記述言語：英語  

CiNii Research

記述言語：英語  

In this study, we discover a turbulence transition and its role in the isotope effects in the Large Helical Device (LHD). The turbulence levels and turbulence-driven transports decrease with an increase in the electron density to a specific density; however, they increase in regimes above the density. Density scan experiments and gyrokinetic/MHD simulations reveal that the observed ion-scale turbulence is the ion-temperature gradient (ITG) turbulence below the density and resistive-interchange (RI) above the density. Negligible isotope effects in the turbulences and transports of hydrogen (H) and deuterium (D) plasma were found in the ITG regime. However, clear isotope effects were observed in the RI regime. In the RI regime, the turbulences and transports were clearly suppressed, and the confinements were improved in D plasma. The reduction of the turbulence level in the RI regime of the D plasma is likely due to the heavier ion mass and lower resistivity in D plasma, which suppress the linear growth rate of RI turbulence.

CiNii Research

記述言語：英語  

P2-15

CiNii Research

記述言語：その他   掲載種別：研究論文（学術雑誌）   出版者・発行元：一般社団法人 プラズマ・核融合学会  

Systematic experiments were carried out in the 23rd Large Helical Device experiment campaign to explore the interaction between fast ions and ion-scale turbulence focusing on the impact of varying the power of the neutral beam (NB) injection. During the NB heating, two distinct phases of turbulence reduction were observed. The first phase was a transient reduction of about 10 msec immediately following the initiation of the NB injection. This reduction in turbulence was noticeable even while the electron density and temperature profiles remained nearly unchanged. The second phase involved a continuous reduction that maintained a lower turbulence level throughout the duration of the NB injection. The physics mechanisms underlying these phenomena are not fully understood yet. However, it is likely that two distinct processes of turbulence stabilization, each corresponding to the transient and continuous reduction phases, are at work due to the influence of the fast ions.

DOI： 10.1585/pfr.18.2402069

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記述言語：その他   掲載種別：研究論文（学術雑誌）   出版者・発行元：一般社団法人 プラズマ・核融合学会  

A digital phase analysis technique was applied to a 119-μm wavelength far-infrared (FIR) laser interferometer on the Large Helical Device (LHD). High-density plasma measurement without phase jumping was achieved, representing an improvement over measurements made using a conventional analog phase counter. Digital phase analysis became operational at a quarter of the threshold of the analog phase counter. The phase sensitivity was also improved using digital analysis because the noise level was reduced to approximately half that of the conventional analog phase counter. The improved phase sensitivity enabled measurement of small-amplitude fluctuations.

DOI： 10.1585/pfr.18.1402062

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記述言語：その他   掲載種別：研究論文（学術雑誌）   出版者・発行元：Nuclear Fusion  

Abstract

Recently an improved confinement regime, characterized by reduced turbulent fluctuations has been observed in the Large Helical Device upon the injection of boron powder into the plasma (Nespoli et al 2022 Nat. Phys.18 350–56). In this article, we report in more detail the experimental observations of increased plasma temperature and the decrease of turbulent fluctuations across the plasma cross section, on an extended database. In particular, we compare powders of different materials (B, C, BN), finding similar temperature improvement and turbulence response for the three cases. Modeling of the powder penetration into the plasma and of neoclassical electric field and fluxes support the interpretation of the experimental results. Additionally, we report evidence of the temperature improvement increasing with powder injection rates and decreasing for both increasing density and heating power. Though, plasma turbulence response varies depending on the initial conditions of the plasma, making it difficult to draw an inclusive description of the phenomenon.

DOI： 10.1088/1741-4326/acd465

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その他リンク： https://iopscience.iop.org/article/10.1088/1741-4326/acd465/pdf

記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：一般社団法人 プラズマ・核融合学会  

CO2 laser interferometers are a promising option for high-density plasma measurements. However, in lowand middle-density measurements, noise due to mechanical vibrations is a serious problem. To remove this noise, we developed a two-color laser imaging interferometer using a CO2 laser and quantum cascade (QC) laser, called the CO2/QC laser imaging interferometer, through benchtop experiments and installed it in the Large Helical Device (LHD). Benchtop experiments provided optical design guidelines for the CO2/QC laser imaging interferometer to minimize the influence of the unstable output wavelength of the QC laser. The optical system in LHD was designed according to this guideline, and the vibration noise was successfully reduced to 2:80 * 1018 m-3. We also demonstrate measurement examples of hollowed and peaked electron density profiles evaluated using Abel inversion and macro-scale instability. This is the first study to present the measurement results of high-temperature plasma using a CO2/QC two-color laser interferometer. The study outcomes provide important insights for the development of two-color laser interferometers in future fusion devices.

DOI： 10.1585/pfr.17.1402107

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リポジトリ公開URL： https://hdl.handle.net/2324/7183303

記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1007/s00339-022-06136-1

記述言語：その他   掲載種別：研究論文（学術雑誌）  

DOI： 10.1088/1741-4326/ac42f3

記述言語：その他   掲載種別：研究論文（学術雑誌）   出版者・発行元：Physical Review Letters  

Turbulence spreading into the edge stochastic magnetic layer induced by magnetic fluctuation is observed at the sharp boundary region in the large helical device. The density fluctuation excited at the sharp boundary region with a large pressure gradient does not propagate into the boundary region due to the blocking of turbulence spreading by the large second derivative of the pressure gradient. Once the magnetic fluctuation appears at the boundary, the density fluctuation begins to penetrate the edge stochastic layer and the second derivative of the pressure gradient also decreases. The increase of density fluctuation in this layer results in the broadening and reduction of the peak divertor heat load. It is demonstrated that magnetic fluctuation plays a key role in controlling the turbulence spreading at the boundary of plasma which contributes to the reduction of divertor heat load.

DOI： 10.1103/PhysRevLett.128.125001

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記述言語：その他   掲載種別：研究論文（学術雑誌）   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

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出版者・発行元：Nature Physics  

In state-of-the-art stellarators, turbulence is a major cause of the degradation of plasma confinement. To maximize confinement, which eventually determines the amount of nuclear fusion reactions, turbulent transport needs to be reduced. Here we report the observation of a confinement regime in a stellarator plasma that is characterized by increased confinement and reduced turbulent fluctuations. The transition to this regime is driven by the injection of submillimetric boron powder grains into the plasma. With the line-averaged electron density being kept constant, we observe a substantial increase of stored energy and electron and ion temperatures. At the same time, the amplitude of the plasma turbulent fluctuations is halved. While lower frequency fluctuations are damped, higher frequency modes in the range between 100 and 200 kHz are excited. We have observed this regime for different heating schemes, namely with both electron and ion cyclotron resonant radio frequencies and neutral beams, for both directions of the magnetic field and both hydrogen and deuterium plasmas.

DOI： 10.1038/s41567-021-01460-4

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記述言語：その他   掲載種別：研究論文（学術雑誌）  

DOI： 10.1088/1741-4326/ac2bbc

記述言語：その他   掲載種別：研究論文（学術雑誌）  

DOI： 10.1088/1361-6587/abffb6

記述言語：その他   掲載種別：研究論文（学術雑誌）  

DOI： 10.21203/rs.3.rs-614131/v1

記述言語：その他   掲載種別：研究論文（学術雑誌）  

DOI： 10.1088/1748-0221/15/01/c01045

リポジトリ公開URL： https://hdl.handle.net/2324/7183307

記述言語：その他   掲載種別：研究論文（学術雑誌）  

DOI： 10.1088/1361-6587/ab5bae

記述言語：その他   掲載種別：研究論文（学術雑誌）  

DOI： 10.1088/1741-4326/ab4237

記述言語：その他   掲載種別：研究論文（学術雑誌）  

DOI： 10.4028/www.scientific.net/msf.910.96

記述言語：その他   掲載種別：研究論文（学術雑誌）  

DOI： 10.1007/s00339-018-1577-6