Recent Publications

Feasibility of fuel-ion temperature and density profile diagnostics Using a neutron spectroscopic camera in JT‑60SA

Shuhei Sumida, Kotaro Iwasaki, Shota Sugiyama, Ryuichi Sano, Nobuyuki Aiba, Mitsuru Honda

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.

Investigation of magnetic field ripple effects on charged particles transport in scrape-off layer of Heliotron J

Furui Cai, Shinichiro Kado, Gakushi Kawamura, Yuhe Feng, Ryota Matoike, Shinsuke Ohshima, Shigeru Inagaki, Fumiyoshi Kin, S Kobayashi, Shinichiro Inagaki, et al.

The impact of helical magnetic ripples on the transport of plasma and carbon impurities in the scrape-off layer (SOL) of a Heliotron J device was investigated using the EMC3-EIRENE code. Comparisons with a simplified one-dimensional fluid model revealed that the transport behavior varied with the Knudsen number (K), defined as the ratio of the ion-ion collisional mean-free path to the ripple scale length. In the weakly collisional regime (K≫1), the qualitative trends suggested by the fluid model indicate that the magnetic mirror effects become dominant, the conductive heat flux governs the heat transport, and the impurity density follows a Boltzmann distribution. Conversely, in the collisional regime (K≪1), the transport exhibits nozzle-like fluid behavior, convective heat flux governs heat transport, and impurities couple to the bulk plasma. These characteristics underscore the importance of charged-particle transport along magnetic field lines in the SOL of fusion devices, where magnetic ripples fluctuate significantly with changes in the magnetic field configuration.

Linear stability analysis of non-resonant kinetic ballooning mode in comparison with the ion temperature gradient/trapped electron mode case

Jin-Yong Kim, Janghoon Seo, Jaemin Kwon, Yasuaki Kishimoto, Hyunsun Han

Unlike in the finite magnetic shear region where turbulent transport is typically dominated by the electrostatic ion temperature gradient and/or trapped electron mode (ITG/TEM), in the weak shear region it is possible for the electromagnetic non-resonant kinetic ballooning mode (KBM), also known as the kinetic infernal mode (KIM) in the low-n limit (here, n is the toroidal mode number), to play a significant role with its very small excitation threshold. Considering the potential relevance to turbulent transport or internal transport barrier (ITB) formation in reverse-shear plasmas, a more systematic study using the gKPSP gyrokinetic code is performed on its linear mode characteristics and excitation threshold, in comparison with the ITG/TEM case. It is shown that, when the magnetic shear decreases, the transition from the resonant to the non-resonant type occurs relatively smoothly in the gradual reduction of global width and coupled slab harmonics number. While this transition feature is similar for ITG/TEM and KBM, the non-resonant mode structure has a notable difference, with the KBM more dominated by a single slab harmonic which then makes it sensitive to the value of nq, allowing clear oscillatory behaviors of its mode center and eigenvalue when n or q is varied (here, q is the safety-factor). Also, while the global radial width typically has a large reduction with the transition, it still appears to have a significant value in the low-n KIM limit. Meanwhile, in accordance to the destabilization of the ideal ballooning mode with decreasing magnetic shear, the non-resonant KBM is found to be more unstable than the resonant KBM, particularly with its threshold temperature gradient (in the broad density profile condition) being smaller than the ITG/TEM one if q is relatively high (>1.5-2.0). These results thus demonstrate that the non-resonant KBM can play a significant role in the weak shear region, emphasizing the necessity of its stabilization for triggering the ITB in reverse-shear plasmas.

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.

Assessment of divertor structures arising from invariant features in Hamiltonian system for three-dimensional magnetic field in Heliotron J

Furui Cai, Shinichiro Kado, Gakushi Kawamura, Yuhe Feng, Fumiyoshi Kin, Ryota Matoike, Shinsuke Ohshima, Shigeru Inagaki, S Kobayashi, Shinichiro Inagaki, et al.

The divertor structures of Heliotron J were investigated by analyzing the three-dimensional magnetic field and plasma transport simulations. The magnetic field of Heliotron J typically exhibits a characteristic resonant structure, where the stable and unstable manifolds emanating from specific X-points determine the divertor legs and the boundary of a confined core region. Regions zoned according to the divertor helical period provide information about the plasma; the boundary of the shadowed region determines the plasma-occupied volume, and the first multi-fold layer determines the fluid acceleration in an island-like configuration. The divertor legs guide the transport of charged particles outside the core. Density accumulates, and the fluid velocity decreases at the intersection points of the legs. The particle and heat depositions on the target plate can be inferred from the lobe structures and their relative toroidal locations on the divertor legs. We found that a chaotic divertor structure can potentially reduce the peak particle and flux densities by creating multiple strike lines, which is preferred for the design of future devices.

Numerical investigation of thermo-fluid dynamics in a high-speed lithium jet under ion beam irradiation for fusion neutron sources

Eiji Hoashi, Daiki Fukunaga, Takafumi Okita, Shunsuke Kenjo, Makoto Oyaizu, Kentaro Ochiai

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.

Revisiting the Growth Rate of the Relativistic Tearing Instability: The Role of the Non-ideal MHD Structure

Kaoru Sugimoto, Kunihito Ioka, Masaru Shibata

Magnetic reconnection in magnetically dominated pair plasmas is a key process in high-energy astrophysical systems. We revisit the relativistic tearing instability in a Harris current sheet and derive an improved analytical expression for its linear growth rate and the most unstable wavenumber. The key modification is the treatment of the vector potential perturbation in the non-ideal magnetohydrodynamic (MHD) region. Instead of the conventional constant-A approximation, we use an extrapolated-A approximation, in which the ideal-MHD solution is linearly extrapolated into the non-ideal region. Comparison with two-dimensional particle-in-cell simulations shows that the revised theory improves the prediction of the most unstable wavenumber. The improvement is most pronounced at low particle drift velocities, where the particle gyroradius is smaller than the current-sheet thickness and the fastest-growing mode shifts to longer wavelength. The resulting analytical expressions provide an updated benchmark for magnetically dominated reconnection and its applications to high-energy astrophysical plasmas, including gamma-ray bursts and fast radio bursts.

Effect of a transparent water-cooled cathode on the neutron production rate in the glow discharge-type fusion neutron source

Yoshimitsu Ito, Toshiro Sakabe, Juro Yagi

Effect of Ti addition on tritium release behavior from neutron-irradiated FLiNaBe

Yuki Setoguchi, Kazunari Katayama, Ao Zhang, Toru Ichikawa, Kenta Akashi, Toshiharu Takeishi, Satoshi Fukada, Yukinori Hamaji, Teruya Tanaka, Yuto Iinuma, et al.

Influence of magnetic shear on kinetic ballooning modes and electromagnetic stellarator turbulence

Paul Mulholland, K. Aleynikova, M J Pueschel, Josefine Proll, Akihiro Ishizawa

The influence of magnetic shear on kinetic ballooning modes (KBMs) and electromagnetic turbulence is reported in the Wendelstein 7-X (W7-X) and Heliotron-J (H-J) stellarators. Gyrokinetic simulations at finite normalized plasma pressure β reveal that sub-threshold KBMs (stKBMs) are present in all configurations, i.e., KBMs that are resonantly destabilized far below the iMHD limit. A reduced KBM model shows that boosting magnetic shear and weakening bad curvature stabilizes (st)KBMs in stellarator geometry. In nonlinear simulations, instead of undergoing nonlinear electromagnetic stabilization, turbulent fluxes in W7-X increase with β when stKBMs are destabilized. Lower-magnetic-shear configurations of W7-X are found to produce lower transport, due to the generation of stronger zonal flows. In H-J, turbulent fluxes reduce with β – despite hosting stKBMs – due to a boosted zonal-flow response at higher β. This highlights the importance of accounting for both linear and nonlinear dynamics when aiming to improve reactor performance.

Non-inductive start-up of spherical tokamak with reduced trapped electrons by electron Bernstein wave heating and current drive on LATE

M. Uchida, H. Tanaka, H. Etou, T. Taiga, H. Shinohara, A. Nakai, R. Hirayama

The start-up and formation of overdense spherical tokamak plasmas by electron Bernstein waves with the microwave injection from a bottom port to suppress trapped electrons have been investigated in the Low Aspect ratio Torus Experiment (LATE). Experimental results show that the development of trapped electrons outside the last closed flux surface (LCFS) is effectively suppressed compared to cases using midplane outboard injection, suggesting improved coupling to current carrying passing electrons and the bulk electrons. Soft x-ray profiles suggest that the electron temperature is higher at the plasma core with the bottom injection than with the midplane outboard injection. These results indicate that the bottom injection is advantageous for core plasma heating, as it avoids the second-harmonic EC resonance heating as well as the Doppler-shifted fundamental EC heating of trapped electrons outside the LCFS.

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

Fumito Okino, Yukinori Hamaji, Juro Yagi, Teruya Tanaka

Sensitivity of a low-shear heliotron configuration to localised ferrite perturbations

A. Matsuyama, Y. Nakamura, S. Inagaki, F. Tanji, Yusuke Yamashita, Akihisa Yamamoto, S. Kobayashi, F. Kin, S. Kado, S. I. Inagaki, et al.

The influence of ferritic steel on low-shear stellarator/heliotron magnetic configurations is investigated for the Heliotron J device using a point dipole magnetisation model. By numerically evaluating ferritic steel plates assumed at several locations inside the Heliotron J vacuum vessel, the changes in the rotational transform and magnetic island width are shown to be sensitive to the installation location. This sensitivity arises from the coupling between the background nonaxisymmetric field and ferrite perturbation, rather than being determined solely by the perturbation amplitude. Ferritic steel plates placed on the outer side of a straight section produce the most significant changes in the magnetic topology and exhibit the highest sensitivity to violations of the $M = 4$ toroidal periodicity. Additionally, we show that appropriate arrangements of passive magnetic dipoles can reduce the effective helical ripple while preserving the vacuum magnetic well depth in Heliotron J, and can induce stellaratorasymmetric boundary perturbation in low-field experiments near the ferrite saturation threshold.

Development of low inductive electric field plasma start-up in JT-60SA

T. Wakatsuki, H. Urano, M. Yoshida, N. Tsujii, Hyun-Tae Kim, T. Nakano, M. Fukumoto, Y. Ohtani, R. Sano, S. Inoue, et al.

Plasma start-up experiments have been performed in JT-60SA under ITER-relevant low inductive electric field conditions. A systematic investigation was conducted to identify the key factors that determined the success or failure of start-up. Ohmic field null configuration (FNC) start-up experiments were found to be strongly limited by high impurity content. In EC-assisted FNC, start-up was hindered by sensitivity to device model uncertainties (e.g. discrepancies between the as-modeled CS/EF coil geometry and the effective coil positions during operation), which induced residual poloidal magnetic fields and outward plasma shifts, reducing the effectiveness of EC heating during burn-through. In contrast to FNC, the trapped particle configuration (TPC) applied a finite vertical magnetic field from the breakdown phase, enabling robust plasma position control and effective utilization of EC heating even when the breakdown timing and the early-phase poloidal field evolution are uncertain. This robustness led to the achievement of the JT-60SA first plasma. Notably, line-integrated spectroscopic diagnostics showed that, although individual differences existed between failed FNC and successful TPC discharges during the breakdown phase, the failed FNC discharges were not systematically characterized by lower breakdown-phase density or electron-temperature indicators. This indicates that, under the present EC-assisted conditions, the start-up outcome was not determined solely by the breakdown-phase plasma parameters, but was strongly affected by the subsequent impurity burn-through phase. Additional experiments demonstrated that TPC also enhanced EC-assisted breakdown, particularly for X2 heating, and enabled successful X2-only start-up. The lower power threshold for X2 start-up was found to be approximately 0.7 MW, set by the requirement for impurity burn-through rather than breakdown itself. These results demonstrate that TPC enables reliable plasma start-up under ITER- and DEMO-relevant conditions through the combined effects of robust position control and efficient EC heating.