Language：English   Publishing type：Research paper (scientific journal)   Publisher：Reviews of Modern Plasma Physics  

This memorial note for Professor Sanae-I Itoh presents her specific achievements in physics research alongside her wider record of accomplishment in the field of magnetically confined plasmas. The topics include bifurcation phenomena (e.g., H-mode and improved confinement modes), turbulence-generated structures (e.g., zonal flows and streamers), and fundamental concepts and processes in plasma turbulence (e.g. nonlinear couplings and energy transfer. The note focuses initially on results obtained through her integration of theory, simulation, and experiment, particularly those arising from a low temperature plasma facility at Kyushu University. We then describe contemporary challenges in plasma turbulence which Sanae addressed with great interest, and consider some of the perspectives that were opened by her achievements.

DOI： 10.1007/s41614-023-00123-6

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Scopus

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

DOI： 10.1585/pfr.16.1201075

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

DOI： 10.2183/pjab.97.006

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

DOI： 10.1038/s41598-021-83191-3

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

DOI： 10.1585/PFR.15.1201018

Language：Japanese  

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

DOI： 10.1063/1.5108717

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

Plasma turbulence tomography is now working to show a two dimensional structure and dynamics of plasma turbulence in a cylindrical plasma device, PANTA. A new method, the Stokes parameter analysis on moment vectors, is being developed to treat the image of plasma turbulence and extract hidden characteristics of polarization or rotation of spatial patterns created in turbulent plasmas. The article presents the explanation of the method and the application results obtained from a PANTA plasma.

DOI： 10.1063/1.5048970

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

A possibility of electron density measurements with heavy ion beam probes (HIBPs) has been demonstrated, along with their capability to measure the potential and magnetic field. A method has been proposed to reconstruct the electron density profile [A. Fujisawa et al., Rev. Sci. Instrum. 74, 3335 (2003)]. In the method, the profile of secondary beam currents is converted into a local density profile by taking into account local brightness and so-called path integral effects which mean the effect of beam attenuation along the beam orbit. Here the article presents the HIBP measurement of the electron density profile after the proposed method was first applied on the real experimental data of compact helical system plasmas. In the real application, the hollow density and the peaked profiles are successfully obtained with sufficiently high temporal resolution (a few ms), in accordance with the electron density profile measured with Thomson scattering for electron cyclotron resonance heating and neutral beam injection plasmas.

DOI： 10.1063/1.5039571

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

Plasma turbulence has been intensively studied in the research of magnetic confinement for realizing a nuclear fusion reactor, and the international research has provided various discoveries of and physics insights into turbulent plasmas, such as turbulence-driven transport, effects of electric field on confinement, generation of flows and fields, and so on, although the research has been performed with a strong weight on achieving fusion-relevant performance plasma. However, at present a project to construct a physics-oriented device, named PLAsma Turbulence Observatory (PLATO), in Kyushu University, aiming at essential physics understanding of turbulent plasmas. The article reviews the major physics achievements up to data, and the steps to reach the concept of a new toroidal device with the major purpose for physics understanding of plasma turbulence.

DOI： 10.1063/1.5048721

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

A system to detect ultraviolet emissions from plasma is developed for multi-point measurement, the target of which is the imaging of turbulence with a high temporal resolution. A fluorescent glass, which converts ultraviolet emissions to visible light with a wavelength of approximately 540 nm, is utilized in the system. Following the conversion, the fluorescent light is transferred with fibre optics, and is converted to electric signals in a location that is sufficiently distant from an electrically noisy environment around the plasma device. This paper describes a prototype system of this diagnostic method and discusses the spectral analysis obtained using a low-aspect-ratio reversed field pinch RELAX

DOI： 10.1063/1.4900660

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

A new paradigm for plasma turbulence has been made; plasma turbulence is regarded as nonlinear　system of drift waves and their generated structures, zonal flows, streamers, and large scale modes.　New paradigm provides ways to clarify unsolved problems in magnetically confined plasma, for　example, confinement improvement, nonlocal transport, isotope effect, and so on. Identification of　these structures should contribute to establishing the first principle laws of transport. Full scale measurement covering the whole plasma is essential for further understanding of plasma turbulence in　laboratory experiments. Special toroidal device should be made for further physical understanding　of turbulence.

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

The modern view of plasma turbulence has been established due to the discovery of zonal flows and other structures which drift-waves generate, and contributes to exploring new manners of understanding turbulence-driven transport and structural formation in magnetized plasmas and astronomic objects. This paper presents the recent development of laboratory experiments, in particular, for drift-wave turbulence which have advanced understanding and have made the paradigm shift. The topics include the discovery of mesoscale structures, such as zonal flows and streamers, the recent development of analyzing techniques to quantify the couplings between different scale structures and methods to elucidate energy transfer direction, and turbulence transport and barrier formation. Finally, future experiments are suggested for establishing the first-principle laws of turbulence transport and structural formation.

DOI： 10.1088/0741-3335/53/12/124015

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

This Letter presents the discovery of macroscale electron temperature fluctuations with a long radial correlation length comparable to the plasma minor radius in a toroidal plasma. Their spatiotemporal structure is characterized by a low frequency of similar to 1-3 kHz, ballistic radial propagation, a poloidal or toroidal mode number of m/n = 1/1 (or 2/1), and an amplitude of similar to 2% at maximum. Nonlinear coupling between the long-range fluctuations and the microscopic fluctuations is identified. A change of the amplitude of the long-range fluctuation is transmitted across the plasma radius at the velocity which is of the order of the drift velocity.

DOI： 10.1103/PhysRevLett.107.115001

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

The present status of zonal flow experiments is reviewed with the historical process to attain the concept of zonal flows, which provides a new framework for understanding turbulence and transport in toroidal plasmas. The existence of zonal flows is experimentally confirmed to present a new paradigm of plasma turbulence. The paper presents contemporary experiments on zonal flows as major topics with a brief presentation of the zonal flow theories, the diagnostics and data processing techniques for turbulence and zonal flows and the peripheral issues of zonal flow physics. The accumulated experimental results introduced in this review include identification of zonal flows (both stationary zonal flows and geodesic acoustic modes), nonlinear interactions between zonal flows and turbulence, quantification of turbulent Reynolds stress, flow dynamics, energy transfer dynamics between turbulent wave components and the effects of zonal flows on plasma transport. These results have given rise to a new paradigm, namely, that the plasma turbulence is a system of zonal flows and drift waves, with an emphasis on the interaction between the disparate scale structures, e.g. zonal flows (mesoscale) and turbulence (micro-scale).

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)  

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 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)  

Spectral changes in electric field fluctuation are measured using twin heavy ion beam probes in compact helical system (CHS) before and after an internal transport barrier is broken down. Wavelet analysis reveals intermittent behaviour of the fluctuations and a significant correlation between fluctuation powers of the low (2.5 < f < 10 kHz) and high (30 < f < 250 kHz) ranges. The high frequency (turbulence) fluctuation increases with a decrease in the low frequency fluctuation after the back-transition. The change in the power distribution between these two frequency ranges may contribute to the improved transport on the barrier.

DOI： 10.1088/0741-3335/48/5A/S36

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

This Letter presents experimental confirmation of the presence of zonal flows in magnetically confined toroidal plasma using an advanced diagnostic system-dual heavy ion beam probes. The simultaneous observation of an electric field at two distant toroidal locations (similar to1.5 m apart) in the high temperature (similar to1 keV) plasma provides a fluctuation spectrum of electric field (or flow), a spatiotemporal structure of the zonal flows (characteristic radial length of similar to1.5 cm and lifetime of similar to1.5 ms), their long-range correlation with toroidal symmetry (n=0), and the difference in the zonal flow amplitude with and without a transport barrier. These constitute essential elements of turbulence-zonal flow systems, and illustrate one of the fundamental processes of structure formation in nature.

DOI： 10.1103/PhysRevLett.93.165002

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

Radial electric field is a key physical quantity in determining the structure of toroidal helical plasmas, owing to the nonlinear dependence of transport on the, radial electric field. It has been observed that the potential profiles show various interesting patterns in space and time in the compact helical system (CHS) plasmas. In particular, the phenomenon of electric pulsation, clearly demonstrates the nature of bifurcation of radial electric field in toroidal helical plasmas.

DOI： 10.1088/0741-3335/41/3A/049

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

A thermal transport barrier resulting in a high central electron temperature of similar to 2 keV is established in the core of electron-cyclotron-resonance heated plasmas of the Compact Helical System Heliotron/Torsatron. The formation of the barrier is correlated with the reduction in the density fluctuations and with a structural change of the radial electric field profile at the barrier location. The results suggest that the decrease in fluctuation should contribute to a reduction of anomalous transport and a drastic increase in electron temperature at the barrier. [S0031-9007(99)08825-0].

DOI： 10.1103/PhysRevLett.82.2669

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

A pulsating behavior of the electrostatic potential has been discovered in a low-density, high-electron-temperature plasma of the CHS heliotron/torsatron. The potential profiles were observed to swing repeatedly between two distinctive states in a constant external magnetic field under a continuous supply of particles and energy. The change of the potential profile, which occurs on a much faster time scale (microseconds) than the diffusive time scale (milliseconds), is accompanied by changes in density and temperature. This discovery clearly demonstrates that even in a low beta regime a toroidal helical plasma can produce a self-sustaining dynamic steady state, like the sawtooth oscillation in tokamaks.

DOI： 10.1103/PhysRevLett.81.2256

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

During combined electron cyclotron and neutral beam heating, an abrupt drop and rise in potential by about one half of the central electron temperature (similar to 400 V) was observed in the core of the compact helical system heliotron/torsatron plasma. Drastic changes of radial electric field between positive and negative states occur over periods as short as 60 mu s, which is much shorter than the energy confinement time scale of about a few milliseconds. A nonlinear relation between the radial electric field and radial current is obtained. This is the first experimental observation of a microsecond time scale spontaneous transition based on an electric field bifurcation in toroidal helical plasmas.

DOI： 10.1103/PhysRevLett.79.1054

Publisher：Physical Review E  

The long-term development of the transitional regime of drift-wave turbulence is studied in a magnetized plasma column by means of the conditional-average technique. In the transitional regime, small changes in the magnetic-field strength as control parameter lead to large changes in the correlation times, indicating the existence of a critical point of an underlying nonequilibrium continuous phase transition. This and the spatiotemporal dynamics shows similarities to puff splitting, slug-gap splitting, and puff jamming.

DOI： 10.1103/PhysRevE.111.045203

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Scopus

Publisher：Physics of Plasmas  

Reliable estimation of equilibrium flow shear from reciprocating probe measurements is challenging since the quantity of interest corresponds to the second derivative of the observable plasma potential. In addition, a time series of the plasma potential obtained by plunging a probe is affected by both the probe head position and plasma fluctuations, complicating the estimation of equilibrium components and their errors. We tackle this problem by employing Gaussian process regression that is able to infer even the derivatives of a spatial or temporal profile in the form of a probability distribution function. The proposed inference framework is validated by using synthetic data generated by gyrofluid simulations. While the inference result based on a single plunge is unstable in certain spatial locations, we have obtained reasonable agreement between the inference result and the true flow shear profile by combining data sets taken from several plunges.

DOI： 10.1063/5.0254473

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Scopus

Publisher：Plasma Physics and Controlled Fusion  

Reliable equilibrium reconstruction is indispensable for understanding and controlling hot magnetized plasmas to achieve fusion reactors. In axisymmetric systems, current and pressure profiles that satisfy the force balance conditions are given by the Grad-Shafranov (GS) equation. While many novel approaches have been developed to swiftly and robustly find an optimum solution of the GS equation, approaches based on a single solution search may not be adaptable if diagnostics fail to provide sufficient constraints. Here, we investigate the solution space of the GS equation when only basic edge magnetic measurements are available. By combining Gaussian process regression and Markov chain Monte Carlo sampling within the Bayesian framework, we treat each current element as an independent variable and evaluate the probability distribution that describes all possible solutions. We have applied this inference frame to the geometry of the PLATO tokamak and shown that the flux surface locations can be determined relatively well only from 16 pick-up coils, 4 flux loops and a diamagnetic loop. On the other hand, the toroidal current density is inferred with limited success, and the inferences of the safety factor and pressure profiles are difficult. The characterization of possible choices of equilibria realized by this inference framework will help optimize diagnostic setups for equilibrium reconstruction.

DOI： 10.1088/1361-6587/ad9521

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

<p>We report on the nonlinear coupling properties of high-wavenumber fluctuation, which exhibits discrete frequency spectra with a frequency span comparable to the ion cyclotron frequency observed in PANTA. Bicoherence analysis is used to investigate this nonlinear coupling. The results indicate a finite coupling between individual discrete peaks but no coupling between the ion cyclotron frequency components of these discrete peaks. This suggests that the observed turbulence with a discrete spectrum is destabilized linearly rather than through nonlinear coupling.</p>

DOI： 10.1585/pfr.1201031

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CiNii Research

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Review of Scientific Instruments  

A preparatory study is underway to investigate the feasibility study of high-energetic, non-thermal electron distribution function measurements using a vertical-viewing electron cyclotron emission (ECE) diagnostic on JT-60SA. The system is designed to detect broad ECE spectra (70-260 GHz) due to the second, third, and fourth harmonics using a focusing optics system with four quasi-optical mirrors in the upper port of JT-60SA. A Gaussian beam optics design is performed in vacuum, and ray tracing calculations are performed in plasma using the TRAVIS code to investigate density characteristics. A ceramic viewing dump is also designed to reduce the effects of multiple reflections from the opposing vacuum vessel surface.

DOI： 10.1063/5.0218359

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PubMed

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Physics of Plasmas  

We investigated the mass dependency of the high-wavenumber turbulence, which occurs at scales smaller than the ion effective Larmor radius, in a partially magnetized plasma column. In this system, two different types of fluctuations were observed: one exhibiting a coherent discrete spectrum, while the other displaying a broadband continuous spectrum. The phase velocities of both types showed a similar mass dependency, approximately matching the ion sound velocity or electron diamagnetic drift velocity. Additionally, we found that the discrete spectrum has a peak interval frequency comparable to the ion cyclotron frequency, which is consistent with ion cyclotron ranges of fluctuations, including ion Bernstein waves.

DOI： 10.1063/5.0187997

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Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Plasma Physics and Controlled Fusion  

Gaussian process regression (GPR) has been utilized to provide fast and robust estimates of plasma parameter profiles and their derivatives. We present an alternative GPR technique that performs profile regression analyses based on arbitrary linear observations. This method takes into account finite spatial resolution of diagnostics by introducing a sensitivity matrix. In addition, the profiles of interest and their derivatives can be estimated in the form of a multivariate normal distribution even when only integrated quantities are observable. We show that this GPR provides meaningful measurements of the electron density profile and its derivative in a toroidal plasma by utilizing only ten line-integrated data points given that the locations of magnetic flux surfaces are known.

DOI： 10.1088/1361-6587/ad074a

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Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Plasma Physics and Controlled Fusion  

The mutual interaction of drift wave-type modes and zonal flows causes the formation of higher-order nonlinear structures. This study focuses on the spatio-temporal behavior of these higher-order structures in a linear magnetized plasma. The structures include a solitary vortex, a long-lived circumnavigating motion localized both radially and azimuthally, and a short-lived packet of finer-scale fluctuations excited at the position of the solitary vortex. Observing the time evolution of the two-dimensional cross-sectional structures revealed that the packet of finer-scale fluctuations is trapped in the solitary vortex. The trapping times found are consistent with the theoretical evaluation.

DOI： 10.1088/1361-6587/acfbb3

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Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Plasma Physics and Controlled Fusion  

Spectral transition between continuous and discrete spectra has been observed in high-wavenumber turbulence excitation experiments. The transition appeared in neutral gas scanning, which shows that continuous spectra dominate at low gas pressures and discrete spectra dominate at high gas pressures. Both spectra have well smaller spatial scale than the ion effective Larmor radius. The discrete spectra have 6-11 peaks and maximum peak values at 4th or 5th peak. Correlation length analysis reveals that the continuous component is turbulent and the discrete component is coherent. The bicoherence analysis shows that the turbulent spectra have finite nonlinear coupling, whereas the discrete spectra have no corresponding coupling.

DOI： 10.1088/1361-6587/acfbf7

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Scopus

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

<p>Recently, a new method that applies vector tomography to laser-induced fluorescence has been introduced, enabling the measurement of quasi-periodic two-dimensional velocity field in plasma [H. Arakawa <i>et al.</i>, Plasma Fusion Res. <b>18</b>, 1401032 (2023)]. In this study, we experimentally demonstrated this method in a linear magnetized plasma and presented the initial measurement results. The observed two-dimensional velocity field allowed the evaluation of Reynolds force and its energy transfer.</p>

DOI： 10.1585/pfr.18.1201086

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CiNii Research

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

<p>Multi-field singular value decompositions (SVDs) is applied to turbulence obtained in a cylindrical magnetized plasma, PANTA. This method enables us to obtain the spatial mode structures with common temporal evolution of different physical quantities, such as the fluctuations of density and flows. Turbulence driven particle transport is evaluated by the method. It is shown that only the coupling of the same mode drives the transport, which stems from the orthogonality of the SVD. Thanks to this characteristics, the number of degrees of freedom which plays roles for the transport dynamics could be significantly reduced.</p>

DOI： 10.1585/PFR.18.1202036

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CiNii Research

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

<p>A new method of vector tomography by laser-induced fluorescence (LIF) spectroscopy is proposed for the evaluation of the velocity field of quasi-coherent structure in linear magnetized plasma. Here, the two-dimensional velocity field was reconstructed with measuring synchronization of azimuthal reference probes and line-integrated LIF. The laser installation condition is designed to move in a radial direction perpendicular to the laser path. We applied the proposed method to a quasi-coherent flow obtained from a direct numerical turbulence simulation and demonstrated that the Reynolds stress and its force can be evaluated if its flow structure is maintained.</p>

DOI： 10.1585/PFR.18.1401032

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CiNii Research

Language：English   Publishing type：Research paper (scientific journal)   Publisher：The Physical Society of Japan  

A procedure to calculate normalized electron temperature and electron density fluctuations using dual wavelength emission intensity tomography in a linear magnetized plasma is proposed. Expression of emission intensity is modeled as ϵk ∞ Tαke n2e where ϵk is emission intensity, Te is electron temperature, ne is electron density, k = 1 for ArI, and k = 2 for ArII. Using the model, normalized Te and ne fluctuations can be expressed as linear function of both the normalized emission intensity fluctuations. To calculate αk, we perform least squares method between real ϵk measured with a tomography system and artificial ϵk reconstructed using Te and ne measured with a Langmuir probe located downstream of the tomography system. The results show that α1 = 2.2 and α2 = 2.6, and normalized Te and ne fluctuations are preliminarily evaluated from emission intensity tomography data.

DOI： 10.7566/JPSJ.92.033501

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CiNii Research

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

<p>A solitary vortex organization process in drift-wave type fluctuations interacting with the zonal flow was identified experimentally in a linear magnetized plasma. An azimuthal probe array was used to evaluate temporal changes in the amplitude and phase in the density fluctuations. Excitation/damping of the solitary vortex is synchronized with zonal perturbation, and the waveform of drift-wave type fluctuation and its harmonics also changes synchronously. The solitary vortex is formed primarily through the phase modulation of the fundamental drift-wave type fluctuation and its harmonics.</p>

DOI： 10.1585/pfr.17.1301106

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CiNii Research

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Plasma Science and Technology  

Enhancements of edge zonal flows, radial electric fields, and turbulence are observed in electron cyclotron resonance heating-heated plasmas (Zhao et al 2013 Nucl. Fusion 53 083011). In this paper, the effects of sawtooth heat pulses on flows and turbulence are presented. These experiments are performed using multiple Langmuir probe arrays in the edge plasmas of the HL-2A tokamak. The edge zonal flows, radial electric fields, and turbulence are all enhanced by sawteeth. Propagation of the zonal flow and turbulence intensities is also observed. The delay time of the maximal intensity of the electric fields, zonal flows, and turbulence with respect to the sawtooth crashes is estimated as ∼1 ms and comparable to that of the sawtooth-triggered intermediate phases. Not only the zonal flows but also the radial electric fields lag behind the turbulence. Furthermore, the intensities of both the zonal flows and electric fields nearly linearly increase/decrease with the increase/decrease of the turbulence intensity. A double-source predator-prey model analysis suggests that a relatively strong turbulence source may contribute to the dominant zonal flow formation during sawtooth cycles.

DOI： 10.1088/2058-6272/ac7c60

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Scopus

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

DOI： 10.1063/5.0065887

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

DOI： 10.1585/pfr.16.1403093

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

DOI： 10.1585/PFR.16.1202081

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

DOI： 10.1585/pfr.16.1201082

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

DOI： 10.1063/5.0041814

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

DOI： 10.1088/1361-6587/abd980

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

DOI： 10.1063/5.0037352

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

DOI： 10.7566/JPSJ.89.093501

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

DOI： 10.1585/pfr.15.1403052

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

DOI： 10.1088/1361-6587/ab8132

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

DOI： 10.1063/1.5144480

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

DOI： 10.1109/TASC.2020.2981075

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

DOI： 10.1088/1361-6587/ab434f

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

DOI： 10.1063/1.5098954

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

DOI： 10.1002/tee.22962

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

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

We experimentally investigate the roles of drift wave type fluctuations interacting with zonal flow. The drift wave type fluctuations examined in this paper are characterized by nonlinear solitary wave, splash, and solitary eddy [Arakawa et al., Sci. Rep. 6, 33371 (2016)]. Compared to the nonlinear solitary wave, splash has a short lifetime, while solitary eddy has a long lifetime. Excitation/damping of the splash and the solitary eddy are synchronized with zonal perturbation. The roles of the splash and the solitary eddy in transport processes are also discussed. Solitary eddy contributes to momentum transport and accelerates zonal flow. The particle flux driven by the solitary eddy and the splash is in the inward and outward directions, respectively, with similar magnitudes.

DOI： 10.1063/1.5094577

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

DOI： 10.1585/PFR.14.1402090

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

The streamer-driven particle flux is evaluated experimentally in a linear cylindrical plasma. The streamers, a radially elongated and azimuthally localized structure created in turbulent plasma, really induce ballistic transport, and directly connect the plasma core to the edge within one order faster time scale than that of turbulence. The occurrence of large transport deviates from a Gaussian distribution and is likely to obey a power law. The fundamental plasma experiment clearly shows the significant and essential contributions of streamers to cross-field transport and structural formation of turbulent plasmas.

DOI： 10.1063/1.5093218

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

Selection rule of the radial propagation direction of geodesic acoustic modes (GAMs) is investigated. Here, we study the influence of nonlinear coupling with drift wave turbulence on the propagation direction of GAMs. Based on wave-kinetic equation for the turbulence, the phase-space dynamics is numerically solved and the nonlinear saturated states are obtained, where the phase-space consists of the real space and the wavenumber space. A wave pattern of the GAM in a nonlinear saturated state varies to form a standing wave, outward and inward propagating waves, depending on the peak radial wavenumber of the turbulence. The impact of nonlinear coupling with turbulence is discussed by deriving the GAM dispersion relation that includes the effect of the turbulence.

DOI： 10.1088/1741-4326/aad251

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

A set of experimental works based on nonlinear data analysis in the JFT-2M is planned in order to verify nonlinearity of edge turbulence in PLAsma Turbulence Observatory (PLATO) tokamak. In this paper, we introduce the preceding experimental works of the JFT-2M tokamak, and show how the revisited works are applied to progress of turbulence research in the new tokamak.

DOI： 10.1063/1.5048716

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

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

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

DOI： 10.1088/1361-6587/aacb69

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

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： 10.1063/1.5027124

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

DOI： DOI: 10.7566/JPSJ.87.034501

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

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DOI： DOI: 10.1063/1.5008541

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

DOI： DOI: 10.1103

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

DOI： DOI: 10.7566

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

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

DOI： 10.1088/1741-4326/aa5d03

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

DOI： DOI: 10.1038/s41598-017-14821-y

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DOI： DOI: 10.1063/1.5000343

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

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

DOI： DOI: 10.1088/1741-4326/aa7c20

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

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DOI： DOI: 10.1063/1.5000744

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

DOI： DOI: 10.1063/1.5000739

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

DOI： DOI: 10.1016/j.nme.2017.03.027

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

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DOI： 10.1088/1402-4896/aa6689

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

DOI： DOI: 10.1088/1009-0630/18/11/03

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

DOI： DOI: 10.1016/j.fusengdes.2016.04.016

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

DOI： DOI: 10.1063/1.4965915

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

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

DOI： DOI: 10.1038/srep33371

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

DOI： DOI: 10.1088/0741-3335/58/11/115004

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

DOI： DOI: 10.7566/JPSJ.85.093501

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

DOI： DOI: 10.1109/TPS.2016.2570815

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

DOI： DOI: 10.1038/srep30720

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

DOI： DOI: 10.1063/1.4960117

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

DOI： DOI: 10.1016/j.vacuum.2016.04.025

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

DOI： DOI: 10.1109/SOFE.2015.7482308

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

DOI： DOI: 10.1088/1748-0221/11/04/C04010

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DOI： DOI: 10.1038/srep22189

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DOI： DOI: 10.1585/pfr.11.1201091

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

DOI： DOI: 10.1016/j.fusengdes.2015.11.007

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

DOI： DOI: 10.1016/j.fusengdes.2015.12.022

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

DOI： DOI: 10.1585/pfr.11.1402002

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

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

DOI： 10.7566/JPSJ.85.063501

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

DOI： 10.7566/JPSJ.85.014501

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

DOI： 10.1088/1748-0221/11/01/C01014

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

A system for plasma turbulence tomography has been developed in a linear cylindrical plasma
as a prototype with aiming at future application on toroidal plasma of higher temperature. This　paper describes the diagnostic system in both aspects of the soft- and hardware, and reports　the first results of tomographic reconstruction that can successfully produce local emission　and its fluctuations. In the reconstruction process, two dimensional view of plasma is obtained　for approximately 0.6 ms in every sampling time of 1 μs using parallel processing of 120 cores　with 10 personal computers. The results include the steady state analysis of local fluctuation　power spectra using fast Fourier transform, analysis of temporal behavior of fluctuation power　spectra with wavelet transform, and analyses of the structural deformation or pattern of local　plasma emission, demonstrating that the success of tomography as a promising diagnostic tool　for plasma turbulence.

DOI： 10.1088/0741-3335/58/2/025005

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

A prototype system for plasma turbulence tomography was tested in linear plasma with aiming at future application to toroidal plasma. The line-integrated plasma emission lines of ArI and ArII were measured with sufficiently high signal-to-noise ratios, and the signals were reconstructed using the maximum likelihood expectation maximization (MLEM) algorithm and parallel computing. Here, the first results for local plasma emissions and fluctuation spectra obtained with the prototype system are reported.

DOI： 10.1585/pfr.10.1201080

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

DOI： 10.1016/j.fusengdes.2015.12.022

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

DOI： 10.1016/j.fusengdes.2015.11.035

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： 10.1063/1.4934537

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： 10.1016/j.jnucmat.2014.12.092

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

DOI： 10.1016/j.jnucmat.2015.01.013

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

DOI： 10.1016/j.jnucmat.2014.11.136

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

DOI： 10.1585

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

Successful production of high βp plasmas (εβp ≥ 1) fully non-inductively (NI) and their long pulse sustainment with the help of modest power (<100 kW) of electron cyclotron waves is demonstrated. High βp plasmas are found for the first time to be naturall

DOI： 10.1088/0029-5515/55/8/083009

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

Heat flux and plasma flow in the scrape-off layer (SOL) are examined for the inboard poloidal field null (IPN) configuration of the spherical tokamak QUEST. In the plasma current (I p) ramp-up phase, high heat flux (>1 MW/m2) and supersonic flow (Mach num

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)  

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.

DOI： 10.1063/1.4914838

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

Non-linear coupling between fluctuations at different radial locations is observed on the Large Helical Device (LHD) plasma. The summed bi-coherence between the low frequency macro-scale fluctuation (frequency is f = 2.75 kHz) at ρ = reff/a99 = 0.63 (wher

DOI： 10.1088/0029-5515/54/11/114014"

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

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

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

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

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

DOI： 10.1088/1742-6596/511/1/012041

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

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)  

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)  

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)  

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

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 (international conference proceedings)  

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

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

DOI： 10.1088/0029-5515/54/2/023010

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

In this article, experimental observations of limit cycle oscillations (LCO) that precede L-to-H transition are discussed. Issues are: (1) the existence of zonal flows, (2) spatio-temporal evolutions of turbulence intensity, and (3) periodic generations/decays of mean radial electric field and density. The role of Reynolds stress to accelerate the LCO flow is also addressed. The propagation of changes of the density gradient and turbulence amplitude into the core is commented. Varieties in experimental reports on these issues are explained, and possible origins of different interpretations are discussed. Problem definitions for the future research for resolution are presented.

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)  

By employing a new method for studying dynamical transport, hysteresis in the flux-gradient relation was recently discovered for modulated heating. In this paper, the new method is compared to the conventional heat-pulse propagation method. We also demonstrate integration of results from the new method with the power balance thermal conductivity.

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)  

We have used the convolution method to obtain higher harmonics of the temperature perturbation in a heating modulation experiment on LHD plasma. In comparison with predictions based on the diffusive model, amplitudes of higher harmonics (such as the seventh) decayed much more slowly as they propagated radially. Changes in the time derivative of temperature at the onset and turning-off of heating power remained much sharper after radial propagation than those predicted by the diffusion model.

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

We report observations of the dynamic response of micro-fluctuations and turbulent flux to a low-frequency heating power modulation in the Large Helical Device. The responses of heat flux and micro-fluctuation intensity differ from that of the change in temperature gradient. This result violates the local transport model, where turbulence is determined by the local temperature gradient. A new relationship between flux, gradient and turbulence is found. In addition to the temperature gradient, the heating rate is proposed as a new, direct controlling parameter of turbulence to explain the fast response of turbulence against periodic modulation of heating power.

DOI： 10.1088/0029-5515/53/11/113006

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

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

DOI： 10.1016/j.fusengdes.2013.03.035

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

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

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)  

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)  

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 (international conference proceedings)  

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 (international conference proceedings)  

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

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

DOI： 10.1016/j.jnucmat.2013.01.106

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

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)  

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)  

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

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

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

DOI： 10.1088/1009-0630/15/3/21

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

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

DOI： 10.1088/0029-5515/52/12/123016

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

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 (international conference proceedings)  

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)  

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

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)  

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 (other academic)  

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

Direct measurements of the electric potential φ, using the Heavy Ion Beam Probing, have been undertaken in the T-10 tokamak, and in the helical devices with various magnetic topology: TJ-II, CHS and LHD. L-mode plasmas were considered. Despite the large differences in machine sizes, heating methods and the topology of the magnetic field, the observed φ shows the striking similarities: (i) Similar magnitudes of Er; (ii) For low densities, ne < 0.5 × 1019 m−3, φ is positive, and an increase in ne is associated with the decrease of positive φ and formation of a negative Er; (iii) For higher densities, ne > (0.5-1)× 1019 m−3 , both φ and Er tends to be negative despite the use of different heating methods: Ohmic and ECR heating in T-10, ECRH and/or NBI in TJ-II, CHS and LHD; (iv) Application of ECRH, causing a rise in Te, results in more positive values for φ and Er. The analysis show that the main features of the φ dependences on the ne and Te agree with neoclassical predictions on the four devices within experimental and simulation precisions.

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 (international conference proceedings)  

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

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

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

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

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

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

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

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

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

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

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

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)  

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)  

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)  

Fluctuation-induced cross-field convective particle flux is studied in the cylindrical magnetized plasma assembly for non-linear turbulence analysis (PANTA). The plasma presents a wave turbulence characterized by a limited number of developing modes. To study the link between mode dynamics and particle transport, frequency-dependent time evolution of transport is obtained by applying complex continuous wavelet decomposition on spatio-temporal data from electrostatic probes. The method is shown to recover transport description consistent with other analysis methods for a weakly turbulent state. For a plasma state featuring a large non-sinusoidal structure, the time evolution of transport on the second harmonic of the main fluctuation shows a slow modulation of azimuthal mode number m = 0 with similarities to a zonal perturbation.

DOI： 10.1088/0741-3335/54/5/055002

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

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)  

Macro-scale temperature fluctuations, which have long radial correlation length (of the order of the plasma radius), have been discovered on Large Helical Device (LHD). In this paper, non-linear coupling between the long-range fluctuations and microscopic fluctuations is investigated using cross bi-coherence analysis. The significant cross bi-coherence between long-range fluctuations and micro-fluctuations is observed in the range of frequency f < 100 kHz, where f is the frequency of microscopic fluctuations. It is found that the squared bi-coherence and bi-phase for this three-wave coupling is weakly dependent on the frequency of microscopic fluctuations. These observations demonstrate that the microscopic fluctuations are coherently interacting with long-range fluctuations.

DOI： 10.1143/JPSJ.81.034501

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

Electron temperature fluctuations with long-distance correlation have been discovered in LHD. This paper reports the extended observations recently made on the spatiotemporal structure of the long-range fluctuations both in quasi-stationary and transient plasmas. The detailed characteristics or spatiotemporal characteristics of long-range temperature fluctuations are revealed successfully using correlation analysis. Particularly, the dynamics of the longrange fluctuations is investigated to find that the amplitude of the fluctuations decreased and their radial correlation lengths shortened during the transient phase induced by pellet injection. Temporal changes of radial correlation structure and amplitude of fluctuations at the onset of change in the plasma state are discussed.

DOI： 10.1088/0029-5515/52/2/023022

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

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

DOI： 10.1016/j.fusengdes.2011.10.001

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

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

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

DOI： 10.1063/1.3662204

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

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

DOI： 10.1109/TPS.2011.2163423

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

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)  

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

DOI： 10.1016/j.fusengdes.2011.04.045

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

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

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

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

A new wavelet analysis method is used to study heat pulse propagation in magnetic confinement plasmas. The wavelet transform is applied to estimate the time evolution of the phase delay in heat pulse propagation with a fine temporal resolution. The procedure of analysis is explained in detail. The method is examined by taking an exemplary heat pulse propagation in the Large Helical Device. Various wavelet parameters are tested and the parameter region where relevant results are obtained is identified. Temporal resolution of the phase variation is evaluated from a convergence study.

DOI： 10.1088/0741-3335/53/9/095012

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

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

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

DOI： 10.1063/1.3640494

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

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

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)  

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

DOI： 10.1016/j.jnucmat.2010.12.225

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

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 (international conference proceedings)  

Language：English  

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 (international conference proceedings)  

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

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

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

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

DOI： 10.1088/1009-0630/13/3/08

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

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)  

We report a detailed correlation technique to identify the long-range temperature fluctuation in the Large Helical Device. Correlation hunting has successfully realized the observation of electron temperature fluctuations, which are characterized by their correlation length comparable to the plasma minor radius, with low frequency of ∼ 1-3 kHz, ballistic radial propagation (at a speed of ∼1 km/s, of the order of diamagnetic drift velocity), spatial mode number of m/n = 1/1 (or 2/1), and amplitude of ∼2% at the maximum. Bicoherence analysis confirmed their nonlinear coupling with local microscopic turbulent fluctuations. This long-range temperature fluctuation is a possible carrier of fast propagation in transport processes observed so far. We also comment on the theoretical interpretation.

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：Japanese   Publishing type：Research paper (other academic)  

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

Magnetized plasma is an non-equilibrium matter rich in nonlinear phenomena; its structural formations are dominated by turbulence. This article provides a brief review of experiments and observations of phenomena occurring in turbulent plasmas, with an emphasis on the methodologies for characterizing turbulence, and visualizing the invisible structure created by turbulence and the internal couplings between the elemental waves that constitute turbulence. Zonal flows, streamers, blobs, and other phenomena are investigated by using analytical methods such as Fourier transformation, wavelet analysis, probabilistic density function analysis, bicoherence, wavelet bicoherence. Finally, the contemporary view of plasma turbulence is presented with discussion of unsolved transport issues in fusion plasmas, such as transport barrier formation and nonlocal transport.

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

Measurements with a toroidally and poloidally displaced three-dimensional set of Langmuir probe arrays have revealed details of turbulence, geodesic acoustic modes (GAMs), zonal flows and their interactions in the edge region of HL-2A tokamak plasmas. The coexistence of intensive low frequency zonal flows (LFZFs) off < 4 kHz and the GAMs of 7 kHz < f(GAM) < 20 kHz has been unambiguously demonstrated. The poloidal and toroidal symmetries of the flows, including the GAMs, are verified experimentally. In particular, the coherency of the flows over a large toroidal scale of 2100 mm at a magnetic flux surface is emphasized. The LFZF packets are shown to propagate outward and inward as equally likely events, whereas the predominantly outward propagation of the GAM packets is analyzed. The nonlinear three-wave coupling of the flows with ambient turbulence is shown with a bicoherency analysis and an envelope modulation of the latter by the former. The intensity of the LFZFs is observed to increase and decrease with increases in ECRH power (similar to 300-700 kW) and safety factor q similar to (3.5-6.2), respectively, whereas the intensity of the GAMs increases with increases in both ECRH power and q.

DOI： 10.1088/0741-3335/52/12/124008

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)  

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)  

Measurements with a three dimensional set of Langmuir probe arrays have unambiguously demonstrated the coexistence of intensive low frequency zonal flows (LFZFs), geodesic acoustic modes (GAMs), low frequency fluctuations (LFFs) and high frequency ambient turbulence (HFAT) in the edge of HL-2A tokamak plasmas, by verifying the temporal and spatial characteristics of the electrostatic fluctuations. The intensity of the LFZFs is observed to increase and decrease with increases of ECRH power (∼ 300-700 kW) and safety factor q (∼ 3.5-6.2), respectively, and the intensity of the GAMs increases with the ECRH power as well as q. The radial wave number-frequency spectra of the LFZF show that the LFZF packets propagate outwards and inwards as equally likely events, while the GAM packets propagate predominantly outwards. The three wave coupling of the zonal flows, including the GAMs, and the LFFs with the HFAT is investigated in detail.

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)  

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)  

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

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

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)  

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 (international conference proceedings)  

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

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

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

0.0

DOI： 10.1143/JPSJ.79.085001

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

The electron internal transport barrier (ITB) is formed by an edge perturbation induced by a tracer encapsulated solid pellet injection in a net-current free LHD plasma. The ITB trigger by an edge perturbation takes place in the low density regime and the ITB is not accompanied by a significant change in the density. At the onset of the core temperature rise, there is a strong correlation between the heat flux in the central region and the temperature gradient at a distance half of the plasma radius away. A radial temperature profile with a concave curvature abruptly appears in the growing phase of temperature gradient. Turbulence suppression mechanisms via flow and magnetic shears are not sufficient to explain these features.

DOI： 10.1088/0029-5515/50/6/064012

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 (international conference proceedings)  

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

The advantages of stellarators are discussed with an emphasis on the high density limit more than 10 times higher than that of tokamaks with the equivalent magnetic field strength. The comparison between stellerators and tokamaks in the light of the new paradigm of plasma turbulence and zonal flows is presented to propose a new optimization principle of magnetic field configuration. The roles of low-temperature devices should be recognized for further understanding of the plasma confinement even in the era of burning state plasmas

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

Experimental drift turbulence and zonal flow studies in magnetically confined plasma experiments are reviewed. The origins of drift waves, transition to drift turbulence and drift turbulence–zonal flow interactions in open field line and toroidal closed flux surface experiments are discussed and the free energy sources, dissipation mechanisms and nonlinear dynamics of drift turbulence in the core, edge and scrape-off layer plasma regions are examined. Evidence that turbulence across these regions is linked and that turbulence-driven zonal flows exist is presented, and evidence that these flows help regulate the turbulent scale lengths, amplitude and fluxes is summarized. Seemingly contradictory reports exist regarding the scale of turbulent transport events; gyro-Bohm behavior of turbulence correlation lengths as well as evidence for long-range transport phenomena both exist. Changes in turbulence during and after transport barrier formation are summarized and compared. The inferred turbulent particle and heat fluxes due to turbulent transport are usually consistent with global confinement, and edge plasma momentum transport appears to be linked to plasma flows at the last-closed flux surface and in the open field line region. However, inconsistencies between observed transport and turbulence have sometimes been reported and are pointed out here. Special attention is given to open issues, and suggestions for future experimental studies are given.

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)  

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)  

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)  

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)  

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)  

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)  

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)  

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)  

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)  

In a magnetized cylindrical plasma produced with the electron cyclotron resonance (ECR) scheme, a sheared E x B-flow showing zonal flow (ZF) characteristics has been driven by ECR power modulation. This method was used to control the sheared flow, and allowed us to observe that a coherent mode (spontaneously driven in the plasma) becomes stabilized as the induced plasma flow amplitude increases. The stabilization of the mode is caused by the increase in the plasma flow shear; thus, the result demonstrates the fundamental process of the shearing effect of ZFs on turbulence, which is commonly observed in toroidal plasma confinement devices for fusion research.

DOI： 10.1088/0741-3335/50/5/055005

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)  

Edge transport barrier (ETB) formation and a reheat mode have been simultaneously realized on the Compact Helical System (CHS). The new mode is induced by neutral particle reduction in the edge region, which is caused by shutting off fueling after a strong gas-puffing. When both the reheat mode and the ETB are simultaneously realized, the density reduction is suppressed by the ETB in the peripheral region, and the temperature continues to increase by the reheat mode. This mode provides an enhanced confinement in the high density region (bar ne ~ 1.2 × 1020mr-3) compared to the ETB formation without the reheat mode, because of a large suppression of an anomalous transport, which is confirmed with fluctuation measurements in the edge region.

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

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)  

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)  

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 internal structural measurements of electric field and density using twin heavy ion beam probes have been performed to elucidate the nonlinear evolution of the magneto-hydro-dynamic (MHD) bursty phenomenon driven by the interaction with high-energy particles in a toroidal plasma. The results have given the finest observation of the internal structure of plasma quantities, such as electric field, density and magnetic field distortion, which nonlinearly develop during the MHD phenomenon. In particular, the finding of a new kind of oscillating zonal flow driven by interaction between energetic particles and MHD modes should be emphasized for burning state plasmas.

DOI： 10.1088/0741-3335/49/11/013

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

Radiation brightness and impurity behaviors have been studied for reheat mode discharges in the Compact Helical System (CHS) by three different types of impurity diagnostics. Total radiation power measured by a pyroelectric detector significantly reduces after entering the reheat mode, whereas the line-averaged radiation brightness measured by an absolute extreme ultraviolet (AXUV) photodiode array increases especially for a center viewing chord due to the impurity accumulation in the plasma core. One possible reason for this opposite behavior between the two bolometric detectors is the reduced sensitivity of the AXUV photodiode for lower energy photons in vacuum ultraviolet (VUV) region. This speculation is supported by temporal evolutions of VUV spectra measured by a grazing incidence spectrometer. These results demonstrate that the comparison of three impurity diagnostics would be beneficial to the determination of the major impurity radiators and a comprehensive understanding of impurity behaviors in the reheat mode discharges.

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 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)  

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)  

In this article, we discuss the turbulent transport of the momentum (component perpendicular to main magnetic field) in toroidal plasmas. The fluctuations in the quasi-linear response tend to induce the shear of mean poloidal flow, as in the case of driving the zonal flows. The case in which the shear of mean poloidal flow and the mesoscale zonal flows coexist is studied. Zonal flows, which have a more rapid growth rate than the shear of mean flow, reaches the nonlinear saturation state, and modify the interaction between microscopic fluctuations and the shear of mean flow. Taking into account the screening effect of zonal flows, the shear of mean poloidal flow is subject to diffusive damping. Responses to the external force are also studied. A new bifurcation (from the state where mesoscale zonal flows are dominant to the one where zonal flows are suppressed) is found.

DOI： 10.1143/JPSJ.76.084502

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

The role of zonal flows in the formation of transport barrier in helical plasmas is analysed using the transport code. A set of one-dimensional transport equations is analysed, including the effect of zonal flows. The turbulent transport coefficient is shown to be suppressed when the plasma state takes the strong positive radial electric field. This bifurcation of the turbulent transport is newly caused by the change of the damping rate of zonal flows. It is theoretically demonstrated that the damping rate of zonal flows plays an important role in global confinement in toroidal plasmas.

DOI： 10.1088/0029-5515/47/8/024

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

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

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)  

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)  

The role of zonal flows (ZFs) in the formation of an internal transport barrier in a toroidal helical plasma is analyzed. The turbulent transport coefficient is shown to be suppressed when the plasma state changes from the branch of a weak negative radial electric field to the strong positive one. This new transition of turbulent transport is caused by the change of the damping rate of the ZFs. It is clearly demonstrated, theoretically and experimentally, that the damping rate of the ZFs governs the global confinement of toroidal plasmas.(c) 2007 American Institute of Physics.

DOI： 10.1063/1.2435310

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 improved confinement mode with reheat and edge transport barrier is observed in the Compact Helical System (CHS). This mode provides enhanced confinement in high density region (ne ˜ 1.2 × 1020 m-3) due to the increase in temperature while high density is maintained in the peripheral region.

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)  

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)  

An edge transport barrier (ETB) similar to the tokamak H-mode has been observed for beam-heated plasma with two co-injected Neutral Beam Injectors (NBIs) in CHS. The Hα emission showed a clear spontaneous drop followed by an increase of line-averaged electron density at the L-H transition. Stored energy increased by ˜ 40% with H-factor improvement up to ˜ 30% compared to the international stellarator scaling (ISS04v03). A rapid density increase in the edge region to two-times level of the L-mode was observed to accompany a ˜ 15% increase in the density gradient. An ETB was formed when the plasma density exceeded the lower density limit by ˜ 1.5 × 1013 cm-3 and the total NBI power exceeds the threshold level (Pdeposit / ne ˜ 200 × 10-13 kW/cm-3 at BT = 0.95 T, Rax = 92.1 cm, where Pdeposit is the deposited NBI power, ne is electron density, BT is troidal magnetic field strength and Rax is location of magnetic axis). The power threshold increased with the magnetic field strength, as in tokamak scaling, and depends on the magnetic field configuration of the helical plasma.

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)  

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)  

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

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)  

This is a report on fluctuation measurements using twin heavy ion beam probes in CHS. The observation shows that dozens of coherent modes with intermittent nature co-exist in an electron cyclotron heated plasma. The modes are found to have a long-distance correlation in the toroidal direction. The radial structure of the modes is evaluated in potential fluctuation. The characteristics are consistent with those of the geodesic acoustic modes.

DOI： 10.1088/0741-3335/48/4/S03

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

The characteristics of geodesic-acoustic-mode (GAM) are investigated through direct and simultaneous measurement of electrostatic and density fluctuations with a heavy ion beam probe.The amplitude of the GAM changes in relation to the radial position; it is small near the separatrix, reaches a local maximum at 3 cm inside the separatrix and then decreases again to 5 cm inside the separatrix. The frequency is constant in the range, though the predicted GAM frequency varies according to the temperature gradient. The correlation length is about 6 cm and comparable to the structure of the amplitude of the GAM. The results indicate the GAM has a radial structure which reflects. the local condition at about 3 m inside the separatrix.The phase relation between the GAM oscillation indicates that the GAM is a radial propagating wave.The interaction between the GAM and the ambient density fluctuation is shown by the high coherence between the GAM oscillation and the temporal behaviour of the ambient density fluctuation. Moreover, the phase relation between the electric field fluctuation of the GAM (E,,GAM) and the amplitude of the density fluctuation indicates that the modulation of the ambient density fluctuation delays the (E) over tilde (r,GAM). The causality between the GAM and the modulation of the density fluctuation is revealed.

DOI： 10.1088/0741-3335/48/4/S04

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)  

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

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

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

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

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

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

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

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)  

The present paper extends the theory of geodesic acoustic mode (GAM) oscillation, which so far has been applied to tokamaks, to helical systems. By using drift kinetic equations for three-dimensional equilibriums, a generalized dispersion relation is obtained including Landau damping. The oscillation frequency is obtained in terms of the squared sum of Fourier components of the magnetic field intensity expressed by means of magnetic flux coordinates. An analytic form of the collisionless damping rate of GAM is obtained by solving the dispersion relation perturbatively. It is found that the GAM frequency is higher in helical systems than in tokamaks and that damping rate is enhanced in multi-helicity magnetic configurations. However, damping rates are predicted to be small if the temperature of electrons is higher than that of ions.

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

Theoretical and experimental studies associated with electric field effects on the stability and transport are briefly surveyed. The effects of radial electric field on the suppression and/or enhancement of various microinstabilities such as drift waves, flute mode and temperature gradient modes are discussed. The suppression of flow shear on the electron temperature gradient mode in plasmas with slightly hollow density profiles is investigated by solving the gyrokinetic integral eigenvalue equation. Comparison between theoretical predictions and experimental observations based on the HIBP measurements with high temporal and spatial resolutions is made in bumpy tori and heliotron (CHS) devices.

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

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

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

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

DOI： 10.1088/0741-3335/46/5A/004

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

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

DOI： 10.1088/0741-3335/46/5A/003

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

The bifurcated nature of the potential profile of a toroidal helical plasma is investigated in CHS using a heavy ion beam probe. The measurements reveal that three main branches of potential profile exist in ECR heated plasmas with a low density of (n) over tilde (e) approximate to 0.5 x 10(19) m(-3). The branches with higher central potential exhibit a rather strong radial electric field shear that should result in fluctuation reduction and formation of a transport barrier. An analysis of the pulsating behaviour in the potential suggests the existence of a fine structure in the potential profiles between the main branches. Lissajous and histogram expressions are useful in extracting the bifurcation characteristics of the potential structure.

DOI： 10.1088/0029-5515/41/5/310

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

A fully calibrated multilayer mirror (MLM) based soft x-ray spectrometer has been developed and installed on the Compact Helical System (CHS). This MLM based spectrometer is a new approach to devising a soft x-ray spectrometer with medium time and energy resolutions. The spectrometer mainly consists of a MLM as the dispersive element and a 20-channel (p-i-n) pin diode array for the detection of the soft x-rays. Both the MLM and the pin diodes have been calibrated at the KEK (High Energy Accelerator Research Organization) Photon Factory for an energy range of 300-1200 eV. The reflectivity of the mirror was found to increase from 3% (photon energy of 335 eV) to 25% (photon energy of 1050 eV). The pin diodes were found to have almost 100% efficiency for creating electron-hole pairs in the calibrated energy range. The spectrometer was installed on the CHS with the aim of fast electron temperature measurement and study of fast magneto-hydrodynamic (MHD) events occurring in plasmas. The experiments show that in the energy range of measurement, the electron "temperature" determined from the slope of the soft x-ray spectrum is much lower than that measured by the Thomson scattering diagnostic. Analysis showed that the soft x-ray spectrum is highly contaminated by impurity emission. Therefore it may be possible to measure electron temperature with this diagnostic if we choose another energy range where we can measure the continuous spectrum. The present time resolution of the system is of the order similar to0.1 ms, which made it possible to study the behavior of the plasma during fast MHD events. Modulations in the soft x-ray intensity were observed during MHD events in CHS plasmas. Analysis of these shows that these may be due to modulations in the temperature or the impurity concentration. (C) 2001 American Institute of Physics.

DOI： 10.1063/1.1324745

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

The bifurcation nature of the electrostatic structure is studied in the toroidal helical plasma of the Compact Helical System (CHS) [K. Matsuoka , Proceedings of the 12th International Conference on Plasma Physics and Controlled Nuclear Fusion Research, Nice, 1988 (International Atomic Energy Agency, Vienna, 1989), Vol. 2, p. 411]. Observation of bifurcation-related phenomena is introduced, such as characteristic patterns of discrete potential profiles, and various patterns of self-sustained oscillations termed electric pulsation. Some patterns of the electrostatic structure are found to be quite important for fusion application owing to their association with transport barrier formation. It is confirmed, as is shown in several tokamak experiments, that the thermal transport barrier is linked with electrostatic structure through the radial electric field shear that can reduce the fluctuation resulting in anomalous transport. This article describes in detail spatio-temporal evolution during self-sustained oscillation, together with correlation between the radial electric field and other plasma parameters. An experimental survey to find dependence of the temporal and spatial patterns on plasma parameters is performed in order to understand systematically the bifurcation property of the toroidal helical plasma. The experimental results are compared with the neoclassical bifurcation property that is believed to explain the observed bifurcation property of the CHS plasmas. The present results show that the electrostatic property plays an essential role in the structural formation of toroidal helical plasmas, and demonstrate that toroidal plasma is an open system with a strong nonlinearity to provide a new attractive problem to be studied. (C) 2000 American Institute of Physics. [S1070-664X(00)04310-X].

DOI： 10.1063/1.1290483

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 the compact helical system, several bifurcation patterns have been observed in potential profiles of electron-cyclotron-heated plasmas. In a bifurcation pattern with a strong shear of radial electric field, an internal transport barrier is confirmed to be established for electron thermal energy. Probabilistic treatment with realization of the pattern or internal transport barrier is discussed.

DOI： 10.1088/0741-3335/42/5A/309

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

Theoretical analysis of the electric field bifurcation is made for the LHD plasma. For given shapes of plasma profiles; a region of bifurcation is obtained in a space of the plasma parameters. In this region of plasma parameters, the electric field domain interface is predicted to appear in the plasma column. The reduction of turbulent transport is expected to occur in the vicinity of the interface; inducing internal transport barrier. Within the present simple model, the electric field bifurcation associated with the internal transport barrier is predicted to be realized for the parameter of T-e(0) similar or equal to 2 keV and n(0) similar or equal to 10(18)(m(-3)). The boundary of the region of multiple solution is characterized by a scaling T-e(0) similar to [n(0)](0.4).

DOI： 10.1143/JPSJ.69.445

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)  

Transition in radial electric field has been observed in CHS plasmas during a combined heating phase of ECH+NBI, using a heavy ion beam probe. The transition occurs in a timescale of a few dozen or a hundred microseconds. Radial current to induce the transition is evaluated by high temporal resolution of the heavy ion beam probe. Neoclassical dependence of radial electric field on a plasma parameter will be discussed to clarify the nonlinearity to allow the transition.

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

In the CHS heliotron/torsatron, dynamic phenomena associated with transitions in radial electric field were observed during combined ECH+NBI heated plasmas. The observations with high temporal resolution confirmed a nonlinear relationship between radial electric field and radial current as the cause of these phenomena associated with electric field bifurcation.

DOI： 10.1088/0741-3335/40/5/011

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

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

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

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

A pellet injection system with controllability of the injection angle has been developed and installed in the JIPP T-IIU tokamak. A long helical ablation cloud ('tail') has been observed when an ice pellet is injected into the tokamak plasma using the controllable injection angle system. The 'tail' does not flow along the direction of the total magnetic field lines of the torus. In order to understand the details of the ice pellet ablation structure in the 'tail mode', the local parameters of electron density and temperature within the ablation cloud are obtained by a spectroscopic method. The electron density determined from Stark broadening of the Balmer beta line is in the range 1.0 x 10(16) - 1.0 x 10(17) cm(-3), while the temperature obtained from the line-to-continuum intensity ratios is in the range 1-4 eV. It has also been observed by a heavy ion beam probe (HIBP) measurement that the potential at the plasma centre becomes more negative and the potential profile becomes deeper negatively after pellet injection. (C) 1997 Elsevier Science S.A.

DOI： 10.1016/S0920-3796(96)00655-2

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

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

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

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

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

Event date： 2023.6

Language：Japanese   Presentation type：Oral presentation (general)  

Country：Japan  

Event date： 2022.11

Language：English   Presentation type：Oral presentation (invited, special)  

Venue：online   Country：Japan  

Event date： 2022.3

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：オンライン   Country：Japan  

Event date： 2019.11 - 2019.12

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：中部大学   Country：Japan  

Event date： 2019.9

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：核融合科学研究所   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：Japanese   Presentation type：Oral presentation (general)  

Venue：核融合科学研究所   Country：Japan  

Event date： 2019.7 - 2019.8

Language：English   Presentation type：Public lecture, seminar, tutorial, course, or other speech  

Venue：名古屋大学   Country：Japan  

Event date： 2017.11

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：姫路商工会議所   Country：Japan  

Event date： 2016.3

Language：Japanese   Presentation type：Symposium, workshop panel (public)  

Venue：日本学術会議   Country：Japan  

Event date： 2015.9

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：関西大学千里山キャンパス   Country：Japan  

Event date： 2015.3 - 2015.9

Language：Japanese  

Venue：早稲田大学   Country：Japan  

Event date： 2014.11

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：新潟市　朱鷺メッセ   Country：Japan  

The research of plasma turbulence enters into a new era after establishing a modern view of plasma turbulence. Further understanding based on the modern view requires development of a new diagnostics that can measure the whole plasma with fine resolution up to micro-scale. A candidate of such diagnostics is tomography, and a prototype is constructed in linear cylindrical plasma to provide successful and promising results. Therefore, in order to advance the plasma turbulence suffering from topological effects of confinement magnetic field, it is necessary to construct a special toroidal that allows such tomography measurement to cover the whole plasma with fine resolution from micro-scale to macro-scale fluctuations.

Event date： 2014.10

Language：English   Presentation type：Symposium, workshop panel (public)  

Venue：Uji Kyoto   Country：Japan  

Event date： 2014.8

Language：Japanese   Presentation type：Public lecture, seminar, tutorial, course, or other speech  

Country：Japan  

Event date： 2014.6

Language：English   Presentation type：Oral presentation (invited, special)  

Venue：Fukuoka   Country：Japan  

It is well-known that meso-scale and macro-scale structures generated by drift-waves, such as zonal flows [1], streamers [2] and large-scale fluctuating structure [3], exist in turbulent plasmas, and their interaction govern the structural formation and dynamics of turbulent plasmas [4]. This paradigm gives us a guiding principle for basic experiments to investigate the plasma turbulence experimentally. Nowadays, analysis tools have been developed to elucidate steady and dynamic characteristics of plasma turbulence, and to quantify elementary processes, e.g., the nonlinear couplings and energy transfer direction between the elementary waves in turbulence. Accordingly, basic experiments to clarify these fluctuating structures and their interaction are possible to advance fundamental understanding of structural formation and transport in turbulent plasma in presence of suitable experimental environments [5]. Plasma devices of low temperature, such as linear cylindrical and torus devices, provide excellent environment to perform the basic experiments, with these analysis tools, to identify the fundamental structures and their interactions. The devices own high accessibility and flexibility, easy controllability of plasma, moreover, advantages to use traditional probes and to develop new diagnostics. In fact, up to date, a huge number of experiments, e.g., CASTOR, CSDX, HYPER-I, MIRABELLE, PANTA, QT-Upgrade, TJ-K, TORPEX, VINETA, etc., have been done in plasma devices of low temperature.The paper introduces what can we learn from such experiments done in linear cylindrical and toroidal devices with brief description of advanced analyses and diagnostics on turbulence; e.g, quantification of nonlinear couplings in wavenumber in k-ω space, energy transfer analysis in wavenumber space, discovery of streamers, investigation of blob generation, detailed studies on EXB shearing effects, and so on. Finally, future direction of such basic experiments is indicated with their contribution to understanding the magnetically confined plasmas oriented for nuclear fusion.
[1] S. Inagaki et al., Phys. Rev. Lett. 107 115001 (2011)
[2] T. Yamada et al. Nature Phys. 4 721 (01 Sep. 2008).
[3] A. Fujisawa et al., Phys. Rev. Lett. 93 165002 (2004)
[4] G. R. Tynan et al., Plasma Phys. Control. Fusion 51 (2009) 113001. A. Fujisawa, Nucl. Fusion 49 013001 (2009).
[5] A. Fujisawa, Plasma Fusion Res. 5 046 (2010).

Event date： 2013.12

Language：Japanese  

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

Event date： 2013.12

Language：Japanese  

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

Event date： 2013.10

Language：English  

Venue：Kyushu University, Fukuoka, Japan   Country：Japan  

Event date： 2013.9

Language：Japanese  

Venue：徳島大学常三島キャンパス   Country：Japan  

Event date： 2013.8

Language：Japanese  

Venue：九州大学筑紫キャンパス　総合研究棟３階　講義室   Country：Japan  

Event date： 2013.7

Language：English  

Venue：International Conference Hall, Makuhari Messe, Chiba, JAPAN   Country：Japan  

Event date： 2013.7

Language：English  

Venue：Espoo, Finland   Country：Finland  

Event date： 2013.6

Language：Japanese   Presentation type：Oral presentation (invited, special)  

Venue：九州大学筑紫地区   Country：Japan  

Event date： 2013.3

Language：Japanese   Presentation type：Symposium, workshop panel (public)  

Venue：広島大学　東広島キャンパス   Country：Japan  

磁場閉じこめプラズマ、レーザープラズマ、大気圧プラズマ、重相プラズマ、宇宙プラズマなど、現在プラズマ物理学は様々な対象へと展開し、著しい発展を見せている。その応用は脱CO2社会へと貢献する核融合発電等の先進エネルギー、カーボンナノチューブやスーパーダイアモンドなど新機能性物質創成による産業応用、大気圧プラズマによる止血、滅菌等の医療応用の他、多方面への応用が期待されている。このように多彩な発展を遂げているプラズマを「非平衡極限プラズマ」という共通学理から統一的に理解しようとする動きがある。夫々のプラズマ独自の特徴および共通する普遍的な問題や機構を抽出し、そして固有のプラズマに対し用いられる様々な方法を互いに適用することで、プラズマ物理の学術としての更なる進展、新領域の開拓を狙った試みである。このシンポジウムは、乱流磁化プラズマ、気液界面プラズマ、ナノ・バイオプラズマ、レーザー高密度プラズマの領域からそれぞれ最新のトピックスについて講演が準備されている。
今回「界面」を一つのキーワードとなっているが、磁場閉じ込めプラズマで「界面」に関連する代表的現象は輸送障である。核融合を目指したプラズマは乱流状態にあり乱流によってその閉じこめ特性が決定されている。1980代に、プラズマの周辺のごく狭い（メソスケール）領域で突然乱流輸送が低減する現象（Hモードあるいは輸送障壁）が発見され[1]、高性能プラズマの実現とその物理機構の解明のため盛んに研究が行われてきた。その結果、磁場閉じ込めプラズマに内在する自発電場の分岐性の存在が明らかになり、その分岐界面によって生じるシアー流（メソスケール）が乱流を低減する機構が同定されている。最近の実験では、ガスパフ・イメージング等の２次元計測の進歩により輸送障壁—すなわち分岐界面の領域で乱流のパターンが可視化され、その変化の詳細が明らかになってきている。
一方で、より基礎的な部分で、プラズマ乱流と輸送の研究は更なる展開を見せている。これまで、プラズマ乱流状態はドリフト波の集合体と考えられてきたが、最近の研究においてドリフト波は帯状流やストリーマなどのメソスケール構造を作ることが分かってきている。ここでいうメソスケールとは、プラズマ半径とイオンラーマー半径の相乗平均程度の長さ(~ )に当たる。帯状流は磁気軸の廻りの対称な流れであり、磁場を横切る輸送に寄与しない。そのためドリフト波から帯状流にエネルギーが移行することで乱流は抑制され輸送は低減する。すなわち、ドリフト波乱流自身の自己組織化によって輸送が規定される機構が明らかになってきており、プラズマの閉じ込めの根源的理解に結びつく知見をもたらした [2,3]。最近の実験で、乱流が駆動するよりプラズマ副半径に及ぶ巨視的な揺らぎの構造も見つかり上記の乱流の描像はメソスケールからさらにマクロスケール（プラズマ半径程度）の揺らぎを含むまでに至っている[4]。すなわち、プラズマ乱流とは、ドリフト波（ミクロスケール）とドリフト波が創る出すメソスケールからマクロスケール構造が非線形に絡み合う系として認識されるに至っている。
最近の興味深い問題として、リミットサイクルを示す輸送障壁領域での揺動と電場シアーのダイナミクス、「補食—被捕食者モデル」を示唆するような観測が見つかるほか、HモードとLモードの中間状態の存在など輸送障壁についても新しい観測が得られ、測地線音波モードを含む帯状流が輸送障壁の形成にどのように関与しているか、などの問題を提示している。また、プラズマに追加熱を行うと瞬時に乱流状態が変化し輸送が変ることが実験的に示され、加熱—分布関数を変形させようとする力が乱流に影響を及ぼすこと、すなわち位相空間と乱流の重要な関連が示されており、これまで謎であった非局所輸送などの現象にも解明の糸口が与えられる等、乱流と乱流輸送の研究は新しい局面を迎えている。以上、磁場閉じこめプラズマの領域ではプラズマ乱流や輸送障壁—すなわち乱流プラズマの構造形成について重要な知見が得られている。この進展には、先に述べたガスパフ・イメージングを含む計測器（マイクロ波散乱、重イオンビームプローブ、BES, Doppler Reflectometor, ECE Array, etc）や解析法(Bicoherence, Wavelet, Wavelet-Bicoherence, etc)の進歩が不可欠であった [5]。本講演では、輸送障壁に関連し最近のトピックス、および近年得られたプラズマ乱流に関する新しいパラダイム、乱流と構造形成の現代的描像について紹介する。さらに今後研究を進展させるために必要な研究について議論する。

Event date： 2013.1

Language：Japanese   Presentation type：Symposium, workshop panel (public)  

Venue：日本学術会議   Country：Japan  

Event date： 2012.12

Language：Japanese   Presentation type：Oral presentation (invited, special)  

Venue：九州大学　筑紫キャンパス   Country：Japan  

プラズマ乱流の理解が著しく進展している。乱流と平均流の相互作用に始まりドリフト波の集合と考えられてきたプラズマ乱流がドリフト波とドリフト波が生成する構造の系として認識されるに至っている。講演ではこれまでの研究成果を振返り現在得られているプラズマ乱流の描像を解説する

Event date： 2012.11

Language：Japanese   Presentation type：Symposium, workshop panel (public)  

Venue：春日市　クローバープラザ   Country：Japan  

Turbulence plays an important role on structure formation of magnetically confined plasmas by enhancing the transport. Thus the understanding of plasma turbulence is strongly required for realization of fusion reactors. In PANTA in Kyushu University [1], a multi-sight spectroscopic system is made to aim at two-dimensional measurement of turbulence, and the system will be extended for tomographic reconstruction of turbulence images. This poster presents preliminary results from a test of the system on spectral analysis of the line-integrated light emission.
PANTA is a cylindrical linear plasma device with the diameter of ~0.1 m and axial length of 4 m. Plasma is generated by helicon wave (7 MHz, 3 kW). The magnetic field is fixed at 0.09 T.
The spectroscopic system consists of 132 (33 rows x 4 columns) channels and can measure different light bands in wavelength using four kinds of optical filters ; infrared (900±20 nm), red (696.5±30 nm), blue (476.5±30 nm), and UV (294±20 nm). The 33 channels in a set are aligned in the vertical direction with the intervals of 5 mm. The filtered light signals, integrated along the horizontal line-of-sight across the plasma, are transferred to photo-diode detectors through optical fibers. The present measurement is performed for selected 16 channels with weighted on different radial positions. The significant fluctuation signals in blue light are obtained in an argon plasma, and red light in a neon plasma. Fast Fourier Transform (FFT) was applied to the observed signals to obtain fluctuation spectra with the frequency resolution of ~50 Hz. Figure 1 (a) and (b) shows an example of power spectrum of emission intensity fluctuation along the central chord (including r = 0 mm) and deviated chord from the center (including r = 20 mm) in the blue range of wavelength in a argon plasma, respectively. As is shown in Fig. 1, we can find several spectral peaks in low frequencies, for example at ~5 kHz, significantly above the noise level, suggesting a possibility to infer the radial position where such coherent modes are located using the 1D multi-channel measurement.

Event date： 2012.11

Language：Japanese   Presentation type：Symposium, workshop panel (public)  

Venue：春日市　クローバープラザ   Country：Japan  

Understanding of plasma turbulence is an essential issue to realize and control fusion plasmas since turbulence-driven transport affects significantly plasma confinement. Turbulence state consists of various spatio-temporal scale fluctuations, therefore, the plasma turbulence needs to be observed multi-dimensionally with covering a wide range of spatial scales. In a cylindrical plasma device PANTA in Kyushu University[1], a spectroscopic system surrounding the plasma for tomography is being constructed to measure two-dimensional structure of turbulence with a fine resolution, and as its first trial, the reconstruction of local emission profile in steady states is being made from one dimensional measurement in argon plasmas of PANTA.
PANTA produces a linear cylindrical plasma device with the diameter of ~0.1 m and axial length of 4 m by helicon wave (7 MHz, 3 KW). The line-integrated measurements were carried out for four different ranges of wavelength by optical filters; blue (480 nm), red (700 nm), infrared (900 nm), and UV (300 nm) light. The system for each wavelength range consists of 33 channels aligned in the vertical direction. In this experiment, the magnetic field was fixed at 0.9 kG with argon gas pressue of 1 mTorr. The measurement was performed with selecting 16 channels that covered the plasma region from the center to an edge on one side.
Assuming azimuthal symmetry of the cylindrical plasmas, radial profile of local emission is reconstructed by different procedures, i.e., using the Abel inversion after fitting functions to the measured profile of line integrated profile, and solving a linear matrix equation to connect the local emission to the line integrated profile. Figure 1 shows an example of the measured line-integrated profle and the inferred profile of local emission obtained the method of solving the matrix equation for the case of the blue light (ArII emission) for an argon plasma, suggesting a sharply peaked profile around the plasma center of r less than 2cm. The poster presents the initial results obtained with the devloping system for tomography and details of the reconstruciton methods, with the progress and future in the construction of a two-dimensional observation system in PANTA.

Event date： 2012.11

Language：Japanese   Presentation type：Symposium, workshop panel (public)  

Venue：春日市　クローバープラザ   Country：Japan  

プラズマ乱流はプラズマの閉じ込めに関わる極めて重要な課題で、磁場閉じこめ核融合分野では半世紀以上に渡り国際的に研究されてきたが、近年の研究で著しい進展が見られている。７０年代にプラズマ乱流の原因としてドリフト波が同定され、２１世紀にはドリフト波が創るメソスケール構造として帯状流やストリーマが見つかり、プラズマ乱流はドリフト波と帯状流などメソスケール構造から成り立つ系として捉えられるに至っている。また、最近の大型ヘリカル装置でのドリフト波が創る巨視的揺らぎ構造の発見により、このプラズマ乱流の見方はマクロスケールを含む系として拡張されている。本シンポジウムではプラズマ乱流の実験、理論、シミュレーションの夫々についての最新のトピックスを紹介し、プラズマ乱流の研究を今後更に発展させるため、その方向を議論するために企画された。
　シンポジウム最初は最近発見された巨視的揺らぎ構造に関する講演（九州大学の稲垣氏）で加熱パワーモジュレーション時に見られる輸送の瞬間的変化（閉じこめ時間に比べ圧倒的に短い）が揺らぎの変化によることを実験的に検証したことが紹介された。次の講演（伊藤公孝氏　核融合研）では、加熱パワーの変化が直接プラズマに位相（速度）空間上の影響を与え、特に波長の長い不安定性を変化させることで瞬間的揺らぎと輸送の変化が説明できることが理論的に示唆された。またプラズマの特徴である位相空間乱流という概念が紹介された。３番目の講演（矢木雅敏氏　原子力研究所）では、核融合プラズマでは原子レベルの現象から装置サイズにおよぶマルチスケールの現象の解明を目指した統合シミュレーションコードの他に、乱流診断シミュレータが紹介された。乱流シミュレータとはプラズマ乱流を特定の計測器でどのよう観測されるかをシミュレーションしその実体を推測し実験を効率的に推進するためのツールである。最後の講演(Prof. H.PARK、POSTECH 韓国)では、電子サイクロトロン輻射イメージングによる三次元計測が紹介され、KSTARトカマクでのELM時の電子温度観測による時空間（３次元）発展の詳細が示された。イメージング与える力が余すことなく示され、今後の乱流研究にも果たすべき重要な役割が示唆された。
　最後に、これまで計測器の発展とともに、プラズマ乱流研究の研究は進展してきた。マイクロ波散乱によるドリフト波の同定、重イオンビームプローブによる帯状流の発見、ECEの径方向分布計測による巨視的揺動の発見などによって、現在のプラズマ乱流は、ミクロ、メソ、マクロの揺らぎの非線形相互作用によって成り立つ系とする描像に至っている。今後その描像に基づいてプラズマ乱流の理解を本質的に進めるためには、プラズマ乱流は一点のみ観測しているだけでは困難である。最初に掲げた問いへの回答として―プラズマ全域およびすべてのスケールに渡る観測が必要であり、それを支持するシミュレーション・理論との密接な三位一体の研究方法の推進が必要である。

Event date： 2012.4

Presentation type：Oral presentation (invited, special)  

Venue：京都　龍谷大学   Country：Japan  

Event date： 2012.4

Presentation type：Oral presentation (invited, special)  

Venue：関西学院大学　西宮   Country：Japan  

Event date： 2012.2

Presentation type：Symposium, workshop panel (public)  

Venue：生産技術研究所   Country：Japan  

Event date： 2011.11

Presentation type：Oral presentation (invited, special)  

Venue：金沢　石川県立音楽堂   Country：Japan  

The modern view of turbulence has been established due to the discovery of structures drift-waves generate, such as
zonal flows and streamers, providing a new framework for understanding turbulence-driven transport and structural
formation in magnetized plasmas. This paper presents recent development of laboratory experiments to have advanced
the understanding of plasma turbulence and transport, focusing on discoveries of mesoscale structures, such as zonal
flows and streamers, the analyzing techniques to quantify the couplings between different scale structures, and physics
of turbulence transport and barrier formation.

Event date： 2010.9

Presentation type：Symposium, workshop panel (public)  

Venue：岡山大学津島キャンパス   Country：Spain  

EFDA(Europe Fusion Development Agreement) transport topical group meeting（第３回目）とEU-US transport Task Force Meeting(第15回）の合同コンファレンスが2009年の9月７日から10日にスペイン、コルトバにて開催された。その会議に招待されReview講演を行った。合衆国とEUの合同による会議であり、その目的は磁場閉じ込め核融合における乱流輸送の物理理解および制御であり、後者は毎年開催され当該分野では歴史のある会議である。

Event date： 2010.3

Presentation type：Oral presentation (general)  

Venue：岡山大学津島キャンパス   Country：Japan  

Event date： 2010.3

Venue：岡山大学津島キャンパス   Country：Japan  

Event date： 2010.3

Venue：岡山大学津島キャンパス   Country：Japan  

Event date： 2010.3

Venue：岡山大学津島キャンパス   Country：Japan  

Event date： 2010.3

Venue：General Atomics, San Diego   Country：United States  

Event date： 2009.12 - 2010.12

Presentation type：Oral presentation (general)  

Venue：山口大学常盤キャンパス   Country：Japan  

Event date： 2009.12

Venue：山口大学常盤キャンパス   Country：Japan  

Event date： 2009.12

Presentation type：Oral presentation (general)  

Venue：山口大学常盤キャンパス   Country：Japan  

Event date： 2009.12

Venue：山口大学常盤キャンパス   Country：Japan  

Event date： 2009.12

Presentation type：Oral presentation (general)  

Venue：京都市国際交流会館   Country：Japan  

Event date： 2008.12

Presentation type：Oral presentation (general)  

Venue：Toki Japan   Country：Japan  

Event date： 2008.10

Presentation type：Oral presentation (general)  

Venue：Geneve, Switzerland   Country：Switzerland  

Event date： 2008.9

Presentation type：Oral presentation (general)  

Venue：Fukuoka, Japan   Country：Japan  

Event date： 2007.11

Presentation type：Oral presentation (general)  

Venue：Orlando, Florida   Country：United States  

Event date： 2007.10

Presentation type：Oral presentation (general)  

Venue：Toki, Japan   Country：Japan  

Event date： 2006.10

Presentation type：Oral presentation (general)  

Venue：Chengdu, China   Country：China  

Event date： 2006.5

Presentation type：Oral presentation (general)  

Venue：Kiev,Ukraine   Country：Ukraine  

Event date： 2005.4

Presentation type：Oral presentation (general)  

Venue：Kyoto Univ.   Country：Japan  

Event date： 2005.4

Presentation type：Oral presentation (general)  

Venue：Kyoto Univ.   Country：Japan  

Event date： 2005.4

Presentation type：Oral presentation (general)  

Venue：Kyoto Univ.   Country：Japan  

Event date： 2004.12

Presentation type：Oral presentation (general)  

Venue：Southwestern Institute of Physics, Chengdu   Country：China  

Event date： 2004.12

Presentation type：Oral presentation (general)  

Venue：Academy of Sciences, Institute of Plasma Physics Hehei   Country：China  

Event date： 2004.12

Presentation type：Oral presentation (general)  

Venue：Tsinghua University, Beijng   Country：China  

Event date： 2004.11

Presentation type：Oral presentation (general)  

Venue：Vilamoura, Portugal(IAEA-EX/8-5RB)   Country：Portugal  

Event date： 2004.6 - 2004.7

Presentation type：Oral presentation (general)