Ryohsuke Seki

We investigated the phase-space dynamics of helically trapped fast ions during energetic-particle-driven magnetohydrodynamic (MHD) bursts in the Large Helical Device (LHD) using a fast-sampling fast-ion D α (F-FIDA) diagnostic based on charge-exchange recombination spectroscopy. Conditional averaging synchronized with the timing of radio-frequency (RF) bursts accompanying the MHD bursts enabled the extraction of F-FIDA signals associated with the bursts. The phase-space dynamics inferred from the F-FIDA spectra showed two distinct behaviors: a rapid increase in the F-FIDA signal near the plasma edge immediately after the burst, and a delayed decrease inside the plasma occurring a few milliseconds later. The former is likely related to localized deformation and magnetic reconnection during the early phase of the MHD event, while the latter appears to be caused by rotating, downward-chirping MHD modes, leading to phase-space dynamics of fast ions. These results contribute to understanding the fast-ion transport processes during energetic-particle-induced MHD activity in fusion plasmas.

Electron density dependence on the energetic neutrals escaping from the plasma, due to charge-exchange reaction between beam ions and neutrals in the plasma peripheral region, was investigated using an imaging neutral particle analyzer (INPA) on the large helical device. The INPA provides simultaneous spatially and energy-resolved measurements of charge-exchanged energetic neutral flux with temporal resolution and is designed to focus on the behavior of helically trapped beam ions. Neutral beam heated hydrogen plasma experiments were performed by scanning the line-averaged electron density from 1.3×1019 to 3.7×1019 m−3. We observed that the total INPA signal decreases with increasing electron density. This is consistent with the slowing-down behavior of energetic ions as described by the Spitzer slowing-down time. To further understand the phenomenon, a numerical simulation of the INPA signal, based on the orbit-following models combined with line-integrated calculation, was conducted using the experimental data. We qualitatively reproduced the electron density dependence of the INPA signal as observed in the experiments. This work provides the first characterization of the electron-density dependence of charge-exchanged energetic neutral flux measured by an INPA in a stellarator.

Burst events can severely deteriorate the performance of future stellarator fusion reactors by reducing the plasma heating efficiency and confinement. Bursting activity has been observed in Large Helical Device (LHD) operation scenarios strongly heated by neutral beam injectors (NBIs). This study analyzes the performance degradation in LHD discharge 189008 , linked to the destabilization of burst events. To that aim, the stability of Alfvén eigenmodes (AEs) and energetic-ion-driven resistive interchange modes (EICs) is analyzed. Linear and nonlinear simulations using the gyro-fluid code FAR3d reproduce the multiple toroidal AEs (TAEs) measured in the frequency range of 40 − 80 kHz, leading to the destabilization of an MHD burst (induced by passing energetic particles (EPs) injected by tangential NBI), as well as the destabilization of EICs at around 12 kHz, which causes the EIC burst (caused by helically trapped EPs injected by the perpendicular NBI). This study suggests that burst events can be induced by the nonlinear interaction between passing (tangential NBI) and helically trapped (perpendicular NBI) EP populations, creating a feedback effect that enhances the EP drive. The magnetohydrodynamic (MHD) burst causes 30 % of passing EP loss, and the EIC burst up to 90 % of trapped EP loss, an EP confinement degradation that may partially explain the decrease in device performance. The analysis also indicates that the EP transport induced during the burst events is ballistic, and linked to avalanche-like processes, leading to intense EP fluxes. In addition, the bursting activity affects the thermal plasma confinement, observed as a decrease in the thermal electron density, reproduced in the simulations as a decay in the thermal plasma pressure. The simulation output shows reasonable agreement with different diagnostic measurements, including the enhanced magnetic perturbations detected by Mirnov coils once the perpendicular NBI operation and bursting activity begin, the intensity and radial location of the radial electric field and shear flows from charge exchange spectroscopy diagnostic data, as well as the instability frequency range and dominant mode numbers calculated from the magnetics data. The main conclusion of the analysis is that the nonlinear feedback between different EP populations can enhance the EP drive, leading to the onset of bursting events and the degradation of burning plasma performance, particularly in scenarios with a large amount of EPs generated by different heating mechanisms that may destabilize the alpha particle population.

We have developed a simulation code for the plasma current driven by the neutral beam in the Large Helical Device (LHD). To compare the results between the experiment and the simulation, the current component, not only the beam-driven current, but also the other components, must be estimated. In this paper, we have integrated the codes that solve electromagnetic induction, neoclassical transport (DKES/PENTA), and the fast-ion distribution function (TASK3D-a and TASK/FP) to achieve NBCD simulation. The code was validated through model simulations, which assume steady-state beam injection. Then the code was applied to actual LHD discharges. Although the simulated value became similar to the measured one, the time constant of the current evolution disagreed. By considering the mutual induction between the plasma and the external coils, the simulated time constant of the plasma current evolution became closer to the experimental one.

This work reports the first comprehensive simulations of ion cyclotron resonance heating (ICRH) driven tokamak plasmas on the slowing-down timescale using the kinetic-MHD hybrid code MEGA, where shear Alfvén wave induced minority ion transport is self-consistently included during the high-energy tail formation. Bursting toroidal Alfvén eigenmodes (TAEs) are observed in both multi- n and single- n simulations of plasmas with a relatively low magnetic field ( B 0 = 1.5 T ) and an ion-cyclotron-range-of-frequency (ICRF) resonance layer located at the magnetic axis or on the inboard side, while outboard heating always leads to non-bursting TAEs, where n is the toroidal mode number. During bursting events, a series of discrete TAEs with distinct frequencies emerges for each toroidal harmonic, with spatial overlap between adjacent modes. In contrast, in non-bursting cases, harmonics with the same toroidal mode number but centered at different radial locations form a single broad structure with nearly identical frequencies. The results further show that, in the bursting case, minority particles remain far from the radio-frequency (RF) resonance layer, whereas in the non-bursting case they stay close to it and experience strong ICRF-driven velocity-space diffusion. This indicates that ICRF-driven velocity-space diffusion can help suppress bursting TAEs and sustain higher energetic particle beta, highlighting the sensitivity of Alfvén eigenmode (AE) dynamics to the ICRF resonance location, which may be utilized for future AE control strategies.

In the Large Helical Device (LHD) experiments using fast protons and fast deuterons, hybrid simulations were performed with the MEGA code to investigate the synergistic effects of multiple fast-ion species on instabilities and fast-ion transport, where multiple fast ions interact with the magnetohydrodynamic (MHD) fluid. In the MEGA simulations, the burst amplitude of Alfvén eigenmodes (AEs) after the fast-ion stored energy reaches a steady state is larger than that before the steady state, leading to a significant increase in the fast-ion loss rate. The dominant AE frequencies obtained in the simulations are in good agreement with those observed in the experiments. In the case of multiple fast-ion species, additional AEs appear that are absent when only a single fast-ion species drives the AEs, and these modes are caused by the redistribution of multiple fast ions. Consequently, these additional AEs lead to increased loss rates of both fast protons and fast deuterons. The synergistic effect of multiple fast-ion species is thus shown to enhance fast-ion transport and losses in the LHD.

The present study is dedicated to the analysis of energetic particle modes (EPMs) in fusion devices. The aim of the analysis is reproducing the EPM frequency down-sweeping and the transition from energetic particle (EP)-driven to current- or pressure-gradient-driven instabilities observed in the experiments. In addition, the shear flow generation and its effect on the EPM saturation and decay phases are analyzed. Two test cases are considered: Fish-bones destabilized in a JET configuration and energetic-ion-driven resistive interchange modes (EICs) triggered in a Large Helical Device (LHD) discharge. A set of nonlinear simulations using the gyro-fluid code FAR3d are performed, removing the EP drive along the saturation phase of Fish-bones and EIC, analyzing the EPMs' decay with and without the presence of shear flows. The simulations reproduce the EPM's frequency down-sweeping along with the transition from a Fish-bone to the current-driven 1/1 internal kink in the JET case, as well as the transition from EIC to the pressure gradient-driven 1/1 resistive interchange mode in the LHD case. The simulations show the generation of shear flows and radial electric fields in the plasma location where the Fish-bone/EIC decay, linked to the energy transfer from the modes toward the thermal plasma. Linear simulations performed using the evolved profiles from nonlinear simulations highlight the key role of the shear flows in the decay of Fish-bones, pointing out the mode frequency down-sweeping can only be reproduced in simulations including the effect of the shear flows. In addition, the analysis may indicate shear flows have a regulatory effect on the Fish-bone saturation, leading to a smoother mode decay by balancing the energy extraction caused by the continuum damping. On the other hand, the role of shear flows in the EIC decay is smaller compared to the Fish-bones, explained by narrower and almost steady shear flow bands observed in the simulations.

Second harmonic deuteron heating in the ion cyclotron range of frequency (ICRF) was evaluated based on neutron measurements newly installed before a deuterium experimental campaign of the LHD. In deuteron plasmas, it was confirmed that the neutron emission rate increased with higher ICRF injection power. Fast deuterons with energy above 500 keV could be measured by a diamond neutral particle analyzer, when the minority proton ratio was less than 1%. On the other hand, a significant increase in the neutron emission rate, due to fast deuterons produced by ICRF second harmonic heating, was not observed, even in the case of ICRF superimposing on neutral beam injection (NBI). In experiments when the ICRF resonance layer was located near the magnetic axis, fast-ion driven instabilities due to the ICRF heating were observed for the first time at the LHD.

Proton-boron-11 (p- ¹¹ B) fusion is an attractive concept for advanced fusion energy because of its nearly aneutronic characteristics. However, efficient use of beam-injected fast ions may be limited by their slowing down through Coulomb collisions, which lead to lower fusion reactivity. In this study we investigate whether ion cyclotron range of frequencies (ICRF) heating can ameliorate this effect by resonantly accelerating fast ions in the Large Helical Device (LHD) of a stellarator. Using the tangential neutral beam (NB) injection to supply energetic protons at ∼160-170 keV, together with boron seeding, we conducted a series of experiments with superimposed hydrogen minority ICRF heating for beam proton acceleration. Three complementary experimental methods, ICRF power modulation, NB termination, and NB energy scan, consistently showed that ICRF waves accelerate beam ions and enhance the p- ¹¹ B fusion yield. This led to measurable enhancements in the p- ¹¹ B fusion alpha-particle yield, as observed with a dedicated fusion-product detector. These findings provide the first direct experimental evidence that ICRF heating can improve the efficiency of beam-driven p- ¹¹ B fusion reactions.