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A radiative collapse predictor has been developed using a machine-learning model based on high-density plasma experiments in the Large Helical Device (LHD). Concurrently, the physical background of radiative collapse was discussed based on the distinct features extracted by a sparse modeling, which is one of the frameworks of data-driven science. Electron density, CIV and OV line emissions, and electron temperature at the plasma edge have been extracted as the key parameters of radiative collapse. Those parameters are relevant to the physical knowledge that the major cause of radiative collapse is the enhancement of radiative loss by light impurities in the plasma-edge region. Using these four parameters, the likelihood of occurrence of radiative collapse has been estimated. The behavior of plasma at the edge—in particular, the carbon impurities outside the last closed flux surface—has been evaluated using EMC3-EIRENE code for the phase with increasing likelihood, that is, the plasma is getting close to the collapse. It is shown that the radiation caused by the C3+ ion, which corresponds to the CIV emission, is enhanced in the region where electron temperature is around 10 eV., source:https://doi.org/10.1585/pfr.16.2402010

The effects of the stochastic magnetic field in a plasma center produced by electron cyclotron current drive (ECCD) on transport have been revealed. Because the electron temperature profile is flat in the core region, in the case of counter-directed ECCD (ctr-ECCD) against the toroidal magnetic field, the magnetic field is stochastic in the core region with rotational transform ᵼ ∼ 1/3. The particle diffusion coefficient of the ctr-ECCD plasma is approximately 20 times as large as that of the plasma without the stochastic magnetic field produced by co-directed ECCD (co-ECCD) at the maximum. Furthermore, in the stochastic magnetic field with ctr-ECCD, counter-directed intrinsic rotation is observed in the plasma with balanced NBI discharge., source:Yoshiaki Ohtani et al 2021 Nucl. Fusion 61 034002, source:https://doi.org/10.1088/1741-4326/abd6b1, identifier:0000-0003-3646-5427

A radiative collapse predictor has been developed using a machine-learning model with high-density plasma experiments in the Large Helical Device (LHD). The model is based on the collapse likelihood, which is quantified by the parameters selected by the sparse modeling, including ne, CIV, OV, and Te,edge. The control system implementing this model has been constructed with a single-board computer to apply this predictor model to the LHD experiment. The controller calculates the collapse likelihood and regulates gas-puff fueling and boosts electron cyclotron resonance heating in real-time. In density ramp-up experiments with hydrogen plasma, high-density plasma has been maintained by the control system while avoiding radiative collapse. This result has shown that the predictor based on the collapse likelihood has the capability to predict a radiative collapse in real-time., source:http://doi.org/10.1585/pfr.17.2402042

source:47th EPS Conference on Plasma Physics P4.1079, source:https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&cad=rja&uact=8&ved=2ahUKEwifjIT1jLr9AhXNZt4KHd5CBDwQFnoECAwQAQ&url=http%3A%2F%2Focs.ciemat.es%2FEPS2021PAP%2Fpdf%2FP4.1079.pdf&usg=AOvVaw3DxzlCmCRXxf3WdAIC0M6K, identifier:0000-0003-1682-1519

source:47th EPS Conference on Plasma Physics O2.103, source:https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&cad=rja&uact=8&ved=2ahUKEwik5L6aibX9AhVVBN4KHWeHDQAQFnoECAwQAQ&url=http%3A%2F%2Focs.ciemat.es%2FEPS2021PAP%2Fpdf%2FO2.103.pdf&usg=AOvVaw0rcW0sXbthmLb0nMM-tppr

We assess the magnetic field configuration in modern fusion devices by comparing experiments with the same heating power, between a stellarator and a heliotron. The key role of turbulence is evident in the optimized stellarator, while neoclassical processes largely determine the transport in the heliotron device. Gyrokinetic simulations elucidate the underlying mechanisms promoting stronger ion scale turbulence in the stellarator. Similar plasma performances in these experiments suggests that neoclassical and turbulent transport should both be optimized in next step reactor designs., source:https://doi.org/10.1103/PhysRevLett.127.225001, identifier:0000-0001-9585-5201

Controlled particulate injections from the PPPL impurity powder dropper (IPD) into the Large Helical Device have demonstrated positive effects on the wall conditions on both an intra and inter-shot basis. Injections over a range of densities, input powers, pulse lengths, heating schemes, injection quantities and main ion species show conclusive evidence of improvement to plasma wall conditions. Successful injections are confirmed by both spectroscopic measurements as well as real-time visible camera signals. In 7 s long plasmas the responses include a reduction in wall recycling as well as a reduction in native impurity content as observed over the course of several discharges. For plasmas longer than 40 s, improvements to the recycling rate and increased impurity control are observed in real time as a consequence of the extended particulate injections. These experiments demonstrate the extended applicability of this solid particulate conditioning technique to the control and maintenance of the plasma wall conditions. In addition they are an important initial step in the development of the real-time boronization technique as a supplement to standard conditioning scenarios., source:R. Lunsford et al 2022 Nucl. Fusion 62 086021, source:http://doi.orig/10.1088/1741-4326/ac6ff5, identifier:0000-0003-3588-6801

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., source:Nespoli, F., Masuzaki, S., Tanaka, K. et al. Observation of a reduced-turbulence regime with boron powder injection in a stellarator. Nat. Phys. 18, 350–356 (2022). https://doi.org/10.1038/s41567-021-01460-4, source:https://doi.org/10.1038/s41567-021-01460-4, identifier:0000-0001-7644-751X

A new multi-tracer technique in the Tracer-Encapsulated Solid Pellet (TESPEL) method has been developed in order to acquire simultaneously the information about the behaviors of various impurities, i.e., to study concurrently the behaviors of low- and mid/high-Z impurities in magnetically confined high-temperature plasmas. In this new technique, an inorganic compound (for example, lithium titanate, Li2TiO3) is proposed to be used as a tracer embedded in the core of the TESPEL, instead of pure elements. The results of the proof-of-principle experiment clearly demonstrate the applicability of the new multi-tracer technique in the TESPEL method for the simultaneous study of behaviors of low- and mid/high-Z impurities in high-temperature plasmas., source:Review of Scientific Instruments 92, 063516 (2021); https://doi.org/10.1063/5.0043495, source:https://doi.org/10.1063/5.0043495, identifier:0000-0003-1682-1519

A bulk charge exchange spectroscopy (BCXS) system using a grism (grating prism) spectrometer has been applied to measure the profile of the deuterium (D) fraction in deuterium and hydrogen (H) mixture plasma in the Large Helical Device. The observed spectrum can be fitted with four Gaussian functions successfully by reduction of free parameters for the least-squares fit. The plasma flow velocity and ion temperature profile measured by charge exchange spectroscopy using carbon impurity are used for estimation of the wavelength shift of hot components to reduce the free parameter. The ion temperature is used to estimate the apparent wavelength shift due to the energy dependent emission cross section only and is not used to set the Doppler width for H and D in the fitting. The sensitivity of the evaluated D fraction on the velocity is increased for a higher D fraction. The error of the D fraction is calculated from the error in the fitted parameter and sensitivity on the velocity of the hot component. The difference in the profile and time trace of the D fraction with D pellet and H pellet injection was observed clearly by BCXS using a grism spectrometer., source:Review of Scientific Instruments 92, 063509 (2021); https://doi.org/10.1063/5.0043607, source:https://doi.org/10.1063/5.0043607, identifier:0000-0002-5113-9710

0000-0001-5669-1937, In this paper, a tomography approach aiming at reconstructing a phase-space structure is proposed. For the phase-space resolved diagnostic system, a signal must be decomposed in real-space, velocity-space, and time; therefore, it is challenging to obtain a sufficiently high signal intensity in a single detector bin. To overcome this difficulty, three different sets of data having different integration directions in real-space, velocity-space, and time are simultaneously used, and a reconstruction of the original structure in the phase-space is attempted by a tomographic manner. The proposed method is demonstrated using a synthetic dataset in the actual diagnostic setup in the Large Helical Device. Time evolution of a phase-space perturbation induced by the Landau damping, which is caused by energetic particle-driven magnetohydrodynamic bursts, is successfully reconstructed by this method. Robustness against realistic diagnostic noise is also presented.

The abrupt and strong excitation of the geodesic acoustic mode (GAM) has been found in the large helical device (LHD), when the frequency of a chirping energetic particle-driven GAM (EGAM) approaches twice that of the GAM frequency. The temporal evolution of the phase relation between the abrupt GAM and the chirping EGAM is common in all events. The result indicates a coupling between the GAM and the EGAM. In addition, the nonlinear evolution of the growth rate of the GAM is observed, and there is a threshold in the amplitude of the GAM for the appearance of nonlinear behavior. A threshold in the amplitude of the EGAM for the abrupt excitation of the GAM is also observed. According to one theory (Lesur et al 2016 Phys. Rev. Lett. 116 015003, Itoh et al 2016 Plasma Phys. Rep. 42 418) the observed abrupt phenomenon can be interpreted as the excitation of the subcritical instability of the GAM. The excitation of a subcritical instability requires a trigger and a seed with sufficient amplitude. The observed threshold in the amplitude of the GAM seems to correspond with the threshold in the seed, and the threshold in the amplitude of the EGAM seems to correspond with the threshold in the magnitude of the trigger. Thus, the observed threshold supports the interpretation that the abrupt phenomenon is the excitation of a subcritical instability of the GAM., source:T. Ido et al 2017 Nucl. Fusion 57 072009, source:https://doi.org/10.1088/1741-4326/aa665a

In a collisionless plasma, it is known that linearly stable modes can be destabilized (subcritically) by the presence of structures in phase space. However, nonlinear growth requires the presence of a seed structure with a relatively large threshold in amplitude. We demonstrate that, in the presence of another, linearly unstable (supercritical) mode, wave-wave coupling can provide a seed, which is significantly below the threshold, but can still grow by (and only by) the collaboration of fluid and kinetic nonlinearities. By modeling the subcritical mode kinetically, and the impact of the supercritical mode by simple wave-wave coupling equations, it is shown that this new kind of subcritical instability can be triggered, even when the frequency of the supercritical mode is rapidly sweeping. The model is applied to the bursty onset of geodesic acoustic modes in a LHD experiment. The model recovers several key features such as relative amplitude, time scales, and phase relations. It suggests that the strongest bursts are subcritical instabilities, driven by this mechanism of combined fluid and kinetic nonlinearities., source:Nonlinear Excitation of Subcritical Instabilities in a Toroidal Plasma M. Lesur, K. Itoh, T. Ido, M. Osakabe, K. Ogawa, A. Shimizu, M. Sasaki, K. Ida, S. Inagaki, S.-I. Itoh, and the LHD Experiment Group Phys. Rev. Lett. 116, 015003 – Published 8 January 2016, source:https://doi.org/10.1103/PhysRevLett.116.015003

Abrupt and strong excitation of a mode has been observed when the frequency of a chirping energetic-particle driven geodesic acoustic mode (EGAM) reaches twice the geodesic acoustic mode (GAM) frequency. The frequency of the secondary mode is the GAM frequency, which is a half-frequency of the primary EGAM. Based on the analysis of spatial structures, the secondary mode is identified as a GAM. The phase relation between the secondary mode and the primary EGAM is locked, and the evolution of the growth rate of the secondary mode indicates nonlinear excitation. The results suggest that the primary mode (EGAM) contributes to nonlinear destabilization of a subcritical mode., source:Strong Destabilization of Stable Modes with a Half-Frequency Associated with Chirping Geodesic Acoustic Modes in the Large Helical Device T. Ido, K. Itoh, M. Osakabe, M. Lesur, A. Shimizu, K. Ogawa, K. Toi, M. Nishiura, S. Kato, M. Sasaki, K. Ida, S. Inagaki, S.-I. Itoh, and the LHD Experiment Group Phys. Rev. Lett. 116, 015002 – Published 8 January 2016, source:https://doi.org/10.1103/PhysRevLett.116.015002

Effects of low-n magnetohydrodynamic instabilities on plasma performance have been assessed in the regime where an achieved beta value is limited by instabilities. The unstable regime of an ideal interchange mode is characterized by enhanced magnetic hill and reduced magnetic shear. Experiments have clarified that (i) low-n modes are significantly destabilized in the ideal-unstable configurations and lead to degradation of central beta by at most 60%, and (ii) the degree of their damages strongly depends on the mode rotation velocity. The occurrence of the minor collapse is independent of an existence of an error field., source:S. Sakakibara et al 2015 Nucl. Fusion 55 083020, source:https://doi.org/10.1088/0029-5515/55/8/083020, identifier:0000-0002-3306-0531

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., source:K. Ida et al 2022 Nucl. Fusion 62 037001, source:https://doi.org/10.1088/1741-4326/ac3f19, identifier:0000-0002-0585-4561

In recent deuterium experiments on the large helical device (LHD), we succeeded in expanding the temperature domain to higher regions for both electron and ion temperatures. Suppression of the energetic particle driven resistive interchange mode (EIC) by a moderate electron temperature increase is a key technique to extend the high temperature domain of LHD plasmas. We found a clear isotope effect in the formation of the internal transport barrier in high temperature plasmas. A new technique to measure the hydrogen isotope fraction was developed in the LHD in order to investigate the behavior of the isotope mixing state. The technique revealed that the non-mixing and the mixing states of hydrogen isotopes can be realized in plasmas. In deuterium plasmas, we also succeeded in simultaneously realizing the formation of the edge transport barrier (ETB) and the divertor detachment. It is found that resonant magnetic perturbation plays an important role in the simultaneous formation of the ETB and the detachment. Contributions to fusion reactor development from the engineering point of view, i.e. negative-ion based neutral beam injector research and the mass balance study of tritium, are also discussed., source:https://doi.org/10.1088/1741-4326/ac3cda, identifier:0000-0001-5220-947X

Impurity behavior is studied in a variety of LHD (Large Helical Device) long pulse discharges, i.e. standard hydrogen plasmas, super dense core plasmas, helium plasmas with ICH (Ion Cyclotron Frequency Heating), multi-species plasmas mixed with H and He. Density scan experiments show a specific density range of impurity accumulation for only hydrogen discharges. Strong suppression of impurity accumulative behavior is observed in high temperature plasmas with high power heating. The main contributions to impurity transport are extracted by a comprehensive study on impurity behavior, i.e. investigating the critical conditions for impurity accumulation and the parameter dependences. It is found that the impurity behavior is determined by three dominant contributions, i.e. neoclassical transport mainly depending on radial electric field, turbulent transport increasing with heating power and impurity screening at high edge collisionality in the ergodic layer. The mapping of impurity behavior on n-T (electron density and temperature) space at the plasma edge shows a clear indication of the domain without impurity accumulation and provides operation scenarios to build up fusion-relevant plasmas., source:Y. Nakamura, N. Tamura, M. Kobayashi, S. Yoshimura, C. Suzuki, M. Yoshinuma, M. Goto, G. Motojima, K. Nagaoka, K. Tanaka, R. Sakamoto, B.J. Peterson, K. Ida, M. Osakabe, T. Morisaki, A comprehensive study on impurity behavior in LHD long pulse discharges, Nuclear Materials and Energy, Volume 12, 2017, Pages 124-132., source:https://doi.org/10.1016/j.nme.2016.11.005

A transition from an interchange mode to a non-resonant mode is found in the nonlinear magnetohydrodynamic simulation for the partial collapse in a large helical device (LHD) plasma with a net toroidal current. This transition can occur when the magnetic shear is weak and the rotational transform is close to unity in the core region. In this transition, the mode number of the dominant Fourier component is reduced. As a result of the nonlinear evolution, the (m, n) = (1, 1) component can be dominant, where m and n are the poloidal and the toroidal mode numbers, respectively. This transition is considered to be a candidate to explain the observation in the LHD experiments with the net toroidal current that show partial collapses are caused by the (1, 1) mode., source:Citation K. Ichiguchi et al 2021 Nucl. Fusion 61 126056, source:https://doi.org/10.1088/1741-4326/ac3292, identifier:0000-0002-7698-0223

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., source:M. Kobayashi, K. Tanaka, K. Ida, Y. Hayashi, Y. Takemura, and T. Kinoshita Phys. Rev. Lett. 128, 125001 – Published 23 March 2022, source:https://link.aps.org/doi/10.1103/PhysRevLett.128.125001, identifier:0000-0002-0990-7093

In order to explore the compatibility of good core plasma performance with divertor heat load mitigation, the interaction between cold edge plasma and core plasma transport, including the edge transport barrier (ETB), has been analysed in the divertor detachment discharges of deuterium plasmas in LHD with resonant magnetic perturbation (RMP) field application. The RMP application introduces a widened edge stochastic layer and sharp boundary in the magnetic field structure between the confinement region and the edge stochastic layer. The widened edge stochastic layer enhances impurity radiation and provides stable detachment operation as compared with the case without RMP. It is found that ETB is formed at the confinement boundary at the onset of detachment transition. However, as the detachment deepens, the resistive pressure gradient-driven MHD mode is excited, which degrades the ETB. At the same time, however, the core transport decreases to keep global plasma stored energy (Wp) unchanged, showing clear core-edge coupling. After a gradual increase of density fluctuation during the MHD activity, a spontaneous increase of Wp and the recovery of ETB are observed while the detachment is maintained. Then, the coherent MHD mode ceases and ELM-like bursts appear. In the improved mode, impurity decontamination occurs, and the divertor heat load increases slightly. Key controlling physical processes in the interplay between core and cold edge plasma are discussed. A comparison between deuterium and hydrogen plasmas shows that hydrogen plasmas exhibit similar features to the deuterium ones in terms of density and magnetic fluctuations, impurity decontamination towards higher confinement, etc. But most of the features are modest in the hydrogen plasmas and thus no clear confinement mode transition with clear ETB formation is defined. Better global confinement is obtained in the deuterium plasmas than the hydrogen ones at a higher radiation level., source:Citation M. Kobayashi et al 2022 Nucl. Fusion 62 056006, source:https://doi.org/10.1088/1741-4326/ac42f3, identifier:0000-0002-0990-7093

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., source:https://doi.org/10.1038/s41598-022-09526-w, identifier:0000-0001-5669-1937

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., source:Physics of Plasmas 29, 032504 (2022); https://doi.org/10.1063/5.0074351, source:https://doi.org/10.1063/5.0074351, identifier:0000-0002-2983-5920

In this study, the use of a spatial heterodyne spectrometer (SHS) to measure the toroidal flow velocity (Vf ) and the ion temperature (TC6+) of the C6+ impurity ion by charge exchange spectroscopy was explored. The instrumental width (IW) of the SHS (aperture size = 16.77 mm2, etendue = 2.9867 mm2sr) was extrapolated to be 0.09 nm, which is half of the 0.17 nm IW extrapolated for a conventionally used dispersive spectrometer (DS) (aperture size = 2.6 mm2, etendue = 0.2605 mm2sr). The resulting Vf and TC6+ measurements were found to be in good agreement with those measured using the DS., source:F. J. Arellano, M. Yoshinuma, and K. Ida , "Charge exchange spectroscopy using spatial heterodyne spectrometer in the large helical device", Review of Scientific Instruments 93, 033503 (2022) https://doi.org/10.1063/5.0078417, source:https://doi.org/10.1063/5.0078417, identifier:0000-0002-0543-6925

The preceding propagation of turbulence pulses has been observed for the first time in heat avalanche events during the collapse of the electron internal transport barrier (e-ITB) in the Large Helical Device. The turbulence and heat pulses are generated near the foot of the e-ITB and propagate to the peripheral region within a much shorter time than the diffusion timescale. The propagation speed of the turbulence pulse is approximately 10 km/s, which is faster than that of the heat pulse propagating at a speed of 1.5 km/s. The heat pulse propagates at approximately the same speed as that in the theoretical prediction, whereas the turbulence pulse propagates one order of magnitude faster than that in the prediction, thereby providing important insights into the physics of non-local transport., source:Kenmochi, N., Ida, K., Tokuzawa, T. et al. Preceding propagation of turbulence pulses at avalanche events in a magnetically confined plasma. Sci Rep 12, 6979 (2022). https://doi.org/10.1038/s41598-022-10499-z, source:https://doi.org/10.1038/s41598-022-10499-z, identifier:0000-0003-1088-8237

A new Doppler radar using millimeter-waves in the Ka-band, named the “dual-comb Doppler reflectometer”, has been developed to measure the turbulence intensity and its velocity in high-temperature plasmas. For the realization of a fusion power generation, it is very important to know the spatial structure of turbulence, which is the cause of plasma confinement degradation. As a non-invasive and high spatial resolution measurement method for this purpose, we apply a multi-frequency Doppler radar especially with simultaneous multi-point measurement using a frequency comb. The newly developed method of synchronizing two frequency combs allows a lower intermediate frequency (IF) than the previously developed frequency comb radar, lowering the bandwidth of the data acquisition system and enabling low-cost, long-duration plasma measurements. In the current dual-comb radar system, IF bandwidth is less than 0.5 GHz; it used to be 8 GHz for the entire Ka-band probing. We applied this system to the high-temperature plasma experimental device, the Large Helical Device (LHD), and, to demonstrate this system, verified that it shows time variation similar to that of the existing Doppler radar measurements., source:Appl. Sci. 2022, 12(9), 4744; https://doi.org/10.3390/app12094744, source:https://doi.org/10.3390/app12094744, identifier:0000-0001-5473-2109

A plasma density fluctuation signal was measured by the recently-installed Beam Emission Spectroscopy (BES). A time-dependent analysis was performed for a discharge in the Large Helical Device (LHD) and compared with the magnetic fluctuation. While the fundamental frequency peak shows a high correlation between the density fluctuation and the magnetic fluctuation, the higher harmonic components have smaller or even negligible correlation. As a possible mechanism that makes the density fluctuation and the magnetic fluctuation different, the relation between the MHD mode and the transport is discussed., source:https://doi.org/10.1585/pfr.16.2402031

In response to recent upgrade of the raytracing code for the electron cyclotron resonance heating (ECH) in LHD, transient electron thermal transport is reanalyzed. The upgraded code LHDGauss-U takes into account the oblique injection of the ECH ray to magnetic field line. The obtained results show reduced transport hysteresis widths by up to ∼ 20% where the heating absorption is less significant, but qualitative features of the transport hysteresis reported in previous studies are found to be preserved., source:https://doi.org/10.1585/pfr.15.1402072, identifier:0000-0001-5669-1937

Density control is crucial for maintaining stable confined plasma. Divertor pumping, where neutral particles are compressed and exhausted in the divertor region, was developed for this task for the Large Helical Device. In this study, neutral particle pressure, which is related to recycling, was systematically scanned in the magnetic configuration by changing the magnetic axis position. High neutral particle pressure and compression were obtained in the divertor for a high plasma electron density and the inner magnetic axis configuration. Density control using divertor pumping with gas puffing was applied to electron cyclotron heated plasma in the inner magnetic axis configuration, which provides high neutral particle compression and exhaust in the divertor. Stable plasma density and electron temperature were maintained with divertor pumping. A heat analysis shows that divertor pumping did not affect edge electron heat conductivity, but it led to low electron heat conductivity in the core caused by electron-internal-transport-barrier-like formation., source:G Motojima et al 2022 Phys. Scr. 97 035601, source:https://doi.org/10.1088/1402-4896/ac5269, identifier:0000-0001-5522-3082

An indirect effect of the electron cyclotron current drive (ECCD) on the ion temperature in the plasma core region was observed in the Large Helical Device. The reference (no ECCD) discharge with a central ion temperature Ti0 of ∼3.0 keV is operated by a standard high ion temperature discharge procedure. To investigate the ECCD effect, a co- or counter-ECCD was applied to the reference discharge, and was turned off immediately before the Ti0 peaked in the reference discharge. In the co-ECCD and counter-ECCD applications, the Ti0 temporarily increased and decreased by ∼0.5 keV from Ti0 in the reference discharge, respectively. The mechanism of this phenomenon is presently unclear, but may be exploited as a practical knob for controlling the central ion temperature., source:https://doi.org/10.1585/pfr.13.1402124, identifier:0000-0001-6744-1829

Spatial profile of deuterium/hydrogen (D/H) ratio is observed using the charge exchange emission of bulk deuterium/hydrogen ion and neutral beam perpendicularly injected into the torus plasma. Neutral beam injection (NBI) is turned on and off in order to subtract the deuterium/hydrogen emission from the plasma edge. The number of fitting parameters for the subtracted spectral signal is reduced from 12 to 3 by assuming that the bulk ion temperature and velocity are the same as those of the carbon ions (C6+). The initial result observed in Large Helical Device (LHD) plasma indicates that D/H ratio is constant in radial direction and is the same as that observed from passive spectroscopic measurement when the tangential NBI or the pellet injection are absent., source:https://doi.org/10.1585/pfr.13.1202103, identifier:0000-0002-1804-6336

Multi-channel scanning filter spectrometer with narrow band pass of 2 nm is installed in Large Helical Device (LHD) for Beam Emission Spectroscopy (BES) using hydrogen (H) and deuterium beam (D). The spectral shape of transmitted light (half width at half maximum, skewness and kurtosis) is measured using the monitor grating spectrometer coupled with the filter spectrometer. No distortion of the spectrum shape is observed in the wavelength scan range of 8 nm required for the Doppler shift of the BES emission from H-beam and D-beam., source:https://doi.org/10.1585/pfr.14.1305118, identifier:0000-0001-5243-4917

Ion internal transport barrier (ITB) was formed in both hydrogen discharge and helium dominated discharge. The central ion temperature was investigated as a function of the hydrogen density ratio (nH/(nH+nHe)). The central ion temperature increases with the decrease of the hydrogen density ratio, while the electron temperature does not change significantly. The experimentally observed ion temperature was not reproduced by the prediction of TASK-3D modeling. The dominant activity in the density fluctuation measured by the phase contrast imaging is consistent with ion temperature gradient mode and they are almost identical between the hydrogen dominated and helium dominated plasmas., source:https://doi.org/10.1585/pfr.11.2402106, identifier:0000-0002-5892-6047

A beam emission spectroscopy (BES) system for density fluctuation measurements having the sight lines passing through the plasma in the toroidal direction was developed in the Large Helical Device (LHD). The coverage of the area sampled by 15 × 100 optical fibers is nearly from the core to the edge on the horizontally elongated poloidal cross section with the spatial pitch of around 1.0 cm, and 32 channels is detected simultaneously with the 4 × 8 pixel Avalanche Photodiode Detector camera. In order to improve the directional sensitivity for the propagation of the density fluctuation in the radial direction or the poloidal direction, we propose the use of slit-shaped sight lines. An initial result of the density fluctuation measurements for MHD activity in the edge region and its cross-correlation analysis is presented., source:https://doi.org/10.1585/pfr.11.1402115

Plasma rotation plays an important role in the suppression of turbulence, leading to an increase in energy and particle confinement. Significant rotation also leads to a stabilisation of the resistive wall mode. The external momentum input from Neutral Beam Injection (NBI) in current generation fusion plasmas may not be available for future self-heated fusion reactors. Therefore it is important to analyse the phenomenon of spontaneous rotation. At NIFS plasma rotation and momentum transport of the Large Helical Device (LHD) plasma is analysed using a code suite called TASK3D-a. In this work neoclassical effects, which can be especially significant in non-axisymmetric plasmas, were implemented in TASK3D-a. Initial analysis of neoclassical radial momentum flux profiles shows that in NBI-driven momentum input neoclassical effects, especially neoclassical damping, become dominant in the non-center plasma region. It was also found that during and after pellet-injection the neoclassical damping force becomes strong. With the implementation of neoclassical effects new features can be examined in the momentum flux-gradient relations; in the damping-dominated situation following pellet injection a large excursion in momentum flux is found. This work can aid in the search for neoclassical transport-optimised configurations for enhanced (spontaneous) plasma rotation., source:https://doi.org/10.1585/pfr.12.1402006

In order to clarify the mechanism responsible for slowing down the precursor of an instability in Large Helical Device (LHD), whose behavior is similar to the locked mode instability in tokamaks, the spatial structure of the precursor of the locked-mode-like instability, and the relationship between the rotation of the precursor and the E × B rotation were experimentally investigated. The precursor rotates together with the E × B rotation at the resonant surface, and the precursor rotation slows down because of a decrease of the E × B rotation. The multi-channel fluctuation measurement of the precursor suggests that the precursor has a magnetic island, which may be related to the decrease of the E × B rotation. In addition, the reason for the appearance of the precursor with a magnetic island is discussed. The precursor appears when a magnetic island grows initially without rotation but then shrinks and begins to rotate., source:https://doi.org/10.1585/pfr.12.1402028, identifier:0000-0003-3754-897X

A ka-band multi-channel Doppler reflectometer system was constructed for the Large Helical Device (LHD) using a comb frequency generator as a source. A filter bank system is utilized for precise quadrature phase detection, and preliminary back-scattered waves were obtained in LHD plasma experiments. In addition, a direct digital signal acquisition system was successfully demonstrated for providing a greater number of multi-channel measurements., source:https://doi.org/10.1585/pfr.9.1402149, identifier:0000-0001-5473-2109