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Shin Kubo, Toru I. Tsujimura, Ryoma Yanai, Kota Yanagihara, Takashi Shimozuma, Yasuo Yoshimura, Masaki Nishiura, Hiroe Igami, Hiromi Takahashi, Naoki Kenmochi
Masahiko Emoto, Hideya Nakanishi, Kenichi Nagaoka, Ryuichi Sakamoto
J. S. Geng, Y. Q. Chu, B. F. Gao, L. Q. Xu, F. Chen, Z. Y. Qiu, J. P. Qian, X. Z. Gong, Q. Long, Y. X. Li, et al.
Self-organized hybrid scenario mediated by fishbone modes in the Experimental Advanced Superconducting Tokamak (EAST) is reported in this work. In this regime, internal transport barriers(ITBs) and a flat core safety factor profile were maintained simultaneously by fishbone modes. First,the drive of low-frequency flows by fishbone modes is clarified, which subsequently suppresses turbulence transport. Besides, fishbone-induced poloidally symmetric (m = 0) toroidal current sheets were first reported experimentally, which drive outward current redistribution that mitigates core current peaking. Together, these effects enable hybrid operation with stable ITBs and robust magnetohydrodynamic (MHD) stability, revealing a self-organized pathway for sustaining hybrid scenario for fusion reactors.
Satoshi Togo, Gakushi Kawamura, Masahiro Kobayashi, Victoria Winters, Yuhe Feng, Felix Reimold, Heinke Frerichs, Byron Jay Peterson, Hiroyuki Takahashi
A numerical survey was conducted using EMC3-EIRENE to investigate the impact of island resonance and location on detachment characteristics during density ramp-up in Wendelstein 7-X (W7-X), prior to corresponding experiments. The radial position of the islands in the scrape-off layer (SOL) was modulated by varying the planar-coil current I PC within the range −1.5 kA ≤ I PC ≤ 1.5 kA, while keeping the non-planar-coil current fixed in the standard configuration (SDC). Detachment characteristics — including detachment onset density, impurity radiation patterns, total target heat load, and divertor neutral pressure — depend strongly on the values of I PC : an inward-shifted island with I PC = −1.5 kA and an outward-shifted island with I PC = +1.5 kA produce markedly different behavior. The detachment onset occurs at lower density for negative I PC (inward-shifted island) and at higher density for positive I PC (outward-shifted island) relative to the reference SDC. In all configurations, the divertor recycling rollover coincides with a radiation fraction of ≈ 0.5–0.6, indicating that power starvation plays a dominant role in the detachment transition. Impurity radiation peaks near the island O‑point for negative I PC , whereas it peaks near the X‑point in the reference SDC and for positive I PC . Regarding the power and particle control, a clear trade-off is observed: negative I PC configurations are most effective for reducing the total target heat load, whereas the reference SDC shows the highest divertor neutral compression for efficient pumping. These results demonstrate that edge magnetic geometry — specifically the positional relationship between the divertor plate and the island O-point, the strike‑line location relative to the island X‑ or O‑point, and the connection length L c in the island SOL, all of which change with I PC — strongly impact detachment operation in W7‑X and can be potential levers for its control.
Akito Nakano, Kentaro Sakai, Hantao Ji, Lan Gao, Brandon K. Russell, Takeo Hoshi
We apply Bayesian inference to the analysis of collective Thomson scattering (CTS) spectra, a diagnostic technique for determining local plasma parameters. Using the population annealing Monte Carlo (PAMC) method, we evaluate the posterior probability density functions (PDFs) for six plasma parameters. The analysis is performed separately for the ion acoustic wave (IAW) and electron plasma wave (EPW) features and jointly for both features. The results show that the combined analysis provides constraints on all six parameters, although the degree of constraint depends on the parameter, whereas the separate analyses provide strong constraints only on complementary subsets of the parameters. In particular, the combined analysis provides a clear constraint on the ion charge state, which is not determined by either separate analysis alone. These findings demonstrate the importance of Bayesian modeling that accounts for the two-component nature of plasmas, consisting of ions and electrons.
Osamu Mitarai, K Y Watanabe, Yuki Takemura, Nagato Yanagi, Shinsuke Satake, Yoshiro Narushima, Akio Sagara
While a tokamak has a plasma current disruption, a helical system is much robust because the current-less operation is, in principle, possible. However, a bootstrap (BS) current due to the pressure gradient could be concerns because it could change the rotational transform. We conducted experiments on the BS current in the Large Helical Device (LHD) in the 1/ν and plateau regimes using three magnetic axis configurations Rax = 3.75 m, 3.6 m and 3.53 m with completely balanced neutral beam injection (NBI). As a reference, we use the standard configuration of Rax = 3.75 m. We then examine the inward-shifted configuration of Rax = 3.6 m, and the strongly inward-shifted configuration of Rax = 3.53 m, which is referred to the neoclassically optimized configuration [1, 2]. It was demonstrated that a clear reduction of the BS current is observed for Rax = 3.53 m than that for Rax = 3.75 m, together with a confinement improvement. Based on these experimental results, the BS current coefficient has been derived, which helps to predict the BS current in a helical fusion reactors.
Kunihiro Ogawa, Eiji Takada, Siriyaporn Sangaroon, LongYong Liao, Mitsutaka Isobe
1 MeV tritons born due to deuterium-deuterium reaction are regarded as the test particles of deuterium-tritium-born alpha particles in deuterium plasma experiments because kinetic parameters of the tritons are almost the same as the alpha particles. Triton burnup ratio, which is secondary deuterium-tritium reactions divided by deuterium-deuterium reactions, is defined as the alpha particle confinement ability because 1 MeV triton should be well confined to slowdown to ~ 100 keV to experience deuterium-tritium reaction. Newly developed scintillation fiber detectors having different operation ranges have been developed and utilized to perform triton burnup experiments for the first time in stellarator heliotron in Large Helical Device.
Yuichi Kawachi, Kunihiro Ogawa, Masaki Osakabe, Yasuko Kawamoto, Mitsutaka Isobe, Ryosuke Seki, Tatsuya Kobayashi, Katsumi Ida
We investigated the phase-space dynamics of helically trapped fast ions during energetic-particle-driven MHD burst events in the Large Helical Device (LHD) using a fast-sampling fast-ion Dα (F-FIDA) diagnostic based on charge-exchange recombination spectroscopy. Conditional averaging synchronized with the timing of RF bursts accompanying the MHD events 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.
Masayuki Yoshikawa, Naomichi Ezumi, Junko Kohagura, Mafumi Hirata, Akihiko Sugiyama, Yasunori Washo, Yoriko Shima, Yui Nagano, Takuma Okamoto, Haruhiko Himura, et al.
Y.S. Zhang, Y.P. Zhang, J. Zhang, W.L. Zhong, M. Isobe, K. Ogawa, D. Mazon, Q.L. Yang, Z. Gao, J.M. Gao, et al.
M. Shoji, G. Kawamura, J. Romazanov, A. Kirschner, G. Motojima, M. Kobayashi, S. Masuzaki, S. Brezinsek
Masaki Nishiura, Hiroe Igami, Hiromi Takahashi, Naoki Kenmochi, Ryoma Yanai, Kenji Ueda, Yoshinori Mizuno, Toshiki Takeuchi, Yasuo Yoshimura, Takashi Shimozuma, et al.
The electron cyclotron resonance heating (ECRH) system on the Large Helical Device (LHD) played a key role in the progress of helical fusion research from the first plasma in 1998 to the final discharge on December 25, 2025. This paper presents a historical review of ECRH activities on LHD, highlighting the stepwise advances in hardware, including gyrotrons, transmission lines, and launcher systems, together with the exploration of advanced plasma regimes such as overdense heating and long-pulse steady-state operation. Beyond plasma heating, the ECRH system was also used as a high-power probe for advanced diagnostics, including collective Thomson scattering (CTS) as well as correlation electron cyclotron emission (CECE), and as a flexible actuator for transport studies and real-time control experiments. Finally, we discuss how the technological assets and operational expertise accumulated over the nearly three decades of the LHD program are being further developed and applied to next-generation helical devices, particularly CHD and CHD-U, as well as to broader gyrotron and millimeter-wave applications.
Kentaro Sakai, ByungJun Kang, Akito Nakano, Takeo Hoshi
We investigate Bayesian inference of non-Maxwellian electron distribution functions from collective Thomson scattering (CTS) spectra. We represent distribution functions using multiple candidate models of different complexity and perform inference on synthetic spectra generated from known non-Maxwellian distribution functions. Our analysis demonstrates that a sufficiently flexible model can approximate the overall shape of the ground-truth distribution function. The most plausible model is identified based on model evidence, a statistical measure representing the probability to obtain the observed data given the model. When the candidates include the ground-truth model, the model evidence favors the ground-truth model. Without the ground-truth model as a candidate, the model evidence favors the candidate model with the fewest parameters that adequately approximates the data. The posterior probability density function of the most plausible model reveals the characteristic features and associated uncertainties of the underlying distribution function. The origin of the additional spectral peaks is attributed to a combination of modifications to the dispersion relation and reduced Landau damping. This enables the objective and data-driven identification of distribution functions directly from observed CTS spectra.
Miguel Lindero-Hernández, Luis Alvaro Montoya-Santiyanes, Nagato Yanagi, Frederic Trillaud, Gabriel dos Santos, Diego A. Garfias-Dávalos, Yoshiro Narushima
Masayuki Yoshikawa, Kazuma Yoshida, Junko Kohagura, Naomichi Ezumi, Ryutaro Minami, Mafumi Hirata, Mizuki Sakamoto, Yoriko Shima, Yousuke Nakashima, Yuya Kudo, et al.
The transient detachment dynamics under upstream perturbations was investigated in the GAMMA 10/PDX tandem mirror device using a combination of supersonic molecular beam injection (SMBI) and radio-frequency heating. These actuators generated intermittent high-density plasma fluxes directed toward the divertor-simulation module, enabling controlled studies of detachment stability under pulsed loading. The experiments revealed rapid transitions between detached and partially reattached plasma states, accompanied by significant increases in electron density and ion flux. Time-resolved diagnostics – including microwave interferometry, Thomson scattering and high-speed Balmer-line imaging – captured the spatio-temporal evolution of excitation and recombination processes, highlighting localised emission structures potentially related to molecular activated recombination activity and asymmetric plasma modification associated with directional SMBI fuelling. A delayed response in the divertor-simulation module indicated finite axial propagation of particle flux from the central cell. The combined observations demonstrate the sensitivity of detached plasma to upstream particle perturbations and provide insight into the possible role of molecular processes in mediating transient partial reattachment. These results provide new insight into detachment control and transient plasma behaviour in mirror-based divertor-simulation experiments.
Moshan Li, Jie Huang, Yuhong Xu, Akihiro Shimizu, Mitsutaka Isobe, Shoichi Okamura, Haifeng Liu, Xian-Qu Wang, Y Q Wu, X Zhang, et al.
A key feature of the stellarator is its inherent three-dimensional magnetic configuration, which leads to numerous differences in plasma physics between stellarators and tokamaks. The influence of this three-dimensional geometry on ion temperature gradient (ITG) modes in the Chinese First Quasi-axisymmetric Stellarator (CFQS) is investigated through linear gyrokinetic simulations using the gyrokinetic Vlasov (GKV) code. Across different radial and toroidal positions, the ITG mode exhibits a strong toroidal dependence and a weak radial dependence. The eigenmode localization is primarily determined by the perpendicular wavenumber k_{\perp }^{2} and the normal curvature {\kappa }_{y}: the ITG mode peaks where {\kappa }_{y} is negative and k_{\perp }^{2} is minimized. For ITG modes with k_{x}=0, the growth rate mainly exhibits a toroidal dependence. As the toroidal angle ϕ increases, the peak growth rate decreases and shifts to the low k_{y}{\rho }_{i} region. For modes with k_{x}\neq 0, the most unstable modes acquire a finite k_{x} at all three radial positions for \phi ={90}^{\circ }, and at ρ=0.8 for \phi ={45}^{\circ }. This finite k_{x} alters the value of k_{\perp }^{2} and modifies its distribution along the field line, thereby changing the eigenmode structure. By combining the eigenmode structure with the overall distributions of magnetic geometry parameters, the variation of the growth rate across different spatial positions can be qualitatively estimated.
Jianfu Liu, Masahiko Sato, Akihiro Ishizawa
The dynamics of a pressure-gradient driven plasmoid (p-plasmoid) are investigated using numerical simulations based on a two-fluid model of magnetized plasmas, including ion and electron diamagnetic effects. The p-plasmoid is devoid of a pressure gradient confined by locally concentric magnetic flux and is generated in a host magnetic island produced by a resistive interchange mode driven by the pressure gradient across a neutral magnetic sheet. Through a series of parameter scans, the p-plasmoid formation and its ejection from the host island are found to be controlled by the electron diamagnetic velocity inside the island. The ion skin depth and plasma resistivity scans show that the p-plasmoid is formed and ejected from the island for low-density and low-temperature plasmas. By a magnetic configuration scan, a higher magnetic shear and a favourable averaged curvature are found to be effective in avoiding the formation and ejection of the p-plasmoid. It is also found that a strong monopolar vortex flow is induced in the p-plasmoid, which can be in the clockwise and counter-clockwise directions, contributing to the sustainment of the p-plasmoid.
T. Fu, X. Q. Wang, X. Su, J. Wang, Y. Xu, J. Cheng, H. F. Liu, J. Huang, X. Zhang, H. Liu, et al.
Using the nonlinear 3D equilibrium code HINT, we investigate the suppression of magnetic field stochasticity and stochasticity-induced radial electron heat transport by an externally driven current in the Chinese First Quasi-axisymmetric Stellarator configuration. For β0=3%, a Gaussian-profile driven current with I0 = −12 kA effectively suppresses stochasticity within the confinement region, increasing the normalized volume-averaged β while maintaining a magnetic well depth of 7%. The suppression mechanism is attributed to the current reducing the rotational transform inside the n/m = 2/4 rational surface, which weakens or eliminates coupling among low-order satellite island chains. Radial heat transport is evaluated via Rechester–Rosenbluth theory. The radial thermal conductivity χr in the stochastic region decreases with increasing magnitude of I0. In the low-density case, the χr is reduced from 498 m2/s to below 0.1 m2/s; in the high-density case, it decreases from 44 m2/s to below 0.01 m2/s. These results confirm that externally driven current effectively suppresses stochasticity-driven radial heat transport in quasi-axisymmetric configurations.
Nobuyuki Nakamura, Ryohko Ishikawa, Motoshi Goto
We present the linear polarization of the $2s$--$2p_{3/2}$ transition in Li-like N$^{4+}$ excited by electron collisions, measured at electron energies between 85 and 1000~eV. The measured polarizations are compared with theoretical values calculated using the Flexible Atomic Code (FAC). Although the overall trend that the degree of polarization decreases with increasing electron energy is reproduced, the theoretical values are systematically larger in magnitude than the experimental values. After examining possible experimental sources of depolarization, we suggest that the discrepancy likely originates from an overestimation in the theoretical model. Further theoretical and experimental investigations along the isoelectronic sequence are needed to resolve this discrepancy.
Hiroaki Ohtani, Nobuaki Ohno, Meguru Nakamura, Akira Kageyama, Kunihiro Ogawa, Mamoru Shoji, Suguru Masuzaki
At the National Institute for Fusion Science (NIFS), advanced visualization and analysis techniques using large-scale virtual reality (VR) systems and head-mounted displays (HMDs) are being developed to interpret various fusion plasma datasets. In particular, by projecting three-dimensional (3D) time-series trajectories of Li ion particle calculated for impurity powder dropper experiments in the Large Helical Device (LHD) into an immersive VR environment together with magnetic field line data, detailed analyses of Li ion transport phenomena have become possible. In deuterium plasma experiments in the LHD, trajectories of tritons generated by D-D fusion reactions and their collisions with plasma-facing components were computed. As a new visualization method, the collision points and the velocity vectors at the moments of impact were displayed in VR space. Visualizing the computed triton orbits in VR revealed collision events taking place behind the divertor plates, which had remained unnoticed using earlier analytical methods. In addition to these visualization capabilities, the present study provides a quantitative three-dimensional evaluation of particle transport and deposition. The analysis shows that Li ions generated from impurity powder injection are predominantly transported along open magnetic field lines in the peripheral plasma, with only limited penetration into closed flux surfaces. For energetic particles, the evaluation of prompt-loss triton impact density reveals that tritons are strongly localized on closed divertor plates, where the maximum impact density reaches approximately 0.08 m⁻², while smaller but finite fluxes are observed on first-wall components. These results highlight the critical role of three-dimensional magnetic topology and realistic device geometry in determining impurity transport and energetic particle deposition in helical fusion devices. In this context, immersive VR complements conventional visualization techniques by providing an interactive environment in which particle trajectories, magnetic field topology, collision points, velocity vectors, and realistic CAD geometry can be explored simultaneously, thereby facilitating the interpretation of complex 3D plasma transport and plasma–wall interactions. Although the physical conclusions are ultimately derived from the simulation data themselves, immersive VR provides a more effective environment for exploring and interpreting these data than conventional desktop-based visualization.
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