Recent Publications

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.

Frequency-inference method for reduced modeling of energetic particle modes (EPM) utilizing resonant auto-optimization remnants of imperfect time-scale separation

Andreas Bierwage, Vinícius N. Duarte, Pablo Oyola, Kouji Shinohara, Roscoe B. White

Integrated codes simulating interactions between Alfvén waves and fast ions in tokamak plasmas use perturbative models for the relatively slow processes of instability growth, saturation, chirping and bursting, and transport. Faster processes by which an Alfvén mode's spatiotemporal structure forms are assumed to have been completed within the mode's oscillation period, $τ_0 \equiv 2π/ω_0$. This separation of time scales underlies the computational efficiency of perturbative models, where the Alfvén mode's time-dependence is reduced to that of a scalar signal $s(t) = A(t)\sin(-ω_0 t - φ(t))$ with variable amplitude $A(t)$ and phase $φ(t)$. For this, accurate input data in the form of a mode's spatial structure $δΦ({\mathbf x})$, damping rate $γ_{\rm d}$, and initial frequency $ω_0$ are required. For modes residing in dense or continuous spectra, $δΦ$ and $γ_{\rm d}$ could be estimated from the form of the continua and fast ion orbits, but it is difficult to guess the seed frequency $ω_0$. Here, we report results of numerical experiments showing that it is possible to find $ω_0$ using a prompt frequency shift that occurs during the first few $100$ time steps of a simulation. Restarts with the shifted frequency iteratively converge to a value of $ω_0$ that seems to maximize the resonant drive, suggesting an auto-optimization process. The need for iteration is attributed to the fact that the terms required for rapid frequency adjustments were truncated when deriving the perturbative model. Meanwhile, the fact that partial auto-optimization is possible at all is attributed to the fact that series truncation alone (without filter) does not strictly enforce slowness. Remnants of and cross-talk with faster dynamics still occur in numerical implementations. This entails potential for both uncertainty and utility.

A unified theory of transport barriers (TBs) in magnetically confined systems

Swadesh M Mahajan, David R Hatch, Zensho Yoshida, Michael T Kotschenreuther

A thermodynamic model of a plasma boundary layer, characterized by enhanced temperature contrasts is proposed. The theory is constructed to determine the inner boundary temperature $T_1$ for a specified outer (colder) boundary temperature $T_0$, the heat flux $F$ entering the inner boundary, and the parameters defining the layer. The system shows bifurcation and switches to a stable high gradient state if the heat flux $F$ entering through the inner boundary exceeds a critical value $F_c$. However there is an additional stringent condition for the transition to occur; the edge temperature $T_0$ must exceed a critical value $T_c$- no transition is possible if $T_0<T_c$ even for arbitrary large $F$. Equally important is the finding that $F_c$ is not a monotonic function of $T_0$ but has a minimum at $T_{optimum}$ (= $4T_c$ )in the model calculation. The confinement peaks at $T_{optimum}$. The basic conceptual physics is obviously simple: The high contrast state becomes the preferred state when the incoming power into the layer is preferentially converted into coherent motions like the fluid flows and currents (undermining the standard diffusive processes that keep the lower temperature contrast). The purely macroscopic thermodynamic model bears excellent comparison with experimental and detailed microscopic investigations of the H-mode. Deeper plausibility reasons for the workability of this heat engine, creating the simultaneous existence of an ordered state and large entropy production, are suggested.

Evaluation of neutron emission anisotropy by neutral beam injection in ITER deuterium plasmas

Shota Sugiyama, Takeo Nishitani, Hideaki Matsuura, Shuhei Sumida, Kouji Shinohara, Vitaly Krasilnikov, Bruno Coriton

We have investigated neutron emission anisotropy caused by deuterium beam injection in ITER deuterium plasmas. We evaluate the double-differential emission spectrum and emission anisotropy of neutrons produced by the D(d,n)3He reaction, using the energetic deuteron velocity distribution function obtained by following guiding-centre orbits of test particles. We clarify the dependence of neutron emission anisotropy on the electron density and temperature. Anisotropy increases with decreasing electron density and temperature. We examine the effect of neutron emission anisotropy on the neutron incident flux distribution on the first wall. We show that neutron emission anisotropy can affect the measurements of the neutron emission rate and emission profile based on the incident flux distribution, the distribution of neutron emission anisotropy, and the correspondence relationship between the poloidal angular positions and the neutron detectors that are planned to be installed in ITER. It is inevitable to consider neutron emission anisotropy for plasma diagnostics and monitoring of the neutron generation rate in beam-injected deuterium plasmas.

A generalized energy-consistent finite difference scheme for 10-moment magnetohydrodynamics

Keita Akutagawa, Shinsuke Imada, Munehito Shoda

Pressure anisotropy and off-diagonal pressure stresses are ubiquitous and play important roles in collisionless/weakly collisional plasmas. The Chew-Goldberger-Low (CGL) MHD model is often used; however, it can lose hyperbolicity when the pressure anisotropy or plasma beta becomes large, making it hard to develop approximate Riemann solvers. An alternative approach is to use the 10-moment MHD equations, but their eigenmode analysis is also difficult, which similarly hinders the development of less-diffusive Riemann solvers. This paper presents a new energy-consistent finite difference scheme for 10-moment MHD designed to operate over a broad range of plasma beta. The proposed scheme extends the 10-moment MHD model using the energy-consistent finite-difference approach developed for conventional MHD. Nonlinear filtering is applied to all six independent components of the pressure tensor, and the kinetic and magnetic energies dissipated by the filtering are explicitly transferred to the diagonal pressure components under an equipartition assumption to maintain consistency with the total energy balance. The proposed scheme is validated against seven test problems in the isotropic limit, the gyrotropic limit, and without isotropization/gyrotropization. The results demonstrate the expected spatial convergence and total energy behavior, reproduce the linear growth rate, and yield pressure tensor structures qualitatively consistent with theoretical expectations and previous simulations, spanning plasma beta values from $10^{-10}$ to $10^{10}$. The proposed scheme provides a promising framework for large-scale simulations of collisionless plasmas across widely separated plasma beta regimes and opens a path toward applications such as solar wind turbulence and plasmoid-mediated reconnection.

High-speed multi-wavelength tomography of rotating plasma ejection events in detached plasma using gated image intensifier

Koyo Munechika, Hirohiko Tanaka, Shin Kajita, Noriyasu Ohno

We developed an advanced diagnostic system by adding an image intensifier and wavelength filters to the conventional high-speed camera setup to analyze rotating plasma ejection events in detached plasma which can be realized in the liner magnetized plasma device NAGDIS-II. The gating capability of the image intensifier achieved temporal resolution exceeding the camera frame rate, while conditional averaging and tomography processing revealed detailed structures of high-speed phenomena previously undetectable. Multi-wavelength measurements enabled spatial mapping of electron temperature distribution through intensity ratio analysis. This enhanced technique provides the first quantitative correlation between electron temperature evolution and spatial development of rotating ejection events.

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.

Inward particle flux caused by low-frequency electric fluctuations in the edge region of dipole magnetic field configuration

Haruhiko Saitoh, Hiroki Mimura, Shinnosuke Aoyagi, Nozomu Shirokoshi, Kenji Ueda, Masaki Nishiura, Naoki Kenmochi, Naoki Sato, Zensho Yoshida

We report experimental evidence of inward particle transport associated with low frequency fluctuations in the levitated dipole device RT-1. During the occurrence of the low frequency fluctuations that accompany the formation of peaked high-β structures in the dipole magnetic field, multi-point measurements with electrostatic probes detected electric field fluctuations with a toroidal component. Simultaneous measurements of electron density and electric field fluctuations show that these fluctuations drive a steady inward particle flux toward the strong magnetic field side.

Ion Weibel instability in the hybrid framework: The optimal resolution

Luca Orusa, Taiki Jikei

The study of collisionless shocks and their role in cosmic-ray acceleration has gained increasing importance through both observations and simulations. Accurately modeling the shock transition region, where particle injection and energization occur, requires a proper description of the microinstabilities governing its structure. In high-Mach-number astrophysical shocks, such as those associated with supernova remnants, the ion Weibel instability is believed to provide the dominant dissipation mechanism. In this work, we investigate the ion Weibel instability driven by counterstreaming beams in the presence of an external perpendicular magnetic field, with beam velocities significantly exceeding the local Alfvén speed. We employ hybrid simulations, in which ions are treated kinetically while electrons are modeled as a charge-neutralizing fluid. Although hybrid models are widely employed to study collisionless shocks, the resolution requirements needed to accurately capture ion-scale instabilities remain poorly understood. We address this issue by developing a linear theory of the ion Weibel instability tailored to the massless-electron assumption of hybrid models and validating it with one- and two-dimensional simulations over a wide range of Alfvénic Mach numbers. We show that hybrid simulations can reliably reproduce the growth, saturation, and polarization of Weibel-generated magnetic fields in weakly magnetized regimes, provided that the relevant ion-scale modes are properly resolved. From the scaling of the dominant mode, we derive a minimum spatial resolution required as a function of Alfvénic Mach number. We also demonstrate that excessive resolution introduces unphysical small-scale whistler modes inherent to the massless-electron approximation. We validate the analysis by comparing the results with full particle-in-cell simulations. Together, these results provide practical guidance for hybrid simulations of collisionless shocks and beam-driven plasma systems.

Real-Time Control System for Electron Cyclotron Heating Injection on LHD

Naoki Kenmochi, Tohru Ii Tsujimura, Yoshinori Mizuno, Masaki Nishiura, Kota Okada, Yasuo Yoshimura, Hiroe Igami, Hiromi Takahashi, Ryoma Yanai, Toshiki Takeuchi, et al.

Real-time control of electron cyclotron heating (ECH) is critical for optimizing plasma performance and preventing instabilities in advanced fusion devices. We present a comprehensive real-time ECH control system developed on the Large Helical Device (LHD) that enables real-time adjustment of microwave heating in response to evolving plasma conditions. A field-programmable gate array (FPGA) controller actuates the ECH launchers, enabling millisecond-level adjustments of both injection angle and polarization to maximize absorption as plasma density and temperature profiles change. To compute suitable settings rapidly, a generative adversarial network (GAN) model was trained on thousands of past LHD discharges and ray-tracing simulations of ECH to generate control parameters for the deposition position and polarization. In high-density LHD experiments, another machine-learning-based prediction framework enabled the first active avoidance of radiative collapse. The predictor identified an impending collapse about 65 milliseconds in advance, triggering automated ECH power re-targeting and a cutoff of fueling that stabilized the plasma beyond the conventional density limit. These developments demonstrate how real-time ECH control, together with machine-learning-assisted prediction and inference, can sustain stable, high-performance plasmas, highlighting a pathway toward long-duration, steady-state fusion operations.

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.