記述言語：英語  

DOI： 10.1038/s41598-025-04755-1

PubMed

記述言語：英語   出版者・発行元：Review of Scientific Instruments  

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

記述言語：英語   出版者・発行元：Proceedings of the National Academy of Sciences of the United States of America  

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

Web of Science

Scopus

PubMed

出版者・発行元：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

出版者・発行元：Nuclear Fusion  

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 R ax / L T e , were investigated. The electron temperature gradient was successfully varied in the range of − 3 < R ax / L T e < 12 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 R ax / L T e , especially in conditions where Electron Temperature Gradient (ETG) instability is linearly unstable, suggesting the presence of ETG turbulence at high R ax / L T e . The electron heat flux also increases steeply with increasing R ax / L T e . In addition, the electron-scale turbulence is observed even at R ax / L T e ∼ 0 , 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

Web of Science

Scopus

出版者・発行元：Nuclear Fusion  

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

Web of Science

Scopus

記述言語：英語   出版者・発行元：一般社団法人 プラズマ・核融合学会  

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.

DOI： 10.1585/pfr.19.1201014

Web of Science

Scopus

CiNii Research

記述言語：英語   出版者・発行元：一般社団法人 プラズマ・核融合学会  

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.

DOI： 10.1585/pfr.19.1201005

Web of Science

Scopus

CiNii Research

出版者・発行元：Plasma Physics and Controlled Fusion  

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

出版者・発行元：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

Web of Science

Scopus

出版者・発行元：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

Web of Science

Scopus

出版者・発行元：Journal of Applied Physics  

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 e t a l ., 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

Web of Science

Scopus

出版者・発行元：Physics of Plasmas  

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

Web of Science

Scopus

記述言語：英語   出版者・発行元：一般社団法人 プラズマ・核融合学会  

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.

DOI： 10.1585/pfr.18.1202036

Web of Science

Scopus

CiNii Research

記述言語：英語   出版者・発行元：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

Web of Science

Scopus

CiNii Research

記述言語：英語   出版者・発行元：一般社団法人 プラズマ・核融合学会  

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.

DOI： 10.1585/pfr.17.1301106

Web of Science

Scopus

CiNii Research

出版者・発行元：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

Web of Science

Scopus

出版者・発行元：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

Web of Science

Scopus

記述言語：英語   出版者・発行元：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

Web of Science

Scopus

PubMed

出版者・発行元：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

Web of Science

Scopus

出版者・発行元：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

Web of Science

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