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.

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.

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.

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 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.

Analysis of the bursting activity in Large Helical Device plasma with multiple EP populations

Jacobo Varela Rodríguez, Kenichi Nagaoka, Masato Matsuoka, Hideo Nuga, Ryosuke Seki, Kunihiro Ogawa, Kenji Tanaka, Y Todo, Hao Wang, Jialei Wang, et al.

Burst events can severely deteriorate the performance of a future stellarator fusion reactor by reducing the plasma heating efficiency and confinement. Bursting activity is observed in the Large Helical Device (LHD) operation scenarios strongly heated by neutral beam injectors (NBI). This study analyzes the performance degradation in the LHD discharge $189008$ linked to the destabilization of burst events. On that aim, the stability of Alfv'en Eigenmodes (AE) and energetic-ion-driven resistive interchange modes (EIC) is analyzed. Linear and nonlinear simulations using the gyro-fluid code FAR3d reproduce the multiple Toroidal AEs (TAEs) measured in the frequency range of $40-80$ kHz leading to the destabilization of an MHD burst (induced by passing energetic particles injected by the tangential NBI), as well as the EIC destabilized around $12$ kHz that causes the EIC burst (caused by helically trapped energetic particles injected by the perpendicular NBI). This study suggests burst events can be induced by the nonlinear interaction between passing (tangential NBI) and helically trapped (perpendicular NBI) energetic particles (EP) populations, creating a feedback effect that enhances the EP drive. The MHD burst causes $30\%$ of passing EP losses and the EIC burst up to $90\%$ of trapped EP losses, EP confinement degradation that may partially explain the decrease of the device performance. The analysis also indicates the EP transport induced during the burst events is ballistic linked to avalanche-like processes, leading to the intense EP fluxes. In addition, the bursting activity affects the thermal plasma confinement observed as a decrease of the thermal electron density, reproduced in the simulations as a decay of the thermal plasma pressure. The simulations output shows a reasonable agreement with different diagnostic measurements, including the enhanced magnetic perturbations detected by the Mirnov coils once the perpendicular NBI operation and bursting activity begin, the intensity and radial location of the radial electric field and shear flows from charge exchange spectroscopy diagnostic (CXS) data as well as the instabilities frequency range and dominant modes numbers calculated from magnetics data. The main conclusion of the analysis is that, the nonlinear feedback between different EP populations can enhance the EP drive leading to the onset of bursting events and the degradation of burning plasmas performance, particularly in scenarios with a large amount of EPs generated by different heating mechanisms that may destabilize the alpha particle population.

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.

Development of time-evolving NBCD simulation in the LHD, consistent with neutron and impurity measurements

H. Nuga, R. Seki, K. Ogawa, Y. Takemura, S. Satake, H. Yamaguchi, S. Kamio, Y. Fujiwara, Y. Kawamoto, M. Yoshinuma, et al.

We have developed a simulation code for the plasma current driven by the neutral beam in the large helical device (LHD). To compare the results between the experiment and the simulation, not only the beam-driven current, but also the other components, must be estimated. In this paper, we have integrated the codes that solve electromagnetic induction, neoclassical transport (DKES/PENTA), and the fast-ion distribution function (TASK3D-a and TASK/FP) to achieve Neutral beam current drive simulation. The code has been benchmarked against the analytical solution for the evolution of the plasma current. Then the code has been applied to actual LHD discharges. Although the trend and the absolute value of the simulated plasma current became similar to the measured one, the time constant of the current evolution disagreed. By considering the mutual induction between the plasma and the external coils, the simulated time constant of the plasma current evolution became closer to the experimental one.

Review of High-Power Microwave Transmission System for ECH in LHD

Ryoma Yanai, Shin Kubo, Takashi Shimozuma, Yasuo Yoshimura, Masaki Nishiura, Hiroe Igami, Hiromi Takahashi, Toru Ii Tsujimura, Naoki Kenmochi, Sakuji Kobayashi, et al.

Electron cyclotron heating (ECH) is one of the auxiliary heating methods employed in many magnetically confined fusion devices. In the Large Helical Device (LHD), the ECH system - including high-power gyrotrons, transmission lines, and mirror antennas – has played an important role in improving plasma parameters and realizing a variety of plasma experiments. Highly efficient transmission lines capable of withstanding high-power microwaves are essential to deliver high-power microwaves emitted from gyrotrons. In addition, mirror antennas are important for ensuring that the ECH power is absorbed at the intended location. In this paper, the ECH transmission system, focusing on the associated components used in high-power microwave transmission lines and various mirror antennas prepared for the LHD experiment, is reviewed.

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.

Experimental Data Management System for Long-Term Fusion Experiments

Masahiko Emoto, Hideya Nakanishi, Masanobu Yoshida, Ryuichi Sakamoto

The LHD experiment started in 1998 at the National Institute for Fusion Science and has continued for nearly 30 years. The experimental data obtained in these experiments are stored as physical data in the Kaiseki Data Server and made available to the public using an open data server. Through the experience of developing and operating this server, this paper discusses the problems that can occur in data management systems in long-term projects such as the LHD experiment and how they can be improved.