Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：Japan Society of Plasma Science and Nuclear Fusion Research  

DOI： 10.1585/pfr.18.1402058

Authorship：Lead author,　Corresponding author   Publishing type：Research paper (scientific journal)  

DOI： 10.1063/5.0146602

Authorship：Lead author,　Corresponding author   Publishing type：Research paper (scientific journal)  

DOI： 10.1063/5.0137669

Authorship：Lead author,　Corresponding author   Publishing type：Research paper (scientific journal)  

DOI： 10.1103/PhysRevLett.128.085001

Web of Science

Publisher：Journal of Fusion Energy  

Experiments on physics issues in burning plasma were conducted in the deuterium campaign of Large Helical Devices (LHD). One is an isotope mixing experiment, and the other is the collisionless energy transfer from energetic particles to bulk ions. The important finding for ion mixing in the LHD experiment is that the ITG turbulence (but not TEM turbulence) contributes to the isotope and ion mixing, which is beneficial for controlling the isotope ratio (deuterium and tritium) and helium ash exhaust. The experimental identification of energy transfer from energetic particles to bulk ions through Landau damping and transit-time damping suggests a possible ion-heating process for the ion-ITB plasma with () through collisionless energy transfer from alpha particles to bulk ions through energetic particle-driven instability, so-called alpha channeling.

DOI： 10.1007/s10894-026-00573-z

Scopus

Publisher：Journal of Fusion Energy  

In this paper, we show the expansion of the high-temperature regime and related physical phenomena in the Large Helical Device (LHD). At the LHD, we have developed a high-temperature operational regime for both ion (T<inf>i</inf>) and electron (T<inf>e</inf>) temperatures through understanding the physical characteristics of high-temperature plasma, the enhancement of plasma heating, and the development of technical methods such as wall recycling control and impurity control. In particular, the formation of the ion internal transport barrier (i-ITB) and electron internal transport barrier (e-ITB) in the plasma core region is the key to improving plasma performance. In addition, the deuterium experiment that began in 2017 showed that, due to the isotope effect, the thermal transport of both ions and electrons is suppressed in deuterium plasma compared to light hydrogen plasma, and that a high central temperature can be achieved efficiently. Furthermore, the combination of neutral beam injection (NBI) and electron cyclotron heating (ECH) has extended the operating conditions that simultaneously maintain high T<inf>i</inf> and T<inf>e</inf>, and comprehensive research on plasma confinement characteristics with an eye to future fusion reactor conditions has progressed.

DOI： 10.1007/s10894-026-00571-1

Scopus

Language：English   Publisher：Review of Scientific Instruments  

Tomography serves as an advanced diagnostic tool for analyzing plasma fluctuations and turbulence. However, it requires time-consuming calculations, which impede rapid analysis. Integrating a neural network into tomography offers a potential solution. In this work, we present a trial conducted on a tomography system installed on the Plasma Assembly for Nonlinear Turbulence Analysis, a cylindrical plasma device designed for plasma turbulence research. This article reports on the optimization process of a neural network algorithm and on its excellent properties for tomographic reconstruction, including the extraction of plasma fluctuation properties. The neural network-aided tomography is 25 times faster than and provides comparable accuracy to, the standard algorithm, Maximum Likelihood Expectation Maximization.

DOI： 10.1063/5.0304482

Scopus

PubMed

Publisher：Nuclear Fusion  

We investigate drift wave—zonal flow interactions near the density limit and demonstrate transitions of turbulence trapping states. Nonlinear simulations of the modified Hasegawa–Wakatani model reveal two robust types of turbulence energy localization: valley-trapping near zonal flow troughs at small adiabaticity (hydrodynamic regime) and hill-trapping near crests at large adiabaticity (adiabatic regime). Around the transition, turbulence exhibits bistability, dynamically switching between valley- and hill-trapping, and also shows indications of multi-stable behavior depending on the adiabaticity. The Lagrangian auto-correlation time of turbulence changes markedly across the bifurcation between the trapping states, indicating that turbulent vortices become long-lived in the adiabatic limit and short-lived in the hydrodynamic limit. Eigenmode analysis reproduces these patterns and shows that the selection is governed by the relative magnitude of the drift wave phase velocity and the zonal flow amplitude. These findings provide a simple selection rule for turbulence energy localization and offer new insight into drift wave—zonal flow dynamics in regimes relevant to the density limit.

DOI： 10.1088/1741-4326/ae2ee6

Scopus

Publisher：Measurement Science and Technology  

Direct observation of the projection matrix is performed for the visible light tomography system to observe turbulence inside magnetized plasmas. The projection matrix is obtained as the response function of the observation system to the emissions from a narrow light source. Reconstruction using the experimentally observed projection matrix successfully reproduced a smooth two-dimensional emission profile. A novel analysis method is developed to estimate the spatial resolution of the tomography system using the mapping feature of the obtained projection matrix. The analysis revealed that the tomography system can resolve as small as 0.9 cm within the region of interest. Furthermore, the analysis method, together with the Fourier–rectangular function, quantitatively demonstrated the resolution of the azimuthal mode number of the spatial structure as a function of the radius. These analytical results indicate that the present tomography system can resolve the typical drift wave turbulence observed in a linear magnetized plasma device, PANTA, whose wavelength is as short as 2.5 cm.

DOI： 10.1088/1361-6501/ae26b9

Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Journal of Fusion Energy  

<jats:title>Abstract</jats:title>
<jats:p>To study microscale turbulence, two non-invasive scattering instruments that use electromagnetic waves in the microwave to millimeter-wave range have been installed at the Large Helical Device. One instrument is a Doppler reflectometer, which is suitable for observing turbulence with relatively low wavenumbers. Three circuit systems were constructed for this instrument. The Doppler reflectometer allows a very large number of spatial points (more than 30) to be observed simultaneously in the radial direction and toroidal correlation analysis to be conducted. The other instrument is a two-frequency millimeter-wave scattering system, which was developed to observe turbulence at relatively high wavenumbers. This scattering system has multiple antennas in a vacuum vessel. It can be used to study turbulence anisotropy or, in combination with the Doppler reflectometer, the response of turbulence at various scales.</jats:p>

DOI： 10.1007/s10894-025-00523-1

Scopus

Publishing type：Research paper (scientific journal)   Publisher：Springer Science and Business Media LLC  

The structural formation of magnetized high-temperature plasmas, which are ubiquitous in nature, is often controlled by turbulence. Such plasma turbulence is characterized by multiple scales and thus cross-scale nonlinear interactions are key to understanding its dynamics and structural formation. This is also the case for laboratory plasmas, for which multi-scale turbulence data are most abundant. The interactions of components on the meso- and ion-gyroradius (micro)-scales have been extensively studied. Here we report, the experimental discovery of the cross-scale nonlinear interactions between fluctuations at micro-scale and hyper-fine (HF)-scale (whose scale is about an electron gyroradius). Cross-scale bifurcation is found, in which micro-scale turbulence is suppressed, and the amplitude of HF-scale turbulence simultaneously increases. There is also an abrupt change in the isotropy of HF-scale components. We discuss a possible mechanisms in which nonlinear interactions, related to the weakening of the electric field generated by micro-scale turbulence, enhance HF-scale turbulence.

DOI： 10.1038/s42005-025-02245-4

Scopus

Other Link： https://www.nature.com/articles/s42005-025-02245-4

Publisher：Fusion Engineering and Design  

In order to save flux consumption and lessen engineering requirements, the achievable toroidal loop voltage tends to be lowered in modern tokamaks. In such devices, careful optimization of the plasma startup scenario is often required to achieve a successful tokamak discharge. The PLATO tokamak, which is recently built at Kyushu University, also faces challenges in the plasma startup. PLATO employs an air-cored central solenoid that limits the loop voltage and creates stray magnetic fields. In addition, only simple capacitor banks with modest maximum charging voltages are available for the generation of the poloidal magnetic and toroidal electric fields in the initial operation. To realize the breakdown and plasma current ramp-up under those constraints, the configuration of coils and capacitor bank settings has been optimized to produce sufficient loop voltage while minimizing the stray field inside the vacuum chamber at the onset of the discharge. Through this optimization, a tokamak discharge with a plasma current of 30 kA and a pulse duration of 20 ms has been achieved.

DOI： 10.1016/j.fusengdes.2025.115222

Scopus

Publisher：Plasma Physics and Controlled Fusion  

In this study, we investigate the spatiotemporal dynamics of fluctuations in the presence of a toroidally directed zonal flow using two pairs of radial capacitive arrays with finer spatial resolution than the observed toroidal zonal flow. Conditional sampling is employed to evaluate frequency- and time-resolved behavior with respect to the toroidal zonal flow phase. We identify three distinct types of fluctuations, each exhibiting different spatiotemporal dynamics synchronized with the toroidal zonal flow. These modes are modulated in space and time in accordance with the phase of the toroidal zonal flow. The mechanism underlying the formation of spatiotemporal structures is discussed in terms of long-range correlations and wave-kinetic characteristics

DOI： 10.1088/1361-6587/adf009

Scopus

Publisher：Plasma Physics and Controlled Fusion  

A method of decomposition of abrupt transport phenomena in magnetized plasmas is demonstrated by using a data-driven science method. The set of turbulence data is produced for the Kelvin-Helmholtz turbulence in a cylindrical plasma by using a numerical nonlinear simulation, which is based on an extended Hasegawa-Wakatani model. The obtained turbulence state is accompanied by limit-cycle-like dynamics, and the abrupt particle transport phenomena is observed. We apply a singular value decomposition to the turbulence data with a clustering method, and the turbulence state is decomposed into background, zonal flows, and coherent and incoherent modes. The contribution of these decomposed modes to the particle transport is evaluated for all the combinations of the modes. It is found that the abrupt transport is mainly driven by the incoherent mode, and that it determines the spatio-temporal scale of the abrupt transport for the presented case.

DOI： 10.1088/1361-6587/add594

Web of Science

Scopus

Language：English  

DOI： 10.1038/s41598-025-04755-1

PubMed

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

Aiming at fully utilizing the spatial resolution of a fast charge exchange recombination spectroscopy (CXS) system, a full-image operation scenario is newly developed. A data analysis procedure is presented. Calibration using a white light source and a neon-enclosed hollow cathode lamp is performed, which provides vertical-stripe separation, horizontal-pixel vs wavelength conversion, and stripe sensitivity. Data from fast and slow CXS systems are compared, providing qualitatively equivalent emission spectra in the same time-integration duration. An asymptotically decreasing trend of noise level with respect to the amount of ensemble averaging (time integration) is confirmed for the fast CXS system. Radial profiles of emission intensity, ion temperature, and toroidal velocity obtained from slow and fast CXS systems are compared, and reasonable agreement is found.

DOI： 10.1063/5.0251924

Web of Science

Scopus

PubMed

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

<p>The Cormack inversion is a well-known procedure for tomography. However, the Fourier-Bessel function expansion could be preferably used for plasma tomography, since plasma images obtained with the Cormack inversion tend to produce a false plasma image in the boundary without regards to its advantage: the direct structural analysis of plasma is possible using a set of line-integrated data. Here we propose an application of function modification technique to revive the advantage of the Cormack inversion and present successful experimental result in a tomography system of a cylindrical plasma.</p>

DOI： 10.1585/pfr.20.1201052

Scopus

CiNii Research

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

<p>We conducted energetic particle-induced MHD burst excitation experiments on the Large Helical Device (LHD) to investigate how these events—and the associated deformations of the ion velocity distribution—depend on the direction of the magnetic field. Wavelet cross-power spectrum analysis revealed that the toroidal propagation direction of the MHD bursts reverses with the magnetic field. Fast charge exchange recombination spectroscopy (fast-CXRS) was used to measure the temporal evolution of the carbon ion velocity distribution function. A bipolar signature in velocity space, as well as the resonant velocity, varied with the magnetic field direction. A strong correlation was observed between the chirping frequency and the corresponding resonant velocity, and the phase velocity was found to closely match the resonant velocity, including the sign of the propagation direction.</p>

DOI： 10.1585/pfr.20.1202053

Scopus

CiNii Research

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

<p>This paper investigates the density dependence of low-frequency edge harmonic oscillations (LF-EHOs) in the Large Helical Device (LHD) H-mode plasmas. A series of discharges with varying density settings reveal that the L-H mode transition occurs above a certain density threshold, and LF-EHOs appear only above an even higher density threshold. In discharges exhibiting LF-EHOs, the cross-coherence between density and magnetic fluctuations for the fundamental frequency remains consistently high across the entire density range. In contrast, the cross-coherence for the second harmonic exhibits an increasing trend with higher plasma density. These findings suggest that the LF-EHO potentially plays a noticeable role in edge profile saturation.</p>

DOI： 10.1585/pfr.20.1402044

Scopus

CiNii Research

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Proceedings of the National Academy of Sciences  

A key ingredient for realizing a magnetically confined tritium-deuterium plasma fusion reactor is plasma heating by fusion-born high-energy helium ions, as a chained cycle of “nuclear burning.” Efficient collisionless plasma heating by high-energy particles is anticipated when their energy is directly transferred to the plasma through waves. Those processes often involve nonlinear structure formations in phase-space, spanned by real-space and velocity-space coordinates, that significantly influence heating efficiency. To date, experimental knowledge of phase-space structure formation physics is severely limited, due to lack of experimental diagnostic schemes. Here, state-of-the-art hardware and software approaches for phase-space perturbative structure detection are introduced. A bifurcation in the phase-space structure formation is found, which affects the plasma heating efficiency. A confinement magnetic field structure is shown to be a potential knob for nuclear-burning control through phase-space manipulation.

DOI： 10.1073/pnas.2408112121

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Scopus

PubMed

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

Recently, experiments on basic plasma physics issues for solving future problems in fusion energy have been performed on a Large Helical Device. There are several problems to be solved in future devices for fusion energy. Emerging issues in burning plasma are: alpha-channeling (ion heating by alpha particles), turbulence and transport in electron dominant heating helium ash exhaust, reduction of the divertor heat load. To solve these problems, understanding the basic plasma physics of (1) wave-particle interaction through (inverse) Landau damping, (2) characteristics of electron-scale (high-k) turbulence, (3) ion mixing and the isotope effect, and (4) turbulence spreading and detachment, is necessary. This overview discusses the experimental studies on these issues and turbulent transport in multi-ion plasma and other issues in the appendix.

DOI： 10.1088/1741-4326/ad3a7a

Web of Science

Scopus

Publishing type：Research paper (scientific journal)   Publisher：IOP Publishing  

Abstract

Electron-scale turbulence, whose wavelength is the electron Larmor radius, is thought to have the potential to cause stiffness in an electron temperature gradient and degrade the confinement of future burning plasma in which the electron heating by alpha particles is dominant. The dependence of electron-scale turbulence and electron heat flux on the electron temperature inverse gradient length , were investigated. The electron temperature gradient was successfully varied in the range of by controlling the injection power of on/off-axis electron cyclotron heating. The results show a significant increase in the electron-scale turbulence with increasing , especially in conditions where Electron Temperature Gradient (ETG) instability is linearly unstable, suggesting the presence of ETG turbulence at high . The electron heat flux also increases steeply with increasing . In addition, the electron-scale turbulence is observed even at , which is stable in linear GKV calculations. Finding the cause of this phenomenon is an interesting task for the future.

DOI： 10.1088/1741-4326/ad5d7c

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Scopus

Other Link： https://iopscience.iop.org/article/10.1088/1741-4326/ad5d7c/pdf

Publishing type：Research paper (scientific journal)   Publisher：IOP Publishing  

Abstract

A new self-sustained divertor oscillation is discovered in magnetic island induced detached plasmas in the Large Helical Device. The divertor oscillation is found to be a self-regulation of the width of an edge magnetic island accompanied by detachment-attachment transitions. The modified Rutherford equation combined with an ad-hoc bootstrap current equation is introduced to describe the divertor oscillation as a predator–prey model between the magnetic island width and a remnant X-point bootstrap current. The model successfully reproduces the experimental observations in terms of the oscillation frequency, the phase relation between variables, and the oscillation amplitude.

DOI： 10.1088/1741-4326/ad5367

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Scopus

Other Link： https://iopscience.iop.org/article/10.1088/1741-4326/ad5367/pdf

Publishing type：Research paper (scientific journal)  

DOI： 10.1088/1741-4326/ad3971

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

<p>A new method is proposed to analyze plasma image of tomography using modified Fourier-Bessel Functions (FBF) instead of the original FBF analysis. The application of the method to an assumed plasma image shows that the difference between the original and the fitting image is improved considerably to that of FBF, owing to giving a better fitting inside the plasma and eliminating the ghost values outside the plasma, without any disadvantages in analysis of plasma structures and patterns.</p>

DOI： 10.1585/pfr.19.1201014

Web of Science

Scopus

CiNii Research

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

<p>This article presents a new method, called extended Rotational Movement Analysis (e-RoMA), to estimate the velocity field of a cylindrical plasma from its two-dimensional images based on Fourier-Bessel function expansion. The proposed method is applied to tomography images of a plasma produced in a linear cylindrical device PANTA and succeeded in obtaining the two- dimensional velocity field. The first results suggest an issue to be investigated, that is, the presence of azimuthal asymmetry in the velocity field.</p>

DOI： 10.1585/pfr.19.1201005

Web of Science

Scopus

CiNii Research

Publishing type：Research paper (scientific journal)   Publisher：IOP Publishing  

Abstract

Dynamics in magnetically confined plasmas are dominated by turbulence driven by spatial inhomogeneities in density and temperature. Simultaneous measurement of velocity field and density fluctuations is necessary to observe the particle transport, but the measurement of the velocity field fluctuations is often challenging. Here, we propose a method to estimation velocity field fluctuations from density fluctuations by using plasma turbulence simulations and a deep technique learning. In order to take multi-scale characteristics into account, the several number of spatial filters are used in the convolutional neural network. The velocity field fluctuations are successfully predicted, and the particle transport estimated from the predicted velocity field fluctuations is within 93.1% accuracy. The deep learning could be used for the prediction of physical variables which are difficult to be measured.

DOI： 10.1088/1361-6587/acff7f

Web of Science

Scopus

Other Link： https://iopscience.iop.org/article/10.1088/1361-6587/acff7f/pdf

Publishing type：Research paper (scientific journal)   Publisher：IOP Publishing  

Abstract

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

Web of Science

Scopus

Other Link： https://iopscience.iop.org/article/10.1088/1361-6587/ad074a/pdf

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

Publishing type：Research paper (scientific journal)   Publisher：AIP Publishing  

This article presents a method to estimate the rotational velocity of a cylindrical plasma from its two-dimensional images by an extended use of the Fourier-rectangular function transform, which was proposed to analyze the structure and dynamics of a cylindrical plasma [K. Yamasaki etal., J. Appl. Phys. 126, 043304 (2019)]. The proposed method is applied to tomography images of plasmas produced in a linear cylindrical device and succeeds in obtaining the radial distribution of rotational velocity and its fluctuations, providing an interesting finding, that is, the existence of flow modulation associated with m=1 mode fluctuations.

DOI： 10.1063/5.0165318

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Scopus

Publishing type：Research paper (scientific journal)   Publisher：AIP Publishing  

Nonlinear fluid simulation of drift wave turbulence in the presence of symmetry breaking particle source is performed for the cylindrical magnetized plasmas. It is demonstrated that the symmetry breaking of the system directly affects the selection rule of structure formations, the streamers disappear, and the zonal flows are enhanced in the case with the symmetry breaking. The symmetry breaking is introduced in the flux-driven simulation by inducing the particle source whose amplitude depends on the azimuthal angle. The symmetry breaking mode is driven stationarily, and the nonlinear process of the drift waves is significantly modified. By scanning the amplitude of the symmetry breaking source, the structure formation processes are systematically investigated. In addition, the nonlinear forces of the turbulence show the relaxation of the density gradient and the drive of the perpendicular/parallel flows in a two-dimensional manner.

DOI： 10.1063/5.0150748

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Scopus

Language：English   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：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

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

In this paper, a phenomenology of competing behavior between the geodesic acoustic mode (GAM) and the limit-cycle oscillation (LCO) is presented. Before the LCO occurs, the GAM can grow to the observable amplitude via the turbulent Reynolds stress force. Approaching the L-H transition, the LCO is excited and the GAM decays. In the LCO phase, the GAM driving force is possibly suppressed by the nonlocal turbulence amplitude modulation by the LCO.

DOI： 10.1088/1361-6587/ac9333

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Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Springer Science and Business Media LLC  

Abstract

The energy transfer from wave to particle occurs in collisionless plasma through the interaction between particle and wave, associated with the deformation of ion velocity space from Maxwell-Boltzmann distribution. Here we show the direct observation of mass-dependent collisionless energy transfer via Landau and transit-time damping in a laboratory plasma. The Landau and transit-time damping are confirmed by the bipolar velocity-space signature of the ion velocity distribution function, measured by fast charge exchange spectroscopy with a time resolution less than ion-ion collision time. The excellent agreement between the resonant phase velocity evaluated from the bipolar velocity-space signature and the wave’s phase velocity, estimated from the frequency of the magnetohydrodynamics oscillation measured with the plasma displacement is clear evidence for the Landau damping. The energy transfer from solitary wave to fully ionized carbon impurity ions is larger than that of bulk ions 2-3 times due to heavier mass.

DOI： 10.1038/s42005-022-01008-9

Other Link： https://www.nature.com/articles/s42005-022-01008-9

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

Since high density operation is advantageous for building an efficient fusion reactor, understanding the density limit in tokamaks has been seen as one of the most important issues. This paper reports a series of measurements around the last-closed flux surface (LCFS) in L-mode plasmas by using a thermal helium beam diagnostic. Fluctuation analysis has been employed to characterize the poloidal flow and the turbulence structure. A reversal of the poloidal flow in the scrape-off layer and concomitant cooling of the outer divertor plasma are observed as the density is raised. While, in the confined region, the change in the density barely affects the poloidal flow, a higher density shifts the fluctuation power spectral densities toward lower frequencies and wave numbers. The eddy tilting of this region is consistent with what is expected from the magnetic shear effect. A radially coherent low frequency mode appears in the case of the highest density investigated in this study (n ¯ e / n e, GW = 0.51), and higher frequencies near the LCFS are modulated by this mode.

DOI： 10.1063/5.0098294

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Scientific Reports  

Self-organized structure formation in magnetically confined plasmas is one of the most attractive subjects in modern experimental physics. Nonequilibrium media are known to often exhibit phenomena that cannot be predicted by superposition of linear theories. One representative example of such phenomena is the hydrogen isotope effect in fusion plasmas, where the larger the mass of the hydrogen isotope fuel is the better the plasma confinement becomes, contrary to what simple scaling models anticipate. In this article, threshold condition of a plasma structure formation is shown to have a strong hydrogen isotope effect. To investigate the underlying mechanism of this isotope effect, the electrostatic potential is directly measured by a heavy ion beam probe. It is elucidated that the core electrostatic potential transition occurs with less input power normalized by plasma density in plasmas with larger isotope mass across the structure formation. This observation is suggestive of the isotope effect in the radial electric field structure formation.

DOI： 10.1038/s41598-022-09526-w

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PubMed

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

This conference report summarizes the contributions to, and discussions at the joint meeting of the 9th Asia Pacific-Transport Working Group (APTWG) & EU-US Transport Task Force (TTF) workshop held online, hosted by Kyushu University, Japan, during 6-9 July 2021. The topics of the meeting were organized under five main topics: (1) isotope effect on transport and physics on isotope mixture plasma, (2) turbulence spreading and coupling in core-edge-SOL, (3) interplay between magnetohydrodynamic topology/instability and turbulent transport, (4) interaction between energetic particle driven instability and transport, (5) model reduction and experiments for validation.

DOI： 10.1088/1741-4326/ac3f19

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

A heating source with off-axis electron cyclotron heating (ECH) alone produced a plasma with a quasi-steady-state hollow electron-temperature profile in the Large Helical Device. The clear formation of this quasi-steady-state hollow electron-temperature profile can be explained by adding the outward heat convection term to the diffusion term, as a simple model to describe the electron heat flux, using the energy conservation equation. In addition, we directly observed the non-locality of the non-diffusive (convective) contribution in transient electron thermal transport in the condition that power-modulated on-axis ECH was applied to the plasma sustained by off-axis ECH. The experimentally evaluated flux-gradient relation shows two different positive values of the electron heat flux at zero temperature gradient by going back and forth between positive and negative temperature gradient regions in the transport hysteresis phenomenon.

DOI： 10.1063/5.0074351

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