. We also found that fitting errors in the evaluation of the electron temperature itself are less than 10% of fluctuation levels of the electron temperature. Therefore the accuracy of the technique is applicable to study of plasma fluctuations.

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

A new radially movable multichannel azimuthal probe system has been developed for measuring azimuthal and radial profiles of electrostatic Reynolds stress (RS) per mass density of microscale fluctuations for a cylindrical laboratory plasma. The system is composed of 16 probe units arranged azimuthally. Each probe unit has six electrodes to simultaneously measure azimuthal and radial electric fields for obtaining RS. The advantage of the system is that each probe unit is radially movable to measure azimuthal RS profiles at arbitrary radial locations as well as two-dimensional structures of fluctuations. The first result from temporal observation of fluctuation azimuthal profile presents that a low-frequency fluctuation (1-2 kHz) synchronizes oscillating Reynolds stress. In addition, radial scanning of the probe system simultaneously demonstrates two-dimensional patterns of mode structure and nonlinear forces with frequency f = 1.5 kHz and azimuthal mode number m = 1. (C) 2011 American Institute of Physics. [doi:10.1063/1.3557384]

DOI： 10.1063/1.3557384

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

A new technique is used to estimate the amplitude of temperature fluctuation in the edge plasma on TST-2. Langmuir probe current-voltage characteristic curves are conditionally reconstituted in terms of the intensity of the floating potential. High/low electron temperatures are obtained in low/high floating potential phases. As a result, normalized temperature fluctuation levels are found to be about 19 % or larger.

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

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We present a detailed investigation of the geodesic acoustic eigenmode(GAEM) profile in the vicinity of the last closed flux surface (LCFS) in the JFT-2M tokamak. Envelope and spectral peak frequency of the GAEM vanish near the LCFS obtained by an equilibrium reconstitution code. Spatial changes in mean plasma gradients (the electron temperature, electron density and floating potential) are correlated with the boundary. This does not contradict that the GAEM can have a node at the LCFS. Discussions of an inverse problem and the application of this GAEM measurement to an experimental test of the LCFS location calculated by the equilibrium reconstitution code are presented.

DOI： 10.1088/0741-3335/51/6/065019

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

Observation of the parametric-modulational interaction between the drift-wave fluctuation (7-8 kHz) and azimuthally symmetric sheared radial electric field structure (similar to 0.4 kHz) in a cylindrical laboratory plasma is presented. Oscillation of the sheared radial electric field is synchronized at modulations of the radial wave number and Reynolds stress per mass density of the drift-wave spectrum. Bispectral analysis at the location where the sheared radial electric field has finite radial wave numbers shows that nonlinear energy transfers from the drift wave to the sheared radial electric field occur. Nonlocal energy transfers of fluctuations via "channel of the azimuthally symmetric sheared radial electric field" in spectral space as well as real space are discovered.

DOI： 10.1063/1.3078075

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

Spatiotemporal structures of fluctuations with frequencies lower than the ion cyclotron frequency in a cylindrical magnetized plasma are investigated. Drift-wave and low-frequency zonal flow coexist. Electrostatic potentials of the zonal flow and the drift-wave are distributed widely in radius. The radial wave number profile of the zonal flow has a shear structure at the radial location where the drift-wave has a maximal normalized fluctuation amplitude. On the other hand, the radial wave number profile of the drift-wave shows vortex tilting, resulting in the generation of stationary turbulence Reynolds stress gradient per mass density. The envelope and bispectral analyses indicate significant nonlinear interactions between the zonal flow and the drift-wave.

DOI： 10.1143/JPSJ.77.114501

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

Nonlinear energy transfer of the drift-wave spectrum is investigated experimentally in a cylindrical laboratory plasma. Using the Hasegawa-Mima equation, the nonlinear transfer function of electrostatic potential energy in frequency space is estimated by bispectral analysis. The transfer function derived from the drift-wave spectrum indicates a forward cascading of energy transfer from fundamental to higher harmonic modes.

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

A method for zonal flow study by using direct density fluctuation measurements is proposed. When ambient drift-wave turbulence is modulated by zonal flows (i.e. in the drift-wave-zonal flow systems), an envelope of the ambient density fluctuations has spectral peaks around zonal flow frequencies. A spectral peak at the geodesic acoustic mode (GAM) frequency is observed in the envelope of the ambient density fluctuations measured in edge plasma of the JFT-2M tokamak. The significant cross-bicoherence is also found between the ambient density fluctuations and its envelope in the GAM frequency. This result demonstrates that we can measure the GAM only by using density fluctuation data. This method provides a possibility of zonal flow research in burning core plasma by density fluctuation diagnostics such as microwave reflectometry.

DOI： 10.1088/0741-3335/49/10/002

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

This paper presents experimental studies of nonlinear processes between meso-scale structures and turbulent fluctuations, by use of bispectral analysis, in the edge region of toroidal plasmas. In ohmically heated plasmas of the JFT-2M tokamak, the drift wave-zonal flow system is investigated by observation of the potential fluctuations. The total bicoherence is composed of two components, i.e., the peak at the geodesic acoustic mode (GAM) frequency and broadband distribution. The biphase is also obtained, which indicates the phase angle of coherent interaction between GAM oscillations and broadband fluctuations. Bispectral functions (e.g., magnitude and phase) give an order-of-estimate agreement for the drift wave-zonal flow systems. Bicoherence analysis is also applied to the edge plasmas of the high-confinement mode (H-mode) of the Compact Helical System (CHS). In the case of CHS plasmas, nonlinear couplings in a system composed of coherent global magneto-hydrodynamic (MHD) oscillations and broadband fluctuations (up to hundreds kHz range) have been observed. Bispectral analysis enhances our understanding of the self-regulation systems of various toroidal plasmas in which strong turbulence and meso-scale structures coexist.

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

Two types of small-poloidal wavelength low frequency floating potential fluctuations (similar to 1 kHz, and 10-15 kHz) are observed in the edge region of ohmically heated plasmas in the JFT-2M tokamak. The higher frequency fluctuations are inferred as geodesic acoustic modes (GAMs). Significant bicoherences between both modes and the broad-band fluctuations are observed simultaneously; however, frequency ranges are different. A significant bicoherence range is from 70 to 125 kHz at the lower frequency modes (LFM) and is from 20 to 125 kHz at the GAM. Phase angles at the significant bicoherences are coherent around pi. These results indicate that both modes are nonlinearly coupled to the broad-band fluctuations. Two spectral peaks are observed around the GAM frequency and the difference in peak frequencies is around the LFM frequency, suggesting nonlinear couplings between the LFM and the GAM. However, the modulation of the GAM by the LFM is not conclusive. In this paper, the results of the letter (Nagashima et al 2005 Phys. Rev. Lett. 95 095002) are presented in detail.

DOI： 10.1088/0741-3335/48/5A/S38

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

A convergence study of bispectral analysis on toroidal plasma turbulence is presented with a theoretical estimate of bicoherence for the system of the drift wave and zonal flow. Examples are chosen from the analysis of floating potential fluctuation in the edge region of the JFT-2M tokamak. The convergence properties of the bispectral functions are clarified for an oscillatory branch of zonal flow (or geodesic acoustic mode) and drift waves. The total squared bicoherence or the biphase converges inversely proportional to the realization number or to the square root of the realization number, respectively. The numbers of realizations to qualitatively converge and obtain a quantitatively significant value of bicoherence are discussed for the drift wave fluctuations and for the zonal flow. (c) 2006 American Institute of Physics.

DOI： 10.1063/1.2194016

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

This paper presents results of bispectral analysis applied to floating potential fluctuations in the edge region of ohmically heated plasmas in the JAERI Fusion Torus-2 Modified (JFT-2M) tokamak. Inside the outermost surface of plasmas, coherent mode fluctuations, (CMs) in floating potential were observed around the frequency of the geodesic acoustic mode. The squared bicoherence shows significant nonlinear couplings between the CMs and background fluctuations. The biphase at the frequency of the CMs is localized around pi, while that at frequencies of background fluctuations distributes in a wide range. The total bicoherence at the frequency of the CMs is proportional to the squared amplitude of the CMs. These observations are consistent with the theoretical prediction on the drift wave-zonal flow systems. Interpretation of the absolute value of the total bicoherence is also discussed.

DOI： 10.1088/0741-3335/48/4/S01

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DOI： 10.1088/1741-4326/ad3d6e

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DOI： https://doi.org/10.1063/5.0187997

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DOI： 10.1585/pfr.19.1201014

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DOI： 10.1585/pfr.19.1201005

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DOI： https://doi.org/10.1585/pfr.18.1201086

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DOI： https://doi.org/10.1063/5.0165318

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DOI： 10.1088/1361-6587/acfbf7

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DOI： 10.1088/1361-6587/acfbb3

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DOI： 10.1585/pfr.18.2402066

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DOI： 10.1088/1741-4326/acd4d1

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DOI： 10.1585/pfr.18.2402025

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DOI： 10.1585/pfr.18.1405012

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DOI： https://doi.org/10.1063/5.0101770

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DOI： https://doi.org/10.1585/pfr.17.1301106

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DOI： 10.1088/2058-6272/ac7c60

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DOI： https://doi.org/10.1063/5.0065887

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DOI： 10.1585/pfr.16.1403093

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

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DOI： 10.1585/pfr.16.1202081

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DOI： 10.1585/pfr.16.1201082

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DOI： https://doi.org/10.1585/pfr.16.1201075

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DOI： https://doi.org/10.1063/5.0041814

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DOI： https://doi.org/10.1063/5.0037352

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DOI： 10.1585/pfr.16.2402034

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DOI： https://doi.org/10.1585/pfr.16.1402027

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

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

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DOI： https://doi.org/10.1088/1361-6587/abd980

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

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

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DOI： https://doi.org/10.1585/pfr.15.1403052

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

Other Link： https://iopscience.iop.org/article/10.1088/1741-4326/ab1858/meta

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DOI： https://doi.org/10.1016/j.nme.2019.03.015

Other Link： https://www.sciencedirect.com/science/article/pii/S2352179118302242

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DOI： https://doi.org/10.1063/1.5094577

Other Link： https://aip.scitation.org/doi/abs/10.1063/1.5094577

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DOI： 10.1585/pfr.14.1402090

Other Link： http://www.jspf.or.jp/PFR/PFR_articles/pfr2019/pfr2019_14-1402090.html

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

Other Link： https://www.sciencedirect.com/science/article/pii/S0920379619305770

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DOI： 10.1585/pfr.14.1402072

Other Link： http://www.jspf.or.jp/PFR/PFR_articles/pfr2019/pfr2019_14-1402072.html

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DOI： https://doi.org/10.1585/pfr.14.1402072

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DOI： https://doi.org/10.1016/j.fusengdes.2019.04.059

Other Link： https://www.sciencedirect.com/science/article/pii/S0920379619305939

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DOI： https://doi.org/10.1063/1.5093218

Other Link： https://aip.scitation.org/doi/abs/10.1063/1.5093218

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DOI： 10.1585/pfr.14.1205038

Other Link： http://www.jspf.or.jp/PFR/PFR_articles/pfr2019/pfr2019_14-1205038.html

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DOI： https://doi.org/10.1016/j.fusengdes.2019.02.027

Other Link： https://www.sciencedirect.com/journal/fusion-engineering-and-design/articles-in-press

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DOI： https://doi.org/10.1585/pfr.13.1102113

Other Link： http://www.jspf.or.jp/PFR/PFR_articles/pfr2018/pfr2018_13-3401105.html

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DOI： 10.1585/pfr.13.3401105

Other Link： http://www.jspf.or.jp/PFR/PFR_articles/pfr2018/pfr2018_13-3401105.html

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DOI： 10.1585/pfr.13.3402087

Other Link： http://www.jspf.or.jp/PFR/PFR_articles/pfr2018/pfr2018_13-3402087.html

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： https://doi.org/10.1016/j.fusengdes.2016.04.016

Other Link： https://www.sciencedirect.com/science/article/pii/S0920379616303064?via%3Dihub

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

It is a critical issue to find the best set of fitting function bases in mode structural analysis of two dimensional images like plasma emission profiles. The paper proposes a method to optimize a set of the bases in the case of Fourier-Bessel function series, using their orthonormal property, for more efficient and precise analysis. The method is applied on a tomography image of plasma emission obtained with the Maximum-likelihood expectation maximization method in a linear cylindrical device. The result demonstrates the excellency of the method that realizes the smaller residual error and minimum Akaike information criterion using smaller number of fitting function bases.

DOI： https://doi.org/10.1063/1.5000744

Other Link： https://aip.scitation.org/doi/abs/10.1063/1.5000744

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

Multiple free energy sources for instabilities coexist in magnetized plasmas with density gradient and velocity shear. Linear stabilities are investigated, and the mutual relation between resistive drift wave, D'Angelo mode and flute mode is systematically clarified. By evaluating the linear growth rates, dominant instability is categorized in a parameter space. The parallel wavenumber spectrum could be used as a guideline for the identification of the instabilities.

DOI： https://doi.org/10.1585/pfr.12.1401042

Other Link： https://www.jstage.jst.go.jp/article/pfr/12/0/12_1401042/_article/-char/ja/

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

Streamers and their mediator have been known to exist in linear cylindrical plasmas [Yamada et al., Nat. Phys. 4, 721 (2008)]. Conditional averaging is applied to extract the nonlinear characteristics of a mediator, which has been simply treated as a linear wave. This paper reports that a mediator should have higher harmonic components generated by self-couplings, and the envelope of a streamer should be generated with not only fundamental but also higher harmonic components of the mediator. Moreover, both the mediator and the envelope of the streamer have common features with solitary waves, i.e., the height should increase inversely as the square of their localization width.

DOI： https://doi.org/10.1063/1.5027124

Other Link： https://aip.scitation.org/doi/abs/10.1063/1.5027124

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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 (SMT) with tight aspect ratio. Slab annular plasmas feature open magnetic field lines and can mimic the tokamak edge-SOL-like plasma attributes reasonably well. Three magnetic shear regimes are realized using three poloidal field (PF) coil pairs. A whole range of connection lengths (~   Lc  5.5 m) is scanned by varying the PF strength for a given toroidal field for each magnetic shear regime. This for the first time a systematic study of the effect of magnetic shear and field line pitch together on edge-scrape off layer (SOL)-like plasma fluctuations is being reported. Slab plasmas with intermediate magnetic shear is observed to be more susceptible to generate distinct blobs when Lc 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 Lc. 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 the coherent mode. This study may provide a better insight on the effect of magnetic configuration in tokamak edge and SOL turbulence and can help searching better tools to control cross-field convective intermittent transport in tokamaks.

DOI： https://doi.org/10.1088/1361-6587/aacb69

Other Link： https://doi.org/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： https://doi.org/10.1016/j.fusengdes.2018.02.069

Other Link： https://www.sciencedirect.com/science/article/pii/S0920379618301704?via%3Dihub

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

The three-dimensional structures of the streamer and its carrier waves and mediator were observed in a linear plasma device for the first time. Not only the cross-sectional structure but also the axial structure was observed. While carrier drift waves, which form the streamer structure by nonlinear interaction, had an axial mode number of 1, the streamer and mediator had an axial mode number of 0. The relationship of these axial mode numbers is well explained by the matching condition of the nonlinear interaction between the carrier waves and the mediator.

DOI： https://doi.org/10.7566/JPSJ.87.034501

Other Link： https://journals.jps.jp/doi/10.7566/JPSJ.87.034501

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

We investigate spatial structures of turbulence and turbulent transport modulated by the geodesic acoustic mode (GAM), from which the excitation mechanism of the GAM is discussed. The GAM is found to be predominantly excited through a localized Reynolds stress force, rather than the dynamic shearing force. The evaluated growth rate is larger than the linear damping coefficients and is on the same order of magnitude as the effective growth rate evaluated from time evolution in the GAM kinetic energy.

DOI： https://doi.org/10.1103/PhysRevLett.120.045002

Other Link： https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.120.045002

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In order to understand the plasma turbulence, experimental studies on the subject of “symmetry-breaking of turbulence structure” has been designed recently. The accuracy of the position identification, which is required for such experimental studies, is analyzed. Possible approach to fulfill the demands is discussed.

DOI： https://doi.org/10.7566/JPSJ.87.025002

Other Link： https://journals.jps.jp/doi/10.7566/JPSJ.87.025002

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

DOI： https://doi.org/10.1585/pfr.12.1201045

Other Link： https://www.jstage.jst.go.jp/article/pfr/12/0/12_1201045/_article/-char/ja/

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

The topological bifurcation of the flow in non-equilibrium magnetized plasmas is demonstrated by a turbulence simulation. A system with two generic sources of turbulence, the gradients of density and parallel flow, is considered. Topological index of the flow is introduced, in order to indicate the chirality of flow pattern. We here report that the turbulence-driven flow forms the structure of co-axial helixes with opposite chirality. By changing the source of plasma particles, which modifies the density gradient, the transition between three turbulent states is obtained. In addition to the two turbulent states, which are dominated by the drift wave and the D'Angelo mode, respectively, the new state is found. In this third state, fluctuations are driven by both of the free energy sources simultaneously, and compete with the others. The result illustrates the generic feature of turbulence flow generation in non-equilibrium magnetized plasmas.

DOI： https://doi.org/10.1063/1.5000343

Other Link： https://aip.scitation.org/doi/abs/10.1063/1.5000343

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Spatiotemporal evolutions of radial electric field and turbulence are measured simultaneously in the H-mode transition, which is a prototypical example of turbulence structure formation in high-temperature plasmas. In the dynamical phase where transport barrier is established abruptly, the time-space-frequency-resolved turbulent particle flux is obtained. Here we report the validation of the mechanism of transport barrier formation quantitatively. It is found that the particle flux is suppressed predominantly by reducing density fluctuation amplitude and cross phase between density fluctuation and potential fluctuation. Both radial electric field shear and curvature are responsible for the amplitude suppression as was predicted by theory. Turbulence amplitude reduction immediately responds to the growth of the radial electric field non-uniformity and saturates, while cross phase continuously approaches zero.

Other Link： https://www.nature.com/articles/s41598-017-14821-y

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

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.

DOI： https://doi.org/10.1088/1741-4326/aa7c20

Other Link： http://iopscience.iop.org/article/10.1088/1741-4326/aa7c20/meta

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

In this paper, we elaborate the electric field excitation mechanism during the L–H transition in the JFT-2M tokamak. Using time derivative of the Poisson's equation, models of the radial electric field excitation is examined. The sum of the loss-cone loss current and the neoclassical bulk viscosity current is found to behave as the experimentally evaluated radial current that excites the radial electric field. The turbulent Reynolds stress only plays a minor role. The wave convection current that produces a negative current at the edge can be important to explain the ambipolar condition in the L-mode

DOI： https://doi.org/10.1088/1741-4326/aa5d03

Other Link： http://iopscience.iop.org/article/10.1088/1741-4326/aa5d03/meta

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： https://doi.org/10.1088/1741-4326/aa8121

Other Link： http://iopscience.iop.org/article/10.1088/1741-4326/aa8121/meta

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

DOI： https://doi.org/10.1063/1.5000739

Other Link： https://aip.scitation.org/doi/10.1063/1.5000739

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

The temporal-spatial structures of plasma flows and turbulence around tearing mode islands are presented. The experiments were performed using Langmuir probe arrays in the edge plasmas of J-TEXT tokamak. The correlation analyses clearly show that the flows have similar structures of m/n  =  3/1 as the magnetic island does (m and n are the poloidal and toroidal mode numbers, respectively). The sign of the potential fluctuations for the flows inverses and the powers significantly reduce at q  =  3 surface. Approaching to the last closed flux surface for the magnetic islands, the radially elongated flow structure forms. The flows are concentrated near separatrix and show quadrupole structures. The turbulence is concentrated near X-point and partly trapped inside the magnetic islands.

DOI： https://doi.org/10.1088/1741-4326/aa8341

Other Link： http://iopscience.iop.org/article/10.1088/1741-4326/aa8341/meta

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

A new type of nonlinear coupling among multi-time scale of fluctuations is observed on the linear plasma device, PANTA. The low frequency drift waves (DWs), a few kHz and a high frequency fluctuation (the HF fluctuation) (∼ 30 kHz) are simultaneously observed. These fluctuations nonlinearly couple with each other confirmed by bi-coherence analysis. The characteristics of HF fluctuation are also discussed.

DOI： https://doi.org/10.1585/pfr.12.1201034

Other Link： https://www.jstage.jst.go.jp/article/pfr/12/0/12_1201034/_article/-char/ja/

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

This article assesses current understanding of hysteresis in transport relations, and its impact on the field. The rapid changes of fluxes compared to slow changes of plasma parameters are overviewed for both core and edge plasmas. The modulation ECH experiment is explained, in which the heating power cycles on-and-off periodically, revealing hysteresis and fast changes in the gradient–flux relation. The key finding is that hystereses were observed simultaneously in both the the gradient–flux and gradient–fluctuation relations. Hysteresis with rapid timescale exists in the channels of energy, electron and impurity densities, and plausibly in momentum. Advanced methods of data analysis are explained. Transport hysteresis can be studied by observing the higher harmonics of temperature perturbation $\delta T_{
m m}$ in heating modulation experiments. The hysteresis introduces the term $\delta T_{
m m}$ , which depends on the harmonic number m in an algebraic manner (not exponential decay). Next, the causes of hysteresis and its fast timescale are discussed. The nonlocal-in-space coupling works here, but does not suffice. One mechanism for 'the heating heats turbulence' is that the external source S in phase space for heating has its fluctuation in turbulent plasma. This coupling can induce the direct input of heating power into fluctuations. The height of the jump in transport hysteresis is smaller for heavier hydrogen isotopes, and could be one of the origins of isotope effects on confinement. Finally, the impacts of transport hysteresis on the control system are assessed. Control systems must be designed so as to protect the system from sudden plasma loss.

DOI： https://doi.org/10.1088/1741-4326/aa796a

Other Link： http://iopscience.iop.org/article/10.1088/1741-4326/aa796a/meta

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

New mesoscale electric fluctuations (MSEFs) are identified in the edge plasmas of the HL-2A tokamak using multiple Langmuir probe arrays. The MSEFs, resulting from the synchronization and having components of dominant geodesic acoustic mode (GAM) and m/n  =  6/2 potential fluctuations, are found at the same frequency as that of the magnetic fluctuations of m/n  =  6/2 (m and n are poloidal and toroidal mode numbers, respectively). The temporal evolutions of the MSEFs and the magnetic fluctuations clearly show the frequency entrainment and the phase lock between the GAM and the m/n  =  6/2 magnetic fluctuations. The results indicate that GAMs and magnetic fluctuations can transfer energy through nonlinear synchronization. The nonlinear coupling analyses show that the MSEFs couple to turbulence and low frequency zonal flows (LFZFs). This suggests that the MSEFs may contribute to the LFZF formation, reduction of turbulence level, and thus confinement regime transitions. The analysis of the envelope modulation demonstrates that both the MSEFs and the LFZFs modulate the turbulence while the MSEFs are modulated by the LFZFs.

DOI： https://doi.org/10.1088/1741-4326/aa6f35

Other Link： http://iopscience.iop.org/article/10.1088/1741-4326/aa6f35/meta

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

Measurements of edge plasma parameters such as current density, electron density, and electron temperature were performed during internal reconnection events in TST-2 Ohmic plasmas. The measured current density consists of two components: a slowly varying component and a spiky bipolar component. The magnitude of the slowly varying component is comparable to the mean current density averaged over the poloidal cross section, and it seems to represent the global transport from the core to the edge. The spiky bipolar component is about an order of magnitude larger than the slowly varying component, but the spatial structure seems to be localized and its effect on plasma confinement is not catastrophic.

DOI： https://doi.org/10.1063/1.4985077

Other Link： https://aip.scitation.org/doi/abs/10.1063/1.4985077

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

Changing the experimental conditions such as the neutral pressure and the magnetic field strength provides a drastic change in the turbulence states in linear magnetized helicon
plasmas. In order to define the turbulence states and their occurrence region a throughout parameter scan in the two-dimensional parameter space was performed. The classification
of the turbulence states is carried out phenomenologically based on the turbulence spectrum and the waveform.

DOI： https://doi.org/10.1585/pfr.12.1401019

Other Link： https://www.jstage.jst.go.jp/article/pfr/12/0/12_1401019/_article/-char/ja/

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

Experimental plasma discharges in linear PANTA device are studied by Mach probe measurements, providing floating potential, ion saturation current, and parallel flow velocity time evolution, at different radii of the device. Spectral analysis indicates that drift waves and D'Angelo modes exist simultaneously in the plasma. A discrimination study shows they are located at different positions in radius and frequency.

DOI： https://doi.org/10.1585/pfr.12.1201008

Other Link： https://www.jstage.jst.go.jp/article/pfr/12/0/12_1201008/_article/-char/ja/

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

Bispectrum and bicoherence analysis is a powerful method to analyze nonlinear interaction of turbulent plasmas. In this review, we explain the difference of the bispectrum and bicoherence analysis and possible research that can be pursued with these methods. Basic concept is explained by using several examples from previous successes brought by bicoherence and bispectrum analysis, such as drift wave-zonal flow interaction. Open questions are discussed that can be challenged by these methods. Problems such as spatial transport of fluctuation energy (i.e. turbulence spreading), momentum transport by the triplet correlation, and so on, are treated. Bispectrum and bicoherence analysis can be utilized to expand the forefront of plasma turbulence research.

DOI： https://doi.org/10.1585/pfr.12.1101003

Other Link： https://www.jstage.jst.go.jp/article/pfr/12/0/12_1101003/_article/-char/ja/

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： https://doi.org/10.1088/1009-0630/18/11/03

Other Link： http://iopscience.iop.org/article/10.1088/1009-0630/18/11/03/meta

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 igh 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 CS 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 dentification is one 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： https://doi.org/10.1016/j.fusengdes.2016.04.016

Other Link： https://www.sciencedirect.com/science/article/pii/S0920379616303064

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

In the present RF-driven (ECCD) steady-state plasma on QUEST (Bt = 0.25 T, R = 0.68 m, a = 0.40 m), plasma current seems to flow in the open magnetic surface outside of the closed magnetic surface in the low-field region according to plasma current fitting (PCF) method. We consider that the current in the open magnetic surface is due to orbit-driven current by high-energy particles in RF-driven plasma. So based on the analysis of current density profile based on the orbit-driven current, plasma equilibrium is to be calculated. We calculated high energy particles guiding center orbits as a contour plot of conserved variable in Hamiltonian formulation and considered particles initial position with different levels of energy and pitch angles that satisfy resonance condition. Then the profile of orbit-driven current is estimated by multiplying the particle density on the resonance surface and the velocity on the orbits. This analysis shows negative current near the magnetic axis and hollow current profile is expected even if pressure driven current is considered. Considering the hollow current profile shifted toward the low-field region, the equilibrium is fitted by J-EFIT coded by MATLAB.

DOI： https://doi.org/10.1016/j.fusengdes.2015.11.035

Other Link： https://www.sciencedirect.com/science/article/pii/S0920379615303604?via%3Dihub

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

In the present RF-driven divertor plasma of QUEST, it has been observed that orbit-driven current flows in the open magnetic surfaces outside of the closed magnetic surfaces. To observe this phenomenon and the characteristics of the orbit-driven current, current density profiles have been calculated on two different equilibrium conditions. We calculated current density profiles from particle guiding center orbits both for the fundamental and the second harmonic resonances for the 8.2 GHz electron cyclotron current drive. From this calculation, hollow current density profiles have been obtained with significant characteristics on both conditions. Only positive current distribution has been observed in the open magnetic surfaces outside of the closed magnetic surfaces.

DOI： https://doi.org/10.1016/j.fusengdes.2015.11.007

Other Link： https://www.sciencedirect.com/science/article/pii/S0920379615303343?via%3Dihub

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： https://doi.org/10.1088/0741-3335/58/11/115004

Other Link： http://iopscience.iop.org/article/10.1088/0741-3335/58/11/115004/meta

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

DOI： 10.1016/j.vacuum.2016.04.025

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： https://doi.org/10.1063/1.4960117

Other Link： https://aip.scitation.org/doi/10.1063/1.4960117

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

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

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

DOI： 10.1016/j.fusengdes.2016.04.016

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

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

DOI： 10.1038/srep22189

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)  

DOI： 10.1016/j.fusengdes.2015.11.035

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

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

DOI： http://dx.doi.org/10.1063/1.4934537

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： 10.1016/j.fusengdes.2015.06.054

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

DOI： http://dx.doi.org/10.1063/1.4928878

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

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

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)  

DOI： 10.1088/0029-5515/55/7/073003

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

DOI： 10.1088/0029-5515/55/6/063009

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

An orthogonal dynamic programming (ODP) based particle image velocimetry (PIV) technique is developed to measure the time resolved flow field of the fluctuating structures at the plasma edge and scrape off layer (SOL) of tokamaks. 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 into 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 plasma to estimate the SOL flow in inboard poloidal null–natural divertor configuration. This technique is also applied to investigate the intricate features of the core of the run-away dominated phase following the injection of a large amount of neutrals in the target Ohmic plasma. Development of the ODP-PIV code and its applicability on actual plasma images is reported.

DOI： 10.1063/1.4914838

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

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

Global particle balance in non-inductive long-duration plasma on QUEST has been investigated. Approximately 70% of the fuel hydrogen (H) was retained in the wall and then was almost exhausted just after the discharge. The global recycling ratio (Rg), defined as the ratio of the evacuated H2 flux to that injected, was found to gradually increase during discharges and subsequently rose rapidly. To study the growth of Rg, the thermal desorption spectra after deuterium implantation in a specimen exposed to QUEST plasma was analyzed with a model which includes reflection, diffusion, solution, recombination, trapping, and plasma-induced desorption in the re-deposition layer. The model reconstructs the growth of Rg during a long-duration plasma and indicates solution plays a dominant role in the growth.

DOI： 10.1016/j.jnucmat.2015.01.013

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

Hydrogen wall pumping is studied in steady state tokamak operation (SSTO) of QUEST with all metal plasma facing materials PFMs at 100 °C. The duration of SSTO is up to 820 s in fully non-inductive plasma. Global gas balance analysis shows that wall pumping at the apparent (retention–release) rate of 1–6 × 1018 H/s is dominant and 70–80% of injected H2 can be retained in PFMs. However, immediately after plasma termination the H2 release rate enhances to ∼1019 H/s. In order to understand a true retention process the direct measurement of retention flux has been carried out by permeation probes. The comparison between the evaluated wall retention and results from global analysis is discussed.

DOI： 10.1016/j.jnucmat.2014.12.092

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

Thermal probe with double function of thermocouples and Langmuir probe has been developed, and the initial data observed in far-SOL in QUEST is obtained. Heat flux of megawatt per square meters related to energetic electrons and sonic plasma flow in far-SOL have been observed in the current rump-up phase although no high power inductive force like ohmic winding is applied. The heat flux and the flow are suppressed after the current is built up. In the quasi-steady state, plasma current starts and keeps sawtooth-like oscillation with 20 Hz frequency. The heat flux and the flow in far-SOL have clear responses to the oscillation.

DOI： 10.1016/j.jnucmat.2014.11.136

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 (scientific journal)  

Meso-scale streamer has a radially elongated structure and is believed to enhance the radial transport. In order to study the control of the streamer, we demonstrated an end plate biasing to the streamer state in the PANTA linear plasma. During the end plate biasing, the electron density profile became more peaked, fluctuation was suppressed, the streamer structure was deconstructed, and the waveform became a periodic solitary state. The radial electric field only induced at around the end plate was found to play an important role for the streamer suppression.

DOI： 10.1088/0029-5515/54/11/114010

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

The multi-pass Thomson scattering (TS) scheme enables obtaining many photons by
accumulating multiple TS signals. The signal-to-noise ratio (SNR) depends on the
accumulation number. In this study, we performed multi-pass TS measurements for
ohmically heated plasmas, and the relationship between SNR and the accumulation number
was investigated. As a result, improvement of SNR in this experiment indicated similar
tendency to that calculated for the background noise dominant situation.

DOI： 10.1063/1.4891707

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

A Rogowski probe consisting of a small multi-layer Rogowski coil, five magnetic pick-up
coils, and a Langmuir probe was developed to measure the local current density and its
direction. It can be moved along the major radius and can be turned around its axis. This
probe was used to measure the current density profile near the last closed flux surface of
Ohmic plasmas in Tokyo Spherical Tokamak-2. The current density profile was measured
successfully with a signal to noise ratio of greater than 20.

DOI： 10.1063/1.4887277

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

A drift error correction technique with machine vision and a real-time equilibrium calculation code have been developed on the QUEST (Q-shu university experiment with the steady-state spherical tokamak) for steady-state operation. The drift error caused by the long time-integration of magnetic raw signals has to be removed. With a captured image of the plasma’s cross section, the plasma’s position is identified by use of image filters. The measured magnetic flux values are corrected to the calculated flux values estimated by using this plasma position. The correction with the captured image work as expected in the preliminary result using a flashlight instead of a plasma.

DOI： 10.3938/jkps.65.1191

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)  

Plasma current start-up and ramp-up using the lower hybrid wave (LHW) were investigated on the TST-2 spherical tokamak. The LHW was launched by a dielectric-loaded waveguide array (grill) antenna. The antenna-plasma coupling of this antenna deteriorates as the input power exceeds several kW. This deterioration is believed to be caused by the density depletion due to the ponderomotive force. This conjecture was confirmed by the measurement of density reduction and the result of a non-linear full wave numerical calculation based on the finite element method (FEM).The plasma current was started and ramped up to 10 kA using this antenna. The ability of this grill antenna to excite the LHW with different n(parallel to) = ck(parallel to)/omega was used to identify the most favourable n(parallel to) spectrum for plasma current ramp-up. It was found that effective current drive can be achieved by the LHW with n(parallel to) less than 6. However, even in this case, the energetic electrons which account for a large fraction of the driven current, are lost rapidly because the poloidal field generated by this level of plasma current is not sufficient to confine high energy electrons.

DOI： 10.1088/0029-5515/54/9/093014

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)  

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)  

In the JFT-2M tokamak (JFT standing for JAERI Fusion Torus), the limit cycle oscillation (LCO), together with several variables, i.e., the electrostatic potential, radial electric field, electron density, turbulence intensity, and D-alpha emission from the divertor region, is observed before the L-to-H transition. The spatiotemporal dynamics of the LCO is analysed in detail. Zonal flows are not seen, while modulation is observed in the edge-localized poloidal flow and density gradient. Modulation is also seen in the Reynolds stress, caused by that in the turbulence intensity and turbulence wavenumber. However, flow acceleration cannot be explained by the modulation in the Reynolds stress. Rapid inward propagation is also observed for the density gradient and turbulence packet. The characteristics of the propagation are verified by means of turbulence spreading theory and diffusion theory.

DOI： 10.1088/0029-5515/54/7/073017

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

In multi-pass Thomson scattering (TS) scheme, a laser pulse makes multiple round trips through the plasma, and the effective laser energy is enhanced, and we can increase the signal-to-noise ratio as a result. We have developed a coaxial optical cavity in which a laser pulse is confined, and we performed TS measurements using the coaxial cavity in tokamak plasmas for the first time. In the optical cavity, the laser energy attenuation was approximately 30% in each round trip, and we achieved a photon number gain of about 3 compared with that obtained in the first round trip. In addition, the temperature measurement accuracy was improved by accumulating the first three round trip waveforms.

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

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

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

In multi-pass Thomson scattering (TS) scheme, a laser pulse makes multiple round trips through the plasma, and the effective laser energy is enhanced, and we can increase the signal-to-noise ratio as a result. We have developed a coaxial optical cavity in which a laser pulse is confined, and we performed TS measurements using the coaxial cavity in tokamak plasmas for the first time. In the optical cavity, the laser energy attenuation was approximately 30% in each round trip, and we achieved a photon number gain of about 3 compared with that obtained in the first round trip. In addition, the temperature measurement accuracy was improved by accumulating the first three round trip waveforms. (C) 2014 AIP Publishing LLC.

DOI： 10.1063/1.4878260

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

RF magnetic probes can be used to measure not only the wavevector, but also the polarization of waves in plasmas. A 5-channel RF magnetic probe (5ch-RFMP) was installed in the TST-2 spherical tokamak and the waves were studied in detail during lower hybrid wave injection experiments. From the polarization measurements, the poloidal RF magnetic field is found to be dominant. In addition to polarization, components of k perpendicular to the major radial direction were obtained from phase differences among the five channels. The radial wavenumber was obtained by scanning the radial position of the 5ch-RFMP on a shot by shot basis. The measured wavevector and polarization in the plasma edge region were consistent with those calculated from the wave equation for the slow wave branch. While the waves with small and large k∥ were excited by the antenna, only the small k∥ component was measured by the 5ch-RFMP; this suggests that the waves with larger k∥ were absorbed by the plasma.

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

A new method is presented for identifying waveforms of fluctuations in turbulent plasmas. The method is based on heartbeat analysis in which the convolution of a waveform is obtained by employing the phase tracking method. Phase tracking is performed by correlating raw time-series data with a template waveform; the template is evaluated through iteration procedure. The method is applied to fluctuations in a PANTA plasma, and the nonlinear waveform and its distribution of periods are obtained.

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 (scientific journal)  

A multi-pass Thomson scattering (TS) system based on confining laser pulses in an optical cavity was constructed for measuring very low-density plasma in the TST-2 spherical tokamak device. This paper describes the setup of the optical system, injection of the laser pulse into the cavity, and properties of the confined laser pulse. A combination of Pockels cell plus polarizer, which serves as an optical shutter, allows us to inject and then confine intense laser pulses in the cavity. A photodiode signal monitoring the very weak light leaking from the cavity mirrors demonstrated that the laser pulse makes many round trips, with a round-trip efficiency of approximately 0.73. The effective number of round trips (i.e., the signal enhancement factor) is approximately 3.7. For an injection efficiency of approximately 0.69, a cavity-confined laser pulse, applied to Thomson scattering, will yield a scattered signal that is five times larger than that from a single-pass laser pulse.

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

Transitions between low (L-mode) and intermediate (I-phase) confinement regimes triggered by sawteeth are investigated using multiple Langmuir probe arrays in the edge plasmas of the HL-2A tokamak. The I-phase is characterized by limit-cycle oscillations (LCOs). Repeated L-I-L transitions induced by sawtooth heat pulses are also observed. Statistical analyses show that the delay time of the L-I transition relative to sawtooth crashes is less than similar to 1 ms. The intensity of turbulence bursts in the I-phase is stronger than those observed in L-mode plasmas. Measurements are consistent with zonal flow reducing turbulence intensity in the frequency band similar to 20-100 kHz, while the LCO turbulence of frequency higher than 100 kHz has more power than that in the L-mode. The analyses of time-resolved power transfers between turbulence and zonal flows suggest that power transfers exist between turbulence and zonal flows. The direction of the power transfers is from turbulence into geodesic acoustic modes in ohmic plasmas.

DOI： 10.1088/0029-5515/53/12/123015

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

We propose the use of a microwave comb for measuring the density profile as a continuous function of radius with high temporal resolution. The analysis focuses on phase-sensitive convolution of the reflected waves: convolution of pulses can eliminate statistical noise. The time resolution for constructing the density profile is also discussed.

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

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A nonlinear simulation of resistive drift wave turbulence in a cylindrical plasma is carried out. Long time evolution of turbulence with formation of a zonal flow is obtained for more than 1000 times of the typical drift wave period, which is sufficient for statistical analyses. Dynamical particle and momentum balance for the formation of the mean reveals that the radial turbulent fluxes are dominant contributors for the evolution of fluctuations. The particle flux is found to precede the momentum flux for 0.4 times of the typical drift wave period with large temporal variance. The analyses of the time series data of 3-D fields give the understanding of the dynamical structural formation mechanism.

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

To study the nonlinear saturation mechanism in turbulent plasmas, we plan to obseve ion temperature gradient (ITG) modes in the Plasma Assembly for Nonlinear Turbulence Analysis (PANTA). Linear growth rates of ITG modes in the linear device are analyzed to determine their excitation conditions using a three-field fluid model to describe the ion motion. Parameter scans of the linear growth rate show the threshold for ITG mode excitation, and the typical plasma parameters of the PANTA are found in the unstable domain.

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.

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)  

The radial structures of multiple sheared flow populations and fluctuations are simultaneously measured using combinations of Langmuir probe arrays in the edge plasmas of the HL-2A tokamak with ohmic and electron cyclotron resonance heating (ECRH). The maximum of geodesic acoustic mode (GAM) amplitude locates at similar to 2-3 cm inside the last close flux surface (LCFS). The low-frequency zonal flow (LFZF) and GAM tend to coexist in the inner region. The dependences of the amplitudes of the multiple sheared flows and turbulence on ECRH heating power are also investigated. The measured turbulent Reynolds stresses are shown to be strongly correlated with the sheared flows, as predicted by theory. The turbulence is modulated at the each frequency of the multiple sheared flows simultaneously, and the particle fluxes induced by the turbulence are significantly reduced near the LCFS and in the GAM peaking region. The analyses also show that the spatial structures of the turbulent envelopes at the LFZF and GAM frequencies are similar to those observed in LFZF and GAM.

DOI： 10.1088/0029-5515/53/8/083011

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

In this Letter, we report analyses of spatiotemporal dynamics of turbulence and structure in the limit-cycle oscillation (LCO) that precedes an L-to-H transition. Zonal flows are not observed during LCO, and the oscillation is the periodic generations or decays of barrier with edge-localized mean flow. Oscillatory Reynolds stress is found to be too small to accelerate the LCO flow, by considering the dielectric constant in magnetized toroidal plasmas. Propagation of changes of the density gradient and turbulence amplitude into the core is also observed.

DOI： 10.1103/PhysRevLett.111.035002

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

Plasma current (I-p) start-up in a spherical tokamak (ST) by waves in the lower-hybrid (LH) frequency range was investigated on TST-2. Alowcurrent (similar to 1 kA) ST configuration can be formed bywaves over a broad frequency range (21 MHz-8.2 GHz in TST-2), but further Ip ramp-up (to similar to 10 kA) is most efficient with waves in the LH frequency range. I-p ramp-up to 15 kA was achieved with 60kW of net RF power P-RF in the fast wave (FW) polarization at 200MHz excited by the inductively coupled combline antenna. X-ray measurements showed that the photon flux and temperature are higher in the direction opposite to I-p, consistent with acceleration of electrons by a uni-directional RF wave. There is evidence that the LH wave is excited nonlinearly by the FW, based on the frequency spectra measured by magnetic probes. Similar efficiencies of I-p ramp-up were obtained with the inductive combline antenna and the dielectric-loaded waveguide array ('grill') antenna, and tendencies for the current drive efficiency to increase with plasma current and toroidal field were observed. During operation of the grill antenna, wavevector components were measured by an array of magnetic probes. Results were qualitatively consistent with expectations based on dispersion relations for the FW and the LH wave. A capacitively coupled combline antenna has been developed to improve coupling to the plasma and the wavenumber spectrum of the excited LH wave, and will be tested in 2013.

DOI： 10.1088/0029-5515/53/6/063006

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

Validation of modeling in meso-scale structure formation is undergoing by using multi-channel probe measurements and advanced analyzing techniques such as bispectrum. Study of meso-scale structures including streamers is important for understanding anomalous transports in magnetized plasmas. A streamer structure and its mediator mode were observed in the Large Mirror Device-Upgrade linear plasma. The mediator mode nonlinearly coupled with many other drift wave modes and generated a streamer structure by phase locking. The radial biphase profiles indicating the phase locking showed the radial elongation of the streamer structure. Modes having similar characteristics to the mediator were also found in a theoretical work based on the Hasegawa-Mima model and a numerical simulation code.

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

Drift wave micro-turbulence is the main source of anomalous transport in a tokamak. Correlation reflectometry is a powerful diagnostic tool which provides information on plasma turbulence and subsequently on underlying instability. In this paper, theoretical expressions for the analysis of radial correlation reflectometry (RCR) data are derived. Integral kernels, which convert the correlation function of two microwave reflectometry signals into a correlation function of plasma turbulence and inverse, are discussed. The analytical expression and the method of combining the RCR diagnostic and another local density fluctuation (e.g. Doppler enhanced scattering or heavy ion beam probe) is proposed. The correlation between reflectometry signals and those from other local fluctuation measurements is analysed. The long-range tail of the correlation decays much more gradually than the turbulence correlation, however, it decays faster than that of two microwave reflectometry signals. The way to calculate turbulence wave number spectrum for this case is also proposed.

DOI： 10.1088/0031-8949/87/04/045502

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

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

This paper proposes a clever usage of bispectral analysis for extracting or reconstructing a quasi-coherent and quasi-periodic structural evolution of plasma fluctuations. The method has been applied on electron temperature fluctuation signals measured with a multi-point electron cyclotron emission (ECE) radiometer in the Large Helical Device (LHD). The method successfully reconstructs the averaged spatiotemporal evolution of fluctuating coherent structure (Inagaki et al 2011 Phys. Rev. Lett. 107 115001), while the structure is buried in (or comparable in power to) background fluctuations. The bicoherence analysis has found the faint fluctuating structure consisting of a fundamental mode and the harmonic modes from the existence of significant couplings between them. The reconstructed spatiotemporal structure with the proposed method is compared with that obtained with the lock-in (conditional) average. Three cases of spatiotemporal evolution of the non-sinusoidal waveform are presented.

DOI： 10.1088/0741-3335/54/11/115004

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

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

Ion flow is studied in the cylindrical magnetized Argon plasma of the PANTA device using several experimental methods. Time delay estimation technique (TDE) is used to measure the azimuthal propagation of density fluctuations between two electrostatic probes. Ion flux is also studied in azimuthal and axial direction using a newly installed Mach probe. TDE shows velocity profiles qualitatively consistent with expected drift wave propagation and E × B rotation. The Mach probe shows a maximum azimuthal flow and an axial flow shear close to the maximum density gradient and maximum fluctuation position. Striking differences exist in the detected axial ion flows depending on the discharge parameters. In high neutral pressure conditions reversal of axial ion flow was detected in the outer part of the plasma column. Temporal evolution of flows and fluctuations are compared as a first step to assess interplay between plasma turbulence and flows in the radial, axial and azimuthal directions.

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

Electron temperature fluctuation amplitude is evaluated with a single probe using conditional sampling technique on the PANTA (linear magnetic configuration device). Electron temperature fluctuations are extracted as a function of floating potential. Normalized temperature fluctuation amplitude of about 15% is observed in low neutral pressure discharge in PANTA.

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 (scientific journal)  

The competing effects of the long-range fluctuation, which has been found on the Large Helical Device (Inagaki et al 2011 Phys. Rev. Lett. 107 115001), on the net turbulent transport are discussed. The transport, which is driven directly by this mode, is evaluated. Then, the associated reduction of fluctuations in the range of drift wave turbulence is analyzed, by employing the predator-prey model. The transport driven by microscopic turbulence is reduced by the appearance of long-range fluctuations. Comparing these two competing processes, the condition is derived for the net transport to be reduced by the appearance of this long-range fluctuation. In addition, the experimental observations of microscopic fluctuations are discussed. The reduction of high-frequency fluctuations occurs in conjunction with the long-range fluctuations.

DOI： 10.1088/0741-3335/54/9/095016

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 (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)  

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 (scientific journal)  

Preliminary observation results are reported for a new discharge regime in the Plasma Assembly for Nonlinear Turbulent Analysis (PANTA), where spontaneous transitions in the equilibrium profile and fluctuation spectra occur. Two different states are defined by using the mean density value. Axial and radial profiles are observed for the two states, and large profile changes are found. The spatiotemporal evolution of the transition front is measured. Changes in the fluctuation spectrum are evaluated using conditional average and lock-in average.

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)  

Intermittent pulse events have been observed in the periphery of the linear cylindrical plasmas produced in Plasma Assembly for Nonlinear Turbulence Analysis (PANTA). A novel combination of 32 azimuthal and 3 radial probes enables us to obtain 32 × 3 = 96 pairs of correlations simultaneously. The two-dimensional (2D) correlation analysis shows that the intermittent structure is well correlated with quasi-periodic bursts inside the plasma, and it reveals that the intermittent pulse is caused by the passage of a radially elongated and azimuthally distorted localized structure rotating in the ion diamagnetic direction.

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

A solitary drift wave structure and its quasi-periodic change are discovered in a linear magnetized plasma. Detailed investigations show that the periodic changes are accompanied by an emission of splash, that is, a short lifetime event characterized by much higher frequencies than that of the fundamental drift waves. Dynamic interaction between the solitary wave and zonal flows is found to cause the emission of splashes since the splashes occur in a particular phase of zonal flows. The findings should give a deep insight into the dynamics and structural formation of magnetically confined plasmas and rotating planets, which are governed by drift and Rossby waves, respectively.

DOI： 10.1088/0741-3335/53/11/115009

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

A double-pass Thomson scattering system, in which a laser pulse makes a round trip through the plasma, was constructed. Using the same optics and a fast detection unit, we can resolve backward and forward scattering pulses in the signal. Because these scatterings reflect velocity distribution along different directions, electron temperature anisotropy can be estimated from the double-pass Thomson scattering system.

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

Electron temperature fluctuations are measured with a rotatable triple probe in PANTA. Effects of fluctuation phase are cancelled by aligning the probe pins in the propagation direction of fluctuations. Evaluated electron temperature fluctuations normalized by average electron temperature are much smaller than normalized density fluctuations and normalized floating potential fluctuations in low electron temperature PANTA plasma. Additionally conditional sampling technique is applied to a single probe measurement.

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

A solitary wave like fluctuating structure is observed in the Large Mirror Device-Upgrade (LMD-U) under relatively high neutral pressure conditions. Ion saturation current fluctuations are measured and Fourier and wavelet power spectra are calculated. Temporal behavior of fluctuation power and strength of nonlinear coupling of low frequency coherent mode and background turbulence in a specific frequency region is analyzed. This is achieved using wavelet spectral and bispectral analyses with conditional averaging.

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

Spherical tokamaks (STs) have the advantage of high beta capability, but the realization of a compact reactor requires the elimination of the central solenoid (CS). The possibility of using the lower hybrid wave (LHW) to ramp up the plasma current (Ip) from zero to a high enough level required for fusion burn in ST is examined theoretically and experimentally. Excitation of a travelling fast wave (FW) by the combline antenna installed on TST-2 was confirmed by a finite element analysis, but efficient current drive requires excitation of the LHW, either directly by the antenna or by mode conversion from the FW. The analysis using the TORLH full-wave solver indicates that core current drive by LHW is possible in the low-density, low Ip plasma formed by electron cyclotron heating (ECH). It is important to keep the density low during Ip ramp-up, and the wavenumber must be reduced as Ip increases. Initial results from TST-2 demonstrate that RF power in the LH frequency range (200MHz) can achieve initial Ip formation, and is more effective than ECH for further ramp-up of Ip. Ip ramp-up to over 12kA was achieved by combining ramp-up of the externally applied vertical magnetic field and ramp-up of the RF power. The significant asymmetry observed between co-current drive and counter-current drive is attributed to the presence of RF driven current. An optimized LHW antenna with appropriate polarization and wavenumber spectrum controllability is being designed. The success of the TST-2 experiment would provide a scientific basis for quantitatively evaluating the required CS capability for a low-aspect-ratio reactor.

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

A new method for estimating the wavenumber of a standing wave system by using three Langmuir probes is proposed. Analytical formulae are derived from a simple model in which two waves of the same frequency and the same wavenumber propagate in opposite directions. The proposed method can estimate the wavenumber correctly even if the two waves have equal amplitude.

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

The turbulence fluctuation spectra of linear magnetized plasmas are measured under various neutral pressure conditions. Strong turbulent waves and symmetric waves are observed in low and high neutral pressure conditions. In this study, we compare the radial structure of the probability density function (PDF) of these fluctuations. The characteristics of the PDFs are related to those of the radial and poloidal turbulent structures.

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

The structure of fluctuations in the Large Mirror Device-Upgrade (LMD-U) depends on the neutral density. In this study, we focus on the high neutral density plasma in which the potential and density fluctuation waveforms are nontrigonometric functions similar to sawtooth waves, and peaks in the power spectrum lie along a straight line when plotted as frequency versus wavenumber. Two-dimentional auto and cross bispectral analyses of Iis and/or Vf, which consider both the matching conditions for both the azimuthal wavenumber and the frequency, have been performed; significant couplings are observed at particular peak frequencies. Similarities and differences between the auto and cross bicoherence of Iis and/or Vf are discussed.

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

In the TST-2 spherical tokamak, non-inductive start-up experiment using electron cyclotron heating (ECH) at 2.45 GHz are performed. Hydrogen and deuterium discharges were compared after sufficient wall cleaning by Ohmic discharges to ensure discharge reproducibility. After the wall cleaning, wall recycling seemed to be reduced, because additional gas puffing was necessary to sustain a discharge. Even in that case, hydrogen emission measurement revealed the existence of finite wall recycling source. A current jump occurred earlier in the deuterium plasma than in the hydrogen plasma. Except for the timing of the jump, the discharge time evolution and various values such as the plasma current were almost the same for the deuterium and hydrogen plasmas.

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

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

Electron cyclotron resonance heating (ECRH) is superimposed on a linear cylindrical helicon plasma in the Large Mirror Device-Upgrade (LMD-U) to study the fluctuation characteristics of high density helicon plasma with/without ECRH injection. The radial profiles of electron density and electron temperature rose by < 10 %, and the drift wave frequency decreased with the ECRH injection. Bicoherence analysis reveals that the nonlinear interaction between drift waves and broad frequency band components exists with or without ECRH injection.

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

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

Nonlinear mode couplings in plasma turbulence generate mesoscale global structures, such as zonal flows and streamers. A streamer is a poloidally localized structure that is radially elongated. In the Large Mirror Device-Upgrade, a streamer structure was observed in the ion saturation-current fluctuation measured with a 64-channel poloidal Langmuir probe array. The bunching of waves was poloidally localized and rotated slowly in the ion diamagnetic direction. The frequency and poloidal mode number of its envelope corresponded to one particular mode, which was strongly coupled with many other modes. By calculating the radial biphase profile of three pronounced modes, including the particular mode, the streamer structure was confirmed to be radially elongated.

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

A streamer, which is a bunching of drift-wave fluctuations, and its mediator, which generates the streamer by coupling with other fluctuations, have been observed in a cylindrical magnetized plasma. Their radial structures were investigated in detail by using the biphase analysis. Their quasi-two-dimensional structures were revealed to be equivalent with a pair of fast and slow modes predicted by a nonlinear Schrodinger equation based on the Hasegawa-Mima model.

DOI： 10.1103/PhysRevLett.105.225002

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The wavelet analyses have been carried out, using a cluster of personal computer, on the signal of electric field fluctuations measured with heavy ion beam probes in the compact helical system stellarator. The results have revealed the intermittent characteristics of turbulence and of the nonlinear couplings between elemental waves of turbulence. The usage of parallel computing is found to successfully reduce the calculation time as inversely proportional to the CPU number used the cluster, which shows the nature of "embarrassingly parallel computation." The present example of the wavelet analyses clearly demonstrates the importance of the advanced analyzing methods and the parallel computation for the modern studies of plasma turbulence. (C) 2010 American Institute of Physics. [doi:10.1063/1.3488244]

DOI： 10.1063/1.3488244

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

Density fluctuation spectra are qualitatively modified depending on filling gas pressure and magnetic field strength in a linear magnetized plasma. Abrupt and spontaneous jumps between fluctuation states during a discharge are observed in the filling gas pressure-magnetic field plane. Those jumps are characterized using a spectrum pattern and have a bifurcation property on the plane.

DOI： 10.1088/0741-3335/52/10/105009

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

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DOI： 10.1143/JPSJ.79.085001

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

A new 50 GHz microwave interferometer was installed, and non-inductively sustained spherical tokamak plasmas in the TST-2 device were measured. The line integrated electron densities (nels) on five chords were compared with visible CCD camera image and equilibrium analysis. It is concluded that the high-density region has a C-like shape along the outboard boundary. This shape suggests that the electron density is not constant on the magnetic flux surface.

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

The amplitudes of electron temperature fluctuations are evaluated in the edge plasma of TST-2 using a new technique. Langmuir probe current-voltage characteristic curves are conditionally reconstituted in terms of the magnitude of the floating potential. High/low electron temperatures are obtained in low/high floating-potential phases. The relationship between the electron temperature fluctuations and the time-averaged electron temperature gradient is discussed.

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

The spatial distribution of the radiofrequency (RF) magnetic field associated with a high harmonic fast wave (HHFW) was measured using an array of magnetic probes in the plasma inside the University of Tokyo Spherical Tokamak (UTST). Data obtained from 25 probes (19 locations for toroidal polarization and 6 locations for vertical polarization) distributed along the poloidal cross section were analyzed. The RF magnetic field is polarized in the toroidal direction, indicating that the HHFW is excited in the plasma. The RF field is weak on the inboard side. Analysis of the group delay suggests that the waves travel long distances in the plasma, probably because of poor absorption. No indication of parametric decay was observed up to 80 kW of injected RF power.

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

Bispectral analysis and multichannel measurement are becoming attractive investigation tools in plasma fluctuation studies. In the Large Mirror Device-Upgrade, the measurement of fluctuations in the ion saturation-current with a 64-channel poloidal Langmuir probe array was performed. The two-dimensional (2D) (poloidal wave number and frequency) power spectrum showed a number of pronounced peaks and broadband fluctuations in the poloidal wave number-frequency space. We applied 2D bispectral analysis, which considers both the matching conditions of poloidal wave number and frequency, to the spatiotemporal waveform, and confirmed the nonlinear couplings between coherent-coherent, coherent-broadband, and broadband-broadband fluctuation components. More than ten peaks were revealed to have as their origins only three original parent modes generated in the plasma. Comparison between the theoretical estimate and experimental observation for the bicoherence showed good agreement. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3429674]

DOI： 10.1063/1.3429674

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

A new method of analyzing the Causal relationship of the interaction between various fluctuating quantities and the mean gradient in turbulent plasmas is proposed. This method involves (1) calculating the correlation coefficient R and the directional area A for the pair Of turbulent quantities of interest, and (2) creating a time ensemble consisting of (R, A) during many fluctuation periods. The statistical features of each ensemble can be used to determine the order Of Occurrence, and hence, the causal relationship between the two fluctuating quantities. We apply this analytical method to the analysis of a plasma in which the drift wave and interchange mode interact with and arc modulated by the density gradient. The Causal relationship between the drift wave and the density gradient is discussed.

DOI： 10.1143/JPSJ.79.024501

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

A transition of the fluctuation spectrum of ion saturation current is observed in linear magnetized plasmas. A power spectrum with regular peaks and a broadband spectrum appear alternately during a discharge. The temporal order of changes in the modes is investigated at the transition. The low poloidal mode number components change before the higher modes begin to change. The delay times of change between modes are determined.

DOI： 10.1088/0741-3335/51/8/085001

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)  

An extension of geodesic acoustic mode (GAM) spectroscopy [Plasma Phys. Control. Fusion 49, L7 (2007)] is proposed. The ratio between the lowest frequency of the co-existing ion acoustic mode (IAM) and the frequency of GAM enables us to identify the safety factor of toroidal plasmas. The lowest frequency can be detected by bispectrum analysis when both GAM and IAM are excited. The possibility of measuring the safety factor is discussed.

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

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

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

Turbulence is a state of fluids and plasma where nonlinear interactions including cascades to finer scales take place to generate chaotic structure and dynamics(1). However, turbulence could generate global structures(2), such as dynamo magnetic field, zonal flows(3), transport barriers, enhanced transport and quenching transport. Therefore, in turbulence, multiscale phenomena coevolve in space and time, and the character of plasma turbulence has been investigated in the laboratory(4-10) as a modern and historical scientific mystery. Here, we report anatomical features of the plasma turbulence in the wavenumber-frequency domain by using nonlinear spectral analysis including the bi-spectrum(11). First, the formation of the plasma turbulence can be regarded as a result of nonlinear interaction of a small number of irreducible parent modes that satisfy the linear dispersion relation. Second, the highlighted finding here, is the first identification of a streamer ( state of bunching of drift waves(12,13)) that should degrade the quality of plasmas for magnetic confinement fusion(14,15). The streamer is a poloidally localized, radially elongated global structure that lives longer than the characteristic turbulence correlation time, and our results reveal that the streamer is produced as the result of the nonlinear condensation, or nonlinear phase locking of the major triplet modes.

DOI： 10.1038/nphys1029

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

Spatiotemporal correlation of heat transport and micro- to meso- scale or macro-scale coupling of plasma turbulence are investigated in LHD plasmas, where fast propagation of a cold pulse and a non-local temperature rise are observed. Evidence of a timescale shorter than the diffusion time and a spatial scale longer than the micro-turbulence correlation length between core heat fluxes and edge temperature gradients is found. At the same time, an envelope of turbulent density fluctuations is also found to be modulated. Observation of low frequency ( 2 kHz) amplitude modulation of density fluctuation suggests the existence of meso- to macro-scale turbulent structures in the plasma, in which a non-local temperature rise takes place. Relationships between the turbulence with long radial correlation length and the edge-core coupling of heat transport are discussed.

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

Zonal flow type oscillation, which is driven by a small modulation of the electron cyclotron resonance (ECR) power in a low-frequency band (< 0.1 kHz), and its impact on drift wave turbulence are observed in linear cylindrical ECR plasmas by simultaneous spatiotemporal measurements with a multi-ring probe array. It is found that a potential low-frequency (< 0.1 kHz) oscillation can have a zonal flow type structure. Bicoherence analysis reveals that this zonal flow type oscillation has a nonlinear interaction with the drift mode. These results indicate that an electric field with zonal flow, which is excited externally, can modulate the drift mode amplitude, similar to spontaneous zonal flow.

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

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

In the LMD-U linear magnetized plasma, fluctuation measurements have been performed with multi-channel poloidal Langmuir probe arrays to investigate the spatiotemporal behaviors of drift wave turbulence. Two-dimensional (poloidal mode number-frequency) power spectrum showed not only fluctuation peaks but also the existence of a broadband fluctuation. The broadband fluctuation developed at high poloidal mode number and high frequency regions and not along the linear dispersion relation curve of drift wave modes. It showed shorter correlation time and poloidal length than the peaked fluctuations. Two-dimensional axial coherence was measured with two poloidal probe arrays separated in the axial direction. The axial coherence was strong for both the broadband fluctuation and peak fluctuations, suggesting the quasi-two-dimensional structure of the drift wave turbulence.

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

Linear plasmas allow multi-point, low-temperature measurements with Langmuir probes. We measured ion saturation-current fluctuations of a Large Mirror Device-Upgrade linear plasma using a poloidal Langmuir probe array. By varying the discharge conditions, the spatiotemporal behavior showed a change from a coherent sine wave to a turbulent waveform through a periodic, modulated sine wave. The two-dimensional (poloidal wave number and frequency) power spectrum for each regime showed a single fluctuation peak, a peak and its harmonics, and a number of peaks in the poloidal wave number-frequency space. Bi-spectral analysis was performed for the turbulent regime, and showed the existence of nonlinear couplings among fluctuation peaks and broadband components.

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

The experimental studies on zonal flows and turbulence have been carried out in Compact Helical System [K. Matsuoka, S. Kubo, M. Hosokawa , in Plasma Physics and Controlled Nuclear Fusion Research, Proc. 12th Int. Conf., Nice, 1988 (International Atomic Energy Agency, Vienna, 1989, Vol. 2, p. 411] using twin heavy ion beam probes. The paper presents the experimental observations of stationary zonal flow, nonlinear couplings between zonal flow and turbulence, and the role of zonal flow in the improved confinement, together with the recent discovery of zonal magnetic field. The presented experimental results strongly support the new paradigm that the plasma transport should be considered as a system of drift wave and zonal flows, and provides the first direct evidence for turbulence dynamo that the structured magnetic field can be really generated by turbulence. (C) 2008 American Institute of Physics.

DOI： 10.1063/1.2889012

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

A competitive coexistence between a drift mode and a flute mode has been investigated experimentally in a bounded cylindrical electron cyclotron resonance ( ECR) plasma. This competitive cycle was repeated during a whole continuous discharge. Competitive phenomena appeared in plasmas when the density scale length was close to the critical value: the drift mode is dominant and the flute mode is suppressed just below a critical value, but above this value, the flute mode grows abruptly and the drift mode is suppressed. In this phase, the density gradient increases temporally. A small crash takes place when the density scale length reaches a limit value, and then the density profile goes back to the previous states and thus the drift mode is excited and the flute mode is suppressed. The direct nonlinear coupling between the drift mode and the flute mode is weaker than the process of the second harmonic generation of each mode, suggesting that the growth and decay of them are closely related to the modification of the radial density profile.

DOI： 10.1088/0741-3335/50/3/035011

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

The internal behavior of fast ions interacting with magnetohydrodynamic bursts excited by energetic ions has been experimentally investigated in the compact helical system. The resonant convective oscillation of fast ions was identified inside the last closed-flux surface during an energetic-particle mode (EPM) burst. The phase difference between the fast-ion oscillation and the EPM, indicating the coupling strength between them, remains a certain value during the EPM burst and drives an anomalous transport of fast ions.

DOI： 10.1103/PhysRevLett.100.065005

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

Multipoint detection is an essential requirement for investigating plasma turbulence which is a highly nonlinear phenomenon in space and time. We have fabricated an array of 64-channel poloidal probes surrounding the linear cylindrical plasma named LMD-U in order to study turbulence properties, particularly the nonlinear mode couplings, in the domain of poloidal wave number and frequency. However, misalignments of probe tips produce spurious modes, which do not exist in the real plasma, to distort the precise wave number measurements. The paper presents the description of the 64-channel poloidal probe array with means to adjust the probe positions, with discussion on the effects of the misalignments on the wave number measurements. (C) 2007 American Institute of Physics.

DOI： 10.1063/1.2818796

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

Measurement of drift wave turbulence using multi-probe systems was performed in the Large Mirror Device-Upgrade linear plasma. We compared a drift wave's two-dimensional (poloidal wave number and frequency) power spectrum with its calculated linear dispersion relation. As a result, a few wave modes that satisfy the linear dispersion relation were observed. Moreover, cascades to non-mode peaks and broadband components that do not satisfy the dispersion relation were identified.

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

In a high-density plasma produced by helicon waves in a linear device, a change from the coherent to broad-band density fluctuations was observed on increasing the magnetic field. The coherent mode exhibited the drift wave features, and a steady oscillation state was observed. When the magnetic field was increased, the fluctuation level increased, and the higher azimuthal Fourier components were excited. When the spectrum became broad at the higher magnetic field, the behavior of the azimuthal structure became complex and changed rapidly with time. The analysis performed using the movable probe array (48ch.-probe) decomposed the short-lived structure into poloidal mode-frequency space and indicated mode-mode coupling in the broad-band spectrum.

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

The present status of experiments on zonal flows in magnetic confinement experiments is examined. The innovative use of traditional and modem diagnostics has revealed unambiguously the existence of zonal flows, their spatio-temporal characteristics, their relationship to turbulence and their effects on confinement. In particular, a number of observations have been accumulated on the oscillatory branch of zonal flows, named geodesic acoustic modes, suggesting the necessity for theories to give their proper description. In addition to these basic properties of zonal flows, several new methods have elucidated the processes of zonal flow generation from turbulence. Further investigation of the relationship between zonal flows and confinement is strongly encouraged as cross-device activity including low temperature, toroidal and linear devices.

DOI： 10.1088/0029-5515/47/10/S19

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

A new kind of plasma diagnostics, geodesic acoustic mode (GAM) spectroscopy, is proposed. Radial eigenmodes of GAMs with discrete eigenfrequencies in toroidal plasmas are analysed. The eigenmode has a wavelength of the order of p(T)(2/3) L-T(1/3) (p(i) the ion gyroradius, L-T the temperature PT T I gradient scale length) and has the highest peak near the radius where the eigenfrequency coincides with the local GAM,frequency. Therefore, observing the radial position of the highest peak of the eigenmode provides a method to measure the local ion sound velocity. A novel method to measure the ion composition is given, combined with densities and temperatures.

DOI： 10.1088/0741-3335/49/8/L01

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

The importance of reducing the neutral density to reach strong drift wave turbulence is clarified from the results of the extended magnetohydrodynamics and Monte Carlo simulations in a linear magnetized plasma. An upper bound of the neutral density relating to the ion-neutral collision frequency for the excitation of drift wave instability is shown, and the necessary flow velocity to excite this instability is also estimated from the neutral distributions. Measurements of the Mach number and the electron density distributions using Mach probe in the large mirror device (LMD) of Kyushu University [S. Shinohara , Plasma Phys. Control. Fusion 37, 1015 (1995)] are reported as well. The obtained results show a controllability of the neutral density and provide the basis for neutral density reduction and a possibility to excite strong drift wave turbulence in the LMD. (c) 2007 American Institute of Physics.

DOI： 10.1063/1.2743030

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

Competition behavior between the drift instability and the flute instability has been investigated experimentally in a bounded linear electron cyclotron resonance (ECR) plasma. Time evolutions of the fluctuation in ion saturation currents clearly show the successive appearance of the two instabilities. The drift instability is excited first, then the flute instability appears along with the suppression of the drift instability, and finally, both fluctuations are stabilized. The cyclic competition process in these instabilities continues periodically, and is closely related to the radial density profile.

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

This paper reports the experimental and theoretical investigation on the coexistence of the collisional drift and flute wave instabilities in the bounded linear electron cyclotron resonance (ECR) plasma. The drift wave instability is excited by the steep density gradient and imposed axial boundary conditions in the device. The flute instability is excited by the bad curvature of the magnetic field. Emphasis is made on the effect of the ion-neutral (i-n) particle collisions. It is observed that the drift wave instability is excited when the i-n particle collision frequency nu(in) is low, but it is stabilized when nu(in) is high. Furthermore, the drift and flute modes are coexistent for the intermediate values of nu(in). The Hasegawa-Wakatani model which describes the dispersion relation of the collisional drift-interchange mode is used for understanding the experimental observations. It is found that the experimentally observed drift frequency is consistent with the numerical calculation. The present result has shown that the i-n particle collisions play an important role in the stabilization of the collisional drift wave instability in the ECR plasma.

DOI： 10.1143/JPSJ.76.054501

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

A zonal magnetic field is found in a toroidal plasma. The magnetic field has a symmetric bandlike structure, which is uniform in the toroidal and poloidal directions and varies radially with a finite wavelength of mesoscale, which is analogous to zonal flows. A time-dependent bicoherence analysis reveals that the magnetic field should be generated by the background plasma turbulence. The discovery is classified as a new kind of phenomenon of structured magnetic field generation, giving insight into phenomena such as dipole field generation in rotational planets.

DOI： 10.1103/PhysRevLett.98.165001

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

An extended application of Gabour's wavelet to bicoherence analysis succeeds in resolving the instantaneous structure of three wave couplings between disparate scale electric field fluctuations in the high temperature core in a toroidal plasma device named the compact helical system. The obtained results quantify an intermittent linkage between turbulence and zonal flows - a highlighted issue in the present plasma research. This is the first demonstration that the intermittent nature of the three wave coupling should underlie the turbulence power modulation due to zonal flows.

DOI： 10.1088/0741-3335/49/3/002

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

Electric field fluctuations are directly measured using twin heavy-ion-beam probes in Compact Helical System. The spectrum of the electric field fluctuation reveals the existence of a zonal flow at a frequency of less than 1 kHz, coherent modes with a long correlation length (conjectured as geodesic acoustic modes), and background turbulence. Analyses using a wavelet elucidate nonlinear interactions between these fluctuation components in disparate scales. The highlighted findings are that the turbulent fluctuations should be modulated in response to the direction of zonal flow, and should be suppressed, on average, by the zonal flow. This is the first observation that demonstrates the causal linkage between zonal flow and turbulence in toroidal plasmas at a high temperature.

DOI： 10.1143/JPSJ.76.033501

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

In JFT-2M, the applicability of reduced activation ferritic steel to a fusion demonstration reactor has been demonstrated mainly concerning the ferromagnetic effect on plasma production, control, stability and confinement. The compatibility of the close ferritic wall with high-normalized beta plasmas has been investigated mainly because the demonstration reactor will utilize the wall stabilization effect. Due to improvements in the operation scenario and the modification of the guard limiter configuration, the region showing good compatibility was extended to the closer wall position (the wall position was normalized to the plasma minor radius; r(wall)/a similar to 1.25) and the higher normalized beta (beta(N) similar to 3.5), compared with the previous works with the relatively far wall position and stronger magnetic field (Tsuzuki K. et al 2003 Nucl. Fusion. 43 1288). A reduction in the growth rate of the instability was also observed in the close wall case, which presumably corresponds to the wall stabilization effect, similar to a resistive wall without ferromagnetism.

DOI： 10.1088/0029-5515/46/11/010

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

The eigenmode of a geodesic acoustic mode in the presence of a temperature gradient is discussed. Eigenmodes are obtained and the characteristic wavelength scales as ρi2/3LT1/3 (ρi: ion gyroradius, LT: temperature gradient scale length). The direction of propagation is discussed.

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

Data for detecting magnetic flux surfaces at the edge plasma in tokamaks are presented here. When magnetic flux surfaces are moved by a plasma jog, it is observed that a current increment appears in the two sets of double probes whose front surfaces are shifted by a small distance and that a difference between these two currents through the differential amplifier giving a signal of the arrival of the separatrix also appears; we refer to consider the separatrix point as a typical position of the magnetic flux surfaces. It is confirmed that magnetic:flux surfaces are located a few cm outward from that calculated by the eqfit-code (EQFIT) and that the absolute value of its deviation is identified. This identification is supported by the result of the observation of the correlation of the floating potential fluctuation between two different points using one probe pin of the same differential as the same double probes. The ion temperature and the space potential are observed by the another use of the differential double probe are modified following the estimated real magnetic surfaces positions.

DOI： 10.1143/JJAP.45.L630

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

Zonal flows, which means azimuthally symmetric band-like shear flows, are ubiquitous phenomena in nature and the laboratory. It is now widely recognized that zonal flows are a key constituent in virtually all cases and regimes of drift wave turbulence, indeed, so much so that this classic problem is now frequently referred to as "drift wave-zonal flow turbulence." In this review, new viewpoints and unifying concepts are presented, which facilitate understanding of zonal flow physics, via theory, computation and their confrontation with the results of laboratory experiment. Special emphasis is placed on identifying avenues for further progress. (c) 2006 American Institute of Physics.

DOI： 10.1063/1.2178779

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

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

Turbulence and zonal flow at, a transport barrier are studied with twin heavy ion beam probes in a toroidal helical plasma. A wavelet analysis is used to extract turbulence properties, e.g. spectra of both density and potential fluctuations, the coherence and the phase between them and the dispersion relation. Particle transport estimated from the fundamental characteristics is found to clearly rise with their intermittent activities after the barrier is broken down. Time-dependent analysis reveals that the intermittency of turbulence is correlated with the evolution of the stationary zonal flow.

DOI： 10.1088/0741-3335/48/4/S15

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

Experimental efforts on JFT-2M have been devoted to understanding the scrape-off-layer (SOL)/divertor plasmas and to investigating power and particle control by boundary plasma modification. Starting in 1985, an open divertor configuration was adopted for the first decade of the JFT-2M experiments. The characteristics of SOLI divertor plasmas such as in/out asymmetry for divertor plasmas, heat and particle diffusivities, and SOL current during an edge-localized-mode event were identified. The power and particle flux was successfully handled by active control methods such as local pumping, boundary plasma ergodization, divertor biasing, electron cyclotron wave edge heating, and fueling optimization. In 1995, to improve the power and particle control capability of the divertor, the JFT-2M divertor was modified to have a closed configuration, which demonstrated the baffling effects with its narrower divertor throat. A dense and cold divertor state (n(e)(div) =4 x 10(19) m(-3) and, T-e(div) = 4 eV), compatible with improved confinement modes (e.g., H-mode), was realized with strong gas puffing in a closed configuration. Edge plasma fluctuations related to the H-mode physics were identified by an electrostatic probe and magnetic measurements. These are reviewed in this paper.

DOI： 0.0

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

The diagnostic system of JFT-2M has consisted of about 30 individual diagnostic instruments, which were used to study plasma production, control, equilibrium, stability, confinement, plasma heating by neutral beam injection and/or by radio-frequency (rf) (lower hybrid, ion cyclotron resonance frequency, electron cyclotron heating), and current drive by rf. In these instruments, the motional Stark effect polarimeter, charge-exchange recombination spectroscopy, heavy ion beam probe, time-of-flight neutral particle analyzer, etc., have helped in further understanding the improved mechanism of confinement such as H-mode and high-recycling-steady H-mode, and operational regimes of these modes. An infrared television camera system and a lost ion probe have played a very important role in investigating the heat load on the walls due to ripple lost particles and escaping ions from the core plasma region, respectively.

DOI： 0.0

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

The mechanism of E x B flow formation, the effect of the E X B flow on parallel flow, the reduction of fluctuations by the shearing effect of the E x B flow shear, and the relation between the geodesic acoustic mode (GAM) and density fluctuations are discussed based on the experiments using various Er measurements in the JFT-2M tokamak. The experiments in plasmas with H-mode and counter-neutral beam injection (NBI) mode show that the feedback loop of the E x B flow shear, the fluctuation suppression, and an increase of ion diamagnetic flow are key to the formation of the transport barrier in toroidal plasmas. Two important effects of the radial electric field are presented: One is fluctuation suppression by the E x B flow shear, and the other is a drive of the parallel flow by radial electric field, which explains the driving mechanism of a spontaneous toroidal flow. The relation between the GAM and the density fluctuations is also discussed. The GAM is observed to be excited by the nonlinear coupling of density fluctuations, while the GAM itself affects the amplitude of the density fluctuations.

DOI： 0.0

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

The bicoherence of fluctuations in a system of drift waves and zonal flows is discussed. In strong drift-wave turbulence, where broadband fluctuations are excited, the bicoherence is examined. A Langevin equation formalism of turbulent interactions allows us to relate the bicoherence coefficient to the projection of nonlinear force onto the test mode. The dependence of the summed bicoherence on the amplitude of zonal flows is clarified. The importance of observing biphase is also stressed. The results provide a basis for measurement of nonlinear interaction in a system of drift waves and zonal flow.

DOI： 10.1063/1.2062627

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

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

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

Event date： 2022.11

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：富山国際会議場   Country：Japan  

Event date： 2022.9

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：東京工業大学   Country：Japan  

Event date： 2022.9

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：オンライン及び京都大学宇治キャンパス   Country：Japan  

Event date： 2022.3

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Venue：オンライン開催   Country：Japan  

Event date： 2020.10

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Venue：Online and Ceratopia Toki, Toki, Gifu   Country：Japan  

Event date： 2020.9

Language：English   Presentation type：Oral presentation (general)  

Venue：Online   Country：Japan  

Event date： 2020.9

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：オンライン開催   Country：Japan  

Event date： 2020.3

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Venue：名古屋大学（現地開催中止）   Country：Japan  

Event date： 2020.2

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Venue：核融合科学研究所   Country：Japan  

Event date： 2019.11 - 2019.12

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：中部大学春日井キャンパス   Country：Japan  

Event date： 2019.9

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：岐阜大学   Country：Japan  

Event date： 2019.9

Language：English   Presentation type：Oral presentation (general)  

Venue：Oubaku Plaza, Uji Campus, Kyoto Univ., Uji, Kyoto   Country：Japan  

Event date： 2018.9

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Venue：Oubaku Plaza, Uji Campus, Kyoto Univ., Uji, Kyoto   Country：Japan  

Event date： 2018.1

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Venue：核融合科学研究所、岐阜県土岐市   Country：Japan  

Event date： 2017.9

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Venue：St. Petersburg   Country：Russian Federation  

Other Link： http://onlinereg.ru/hmws2017

Event date： 2017.6

Language：English   Presentation type：Oral presentation (general)  

Venue：Nagoya University, Nagoya   Country：Japan  

Other Link： http://www.p.phys.nagoya-u.ac.jp/APTWG2017/index.html

Event date： 2016.3

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：東北学院大学泉キャンパス   Country：Japan  

Event date： 2015.3

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Venue：早稲田大学早稲田キャンパス   Country：Japan  

Event date： 2014.11

Presentation type：Oral presentation (general)  

Venue：朱鷺メッセ、新潟   Country：Japan  

Event date： 2014.11

Presentation type：Oral presentation (general)  

Venue：セラトピア土岐、土岐市、岐阜   Country：Japan  

Event date： 2014.6

Presentation type：Oral presentation (general)  

Venue：Kyushu University   Country：Japan  

Event date： 2014.3

Presentation type：Oral presentation (general)  

Venue：東海大学湘南キャンパス   Country：Japan  

Event date： 2013.12

Presentation type：Oral presentation (general)  

Venue：東京工業大学大岡山キャンパス   Country：Japan  

Event date： 2012.12

Presentation type：Oral presentation (general)  

Venue：RIAM, Kyushu University   Country：Japan  

Event date： 2012.11

Presentation type：Oral presentation (general)  

Venue：Ioffe Institute, St. Petersburg   Country：Russian Federation  

Event date： 2012.10 - 2012.11

Presentation type：Oral presentation (general)  

Venue：Providence, Rhode Island   Country：United States  

Event date： 2012.9

Presentation type：Oral presentation (general)  

Venue：Yokohama National University   Country：Japan  

Event date： 2012.3

Venue：Kansei Gakuin University   Country：Japan  

Event date： 2012.3

Venue：Kyushu University   Country：Japan  

Event date： 2012.1

Presentation type：Oral presentation (general)  

Venue：NIFS、土岐   Country：Japan  

Event date： 2011.11

Presentation type：Symposium, workshop panel (public)  

Venue：Kanazawa Ongaku-do   Country：Japan  

Event date： 2011.10

Presentation type：Oral presentation (general)  

Venue：Kyushu University   Country：Japan  

Type：Peer review 

Number of peer-reviewed articles in foreign language journals：2

Type：Peer review 

Number of peer-reviewed articles in foreign language journals：4

Proceedings of International Conference Number of peer-reviewed papers：2

Type：Competition, symposium, etc. 

Type：Competition, symposium, etc. 

Type：Competition, symposium, etc.