Recent Publications

Preliminary shielding design of the neutron cameras for D-T fusion product diagnostics on the HL-3 tokamak

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.

Review of LHD-ECRH activities and beyond

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.

Bayesian inference of non-Maxwellian distribution functions from collective Thomson scattering spectra

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.

Impact of ferritic steel blankets on the magnetic field configuration of the helical fusion reactor

Miguel Lindero-Hernández, Luis Alvaro Montoya-Santiyanes, Nagato Yanagi, Frederic Trillaud, Gabriel dos Santos, Diego A. Garfias-Dávalos, Yoshiro Narushima

Transient detachment dynamics induced by supersonic molecular beam injection and radio-frequency heating in GAMMA 10/PDX

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.

The influence of magnetic geometry on ion temperature gradient modes in the CFQS

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.

Parameter dependence of pressure-gradient driven plasmoid formation and its ejection from host magnetic island

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.

Suppression of stochasticity and radial transport by externally driven current in CFQS quasi-axisymmetric stellarator

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.

Polarization measurements of the $2s$--$2p_{3/2}$ VUV transition in N$^{4+}$ excited by electron collisions

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.

Immersive VR-based visualization and analysis of fusion plasmas using Digital-LHD and Virtual-LHD

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.

Development of systematic exploration of magnetic configurations in Heliotron J

Fumiyoshi Kin, Shinji Kobayashi, Akinobu Matsuyama, Furui Cai, Tatsuya Kobayashi, Takeo Hoshi, Shinsuke Satake, Shinichiro Kado, Shinichiro Inagaki, Shigeru Konoshima, et al.

We develop a systematic configuration-search methodology for the Heliotron J helical-axis heliotron, combining a Bayesian exploration approach with field-line tracing and evaluation of magnetic field structural parameters. The methodology enables efficient exploration of an enormous coil current parameter space (∼10 11 possible combinations) and is consistent to VMEC equilibrium calculations. As a demonstration, the methodology is applied to search three magnetic configurations: σ = +1 (uniform ripple bottom), σ = -1 (uniform ripple top), and quasi-omnigenous configurations. The σ = +1 exploration successfully identifies configurations with a factor-of-two reduction in ε 3/2 eff , associated with an increase in the bumpiness (toroidal mirror) component. The σ = -1 exploration yields configurations with worse ε 3/2 eff ,as expected, providing as a contrasting reference to the σ = +1 case. A practical omnigenous configuration was not found, suggesting that the inherent toroidicity cannot be overcome through coil current optimization alone. The developed framework provides a practical methodology for systematic magnetic configuration search within a single device, providing the way for element-wise experimental studies of how individual magnetic field components affect confinement in stellarator/heliotron devices.

Observation of significant non-collisional ion heating in helical plasmas with dominant electron heating by neutral beam injection on LHD

Kazuo Toi, Shigeru Morita, K Tanaka, Akihiro Shimizu, Masaki Nishiura, Kunihiro Ogawa, Novimir Antoniuk Pablant, Donald A. Spong, Tokihiko Tokuzawa, Ichihiro Yamada, et al.

In LHD, transient but significant increases in the central ion temperature T_io are observed in low density plasmas having a non-monotonic rotational transform profile produced by high energy neutral beam injection (NBI). The T_io-increase realizes T_io ~ T_eo (central electron temperature) on strong electron heating condition. The increase gradually decreases as the line-average electron density increases, and disappears once it exceeds 1×10¹⁹ m⁻³. During the T_io-increase phases, turbulent density fluctuations in the core plasma region are not suppressed but are enhanced slightly. The ion temperature increases are attributed to an addition of non-collisional ion heating, but not confinement improvement due to suppression of turbulent transport. The ion heating power density estimated from the time evolution of T_io is much higher than that of collisional ion heating by NBI. The estimated maximum power density averaged over the plasma volume becomes transiently comparable to or even higher than the volume-averaged total NBI heating power density. The observed amplitude of energetic ion driven geodesic acoustic modes (EGAMs) with a significant value 〖eϕ〗_EGo⁄T_io ~ 1 (ϕ_EGo: peak value of the EGAM amplitude at the plasma centre) decreases clearly during the initial T_io-increase phase but is maintained at a finite level, with some modulation, until the end of the NBI pulse. The EGAM damping rate expected from the observed ion heating power density is much higher than the linear Landau damping rate estimated from the GAM dispersion relation of a helical plasma. Nonlinear ion Landau damping in high-amplitude EGAM is thought to be one of the leading mechanisms for qualitatively explaining the observed significant ion heating. The significant T_io-increases always induced in the upward-sweeping phase of n=1 reversed shear Alfvén eigenmode (RSAE) frequency are suddenly suppressed, when nonlinear wave-wave coupling of EGAM with n=1 RSAEs and n=0 global Alfvén eigenmodes (GAEs) is activated noticeably (n: toroidal mode number) in the latter phase of the upward sweeping and the downward sweeping of the RSAE frequency. This observation shows a potentiality of a new energy channeling scenario based on EGAM in a future fusion plasma.

Hydrogen isotope permeation in liquid Li17Pb83 supported by pure V and V alloy substrates

Yuya Ando, Yuto Murata, Takuya Nagasaka, Teppei Otsuka

Plasma properties in negative-ion beam sources for fusion: recent results and challenges in view of ITER

Emanuele Sartori, Valeria Candeloro, Isabella Mario, Antonio Pimazzoni, Carlo Poggi, Gianluigi Serianni, Pierluigi Veltri, Matteo Brombin, Riccardo Casagrande, Michele Fadone, et al.

Large plasma sources are employed in negative ion based heating neutral beam injectors for fusion applications. A review of the ITER beam source plasma properties is presented, in comparison with negative ion sources of comparable size, based on the latest experimental measurements from SPIDER and supporting numerical simulations. Spatial investigation of plasma parameters using several diagnostics, including the beam itself, retarding field energy analysers, optical emission spectroscopy, movable and fixed Langmuir probes, is key to understanding the source physics, to support the operation and improve the source performances. The influence of multiple rf-driver configuration on uniformity and the improvement flexibility it provides, as well as the implications of control parameters like filter field and plasma grid bias are examined. Similarities and differences with other giant negative ion sources are discussed to highlight the key physical processes leading to the optimal operation and viable paths for possible improvements of the ITER source design.

Dynamic mechanisms across the transition from the L-mode to steady-state H-mode in Large Helical Device

Wei Li, Yuhong Xu, Masahiro Kobayashi, Xian-Qu Wang, Jun Cheng, Akihiro Shimizu, M Yoshinuma, Haifeng Liu, X Zhang, Jie Huang, et al.

Dynamic features across the transition from low (L) to steady-state high (H)-mode in Large Helical Device are investigated. We focus on several transition processes from the L-mode, developing H-mode towards the stable H-mode. It appears that for the initial L-H transition, the mean Er × B flow curvature and nonlinear energy cascading of ambient turbulence both play significant roles for entering the developing H-mode. From the developing to stable H-mode, experimental results reveal essential effects of nonlinear energy coupling between turbulence and large-scale MHD modes on sustaining the steady H-mode, for which the MHD bursts act as a predator whereas turbulence is a prey. These findings provide additional insight into the dynamic evolution from the L-mode to steady-state H-mode.

Effect of the cusp magnets system on the hybrid RF ion source in NIFS by 2D fluid model

Na Wang, Katsuyoshi Tsumori, Masaki Osakabe, Haruhisa Nakano, Zhimin Liu, Yuanlai Xie

A two-dimensional fluid model of the RF ion source in NIFS test stand was established using COMSOL Multiphysics to examine the influence of the cusp magnets on plasma transport and RF power coupling. Particular attention was paid to the magnetic field intensity of the cusp magnets on driver backplane and its impact on plasma behaviour in both the driver and the extraction region. Simulation results reveal that strong magnetic confinement from the upper cusp magnets suppresses cross-field plasma transport, leading to reduced electron density near the plasma grid and less efficient RF power deposition. To explore potential optimization, a modified configuration with two smaller magnets of half the original size was investigated. This adjustment significantly enhances plasma density, increasing the peak density in the driver by nearly a factor of two. However, the enhanced plasma density is accompanied by increased spatial gradients in the extraction region, indicating that plasma uniformity is not improved under the modified magnetic configuration. These results reveal a trade-off between plasma density enhancement and spatial uniformity, arising from the interplay between magnetic confinement and plasma transport. This study provides insight into the role of cusp magnetic fields in regulating plasma generation and spatial gradients, and highlights the importance of balancing plasma density and uniformity in the optimization of RF negative ion sources.

Beam-Tracing-Based Quantitative Reconstruction of Density Fluctuations in QUEST Using Doppler Backscattering

T. Kinoshita, T. Tokuzawa, V. H. Hall-Chen, Y. T. Tan, T. Ido, H. Idei, R. Ikezoe, K. Hanada, M. Hasegawa, T. Onchi, et al.

A three-channel X-/Ku-band Doppler backscattering (DBS) system has been developed and installed on QUEST for turbulence and electric-field measurements. In spherical tokamaks, the large magnetic-field pitch angle increases the geometric mismatch between the probing beam wave vector and the local magnetic-field vector, reducing the effective perpendicular projection and resulting in a systematic underestimation of the measured scattering intensity. In addition, in QUEST, where low plasma density requires a low-frequency probe beam, beam propagation effects become increasingly significant, further complicating the interpretation of the measured DBS power in terms of local density fluctuation amplitude. To address these issues, a quantitative correction methodology based on the synthetic DBS code SCOTTY was established. All relevant diagnostic response effects were evaluated using SCOTTY along ray trajectories, yielding a correction factor for reconstructing the local turbulence amplitude from the measured scattering signal. The correction factor exhibits strong spatial and frequency dependence, varying by up to an order of magnitude between the plasma core and edge regions, highlighting the necessity of frequency-dependent corrections. By applying the derived correction factor to experimental measurements, quantitative density fluctuation amplitudes were reconstructed from the detected scattering signals. Evaluation of the fluctuation amplitude indicates enhanced turbulence activity in the plasma edge region, where a finite negative radial electric field is inferred. This work demonstrates the first quantitative turbulence evaluation using low-frequency X-/Ku-band DBS measurements in QUEST and establishes a framework for quantitative DBS analysis in spherical tokamaks.

Evaluation of spatiotemporal tungsten density profiles using Unresolved Transition Arrays in the Large Helical Device

R. Nishimura, T. Oishi, I. Murakami, D. Kato, H. A. Sakaue, S. Gupta, H. Ohashi, C. Suzuki, M. Goto, Y. Kawamoto, et al.

Tungsten spectroscopic studies have been conducted in the Large Helical Device with a pellet injection technique. Spatiotemporal profiles of tungsten density were evaluated using a space-resolved spectrometer, for plasmas with an electron temperature of below 1 keV and electron density of $10^{19}-10^{20}$ $m^{-3}$. Slice & Stack, a method for reconstructing emissivity, was applied to a line at 191.7 Å , which is a part of the Unresolved Transition Array (UTA) spectrum at 90-250 Å. Tungsten density was obtained using photon emission coefficients of $\mathrm{W}^{17+} - \mathrm{W}^{27+}$, evaluated from collisional-radiative model. The tungsten pellet injected from outside the plasma was first ablated in the edge plasma and subsequently diffused throughout the plasma. This behavior is typical of pellet injection experiments. Furthermore, after an event triggered by NBI breakdown, tungsten accumulated in the core plasma. The radiation power was estimated from the evaluated tungsten density profile and cooling factor dataset, and compared with bolometer measurement. This sequence of processes would be useful for validating atomic data of tungsten ions in low-to-intermediate charge states.

Nonlinear dynamic evolution of energetic particle mode due to nonadiabatic wave-particle interaction

Pengjuan Su, Fulvio Zonca, Matteo Valerio Falessi, Jialei Wang, Yasushi Todo, Zhiyong Qiu

Adopting dedicated phase-space diagnostics, we analyze the nonlinear dynamic evolution of an energetic particle mode (EPM) due to nonlinear wave–particle interactions, including nonlinear frequency chirping and secular particle motion due to trapping and de-trapping. By tracking representative particle orbits, we show that the nonlinear evolution is dominated by continuous turnover of the distinct resonant populations via self-consistent trapping and de-trapping, rather than by adiabatic frequency sweeping of a fixed cohort. The resulting clump motion in phase space follows the instantaneous low-frequency resonance contour, leading to pronounced downward frequency chirping. The measured EPM frequency chirping rate scales linearly with the mode amplitude, as predicted by general theory.

Development and Application of Collective Thomson Scattering Diagnostics for Energetic and Bulk Ion Measurements in the Large Helical Device

Masaki Nishiura, Yuusuke Yamaguchi, Shin Kubo, Kenji Tanaka, Naoki Kenmochi, Ryoma Yanai, Ryosuke Seki, Hideo Nuga, Takashi Shimozuma, Yoshinori Tatematsu, et al.