Recent Publications

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.

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.

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

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.

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.

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.

Conceptual design of vacuum vessel and welding joint between vacuum vessel sectors for a torus fusion reactor

Arata Nishimura

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.

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.

Development of Charge Exchange Spectroscopy in LHD

Mikirou Yoshinuma, Katsumi Ida, Tatsuya Kobayashi

Charge exchange spectroscopy is widely used in fusion plasma research as a method for measuring ion temperature distribution and plasma flow velocity. In the LHD, charge exchange spectroscopy has been applied to observe impurity density distribution, hydrogen/deuterium isotope ratio distribution, and distortion of the ion velocity distribution function using multiple systems that employ high-throughput spectrometers, multi-channel optical fibers, and high-speed image detectors. In the measurement of the hydrogen/deuterium ratio, we succeeded in the analysis by combining it with velocity distribution measurement using a conventional system. In the measurement of the ion velocity distribution function, we were able to grasp the overall situation by combining one with high spatial resolution and a system with high temporal resolution.

Spectroscopy Using a Schwob-Fraenkel Grazing Incidence Spectrometer

Chihiro Suzuki, Hisamichi Funaba, Naoki Tamura

This article reviews the technological details of a 2-m Schwob-Fraenkel soft X-ray multichannel spectrometer (SOXMOS) which has been installed in the Large Helical Device (LHD) to study impurity behaviors. Experimental setup of the SOXMOS operated in the duo-multichannel detector mode is described together with the peripheral equipment and remote control systems customized for the LHD. The absolute wavelength is carefully calibrated using the positions of reference lines based on the two different fitting methods. The data acquisition and registration are completely automated, and the secondary physical data resulting from the wavelength calibration and the peak detection are also made available for all the LHD users. The SOXMOS data have been widely used for various research topics relevant to impurity transport as well as atomic physics.

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.

Deuterium release from lithium-lead droplets formed under 2.0 mm nozzle flow

Fumito Okino, Yukinori Hamaji, Juro Yagi, Teruya Tanaka

Transport in high-performance plasmas of the TJ-II stellarator: From first-principles simulations to experimental validation

José Manuel García-Regaña, Daniel Alegre, Arturo Alonso, Régulo Anton, Enrique Ascasibar, Alfonso Baciero, Alejandro Banon Navarro, Jose-Miguel Barcala, Michael Barnes, M. Scherezade Barquero Balsera, et al.

We provide an overview of activities carried out at the TJ-II stellarator aimed at understanding transport from first principles and power balance analysis. These tasks include gyrokinetic simulations with the codes stella and EUTERPE, neoclassical simulations with the code SFINCS, particle deposition calculations with the code HPI2, and heat source estimates with ASCOT5. All these numerical simulation efforts converge, together with transport analyses, to address the transport mechanisms in plasmas with improved confinement through pellet injection—a scenario studied during the past few TJ-II campaigns bearing resemblance to the pellet-fueled high-performance plasma scenarios of observed in W7-X.

A study toward in-situ calibration for a neutron flux monitor based on the spherical long counter with the support of three-dimensional Monte Carlo neutron transport calculation in EAST

Gong Jiang, Changlin Lan, Mitsutaka Isobe, Kunihiro Ogawa, Ruijie Zhou, Bo Hong, Xiaodong Pan, Jiahao Wang, Haiqing Liu

HL-3 research towards high-performance plasma and power exhaust solution

Wulyu Zhong, Xiao Quan Ji, Wei Chen, Xingyu Bai, Alain Becoulet, Jerome B Bucalossi, Zhe Chen, Shaoyong Chen, Zhipeng Chen, Yihang Chen, et al.

The HL-3 tokamak program addresses critical challenges in developing integrated high-performance scenarios compatible with power exhaust demands for ITER and future reactors. Through systematic facility enhancements including auxiliary heating upgrades to 19.5 MW and AI-enabled control systems achieving 95.5% disruption avoidance, HL-3 finished a new round of exploration and validation for high-performance operation and power exhaust solution. Experiments at mega-ampere plasma currents demonstrated a reactor-relevant hot-ion regime with core ion temperatures exceeding 10 keV and a fusion triple product of . High normalized beta scenarios featuring internal and double transport barriers were successfully established. To address the compatibility of the plasma boundary, various small- or no-edge localized mode (ELM) regimes were achieved, including the enhanced D-alpha (EDA) H-mode, quiescent H-mode (QH-mode), and the quasi continuous exhaust (QCE) regime. Investigations into power exhaust integration highlighted the efficacy of advanced divertor configurations, specifically snowflake and tripod geometries, in significantly reducing peak heat fluxes. Active ELM control was demonstrated via resonant magnetic perturbations (RMPs), lower hybrid waves (LHW), and impurity supersonic molecular beam injection (SMBI), complemented by real-time feedback control of divertor detachment. These developments establish the fundamental physics basis and technical foundations necessary for demonstrating and extrapolating high-performance operations to ITER and next-step devices.