Recent Publications

Multi-beamlet operation of the ITER negative ion source prototype SPIDER towards full beam extraction

Emanuele Sartori, Riccardo Casagrande, Isabella Mario, Antonio Pimazzoni, Basile Pouradier-Duteil, Alastair Shepherd, Riccardo Agnello, Kamran Ahmad, Piero Agostinetti, Matteo Agostini, et al.

The neutral beam heating systems for ITER will be based on radio-frequency driven sources, accelerating a negative ion beam current of tens of amperes for up to one hour. In its second operation phase SO-2, the full-scale prototype source SPIDER was operated for the first time in multi-beamlet configuration at ⁓70 kW/driver, extracting a H– current density of 215 A/m2 over ¼ of the extraction area at 0.3 Pa source filling pressure, providing a total accelerated beam power of ⁓700 kW. Scaling of the results confirms the specifications of the on-going RF generators upgrade, indicating that target current density of 330 A/m2 in hydrogen is achievable by increasing the RF power to 100 kW/driver. Beam uniformity was assessed at the beamlet-group scale under moderate underperveance conditions: at high power row-averaged beamlet currents were within ±10% for fifteen out of sixteen rows. A lower homogeneity was observed at the single-beamlet level. The influence of the magnetic filter field on beam uniformity was also characterised, confirming the beneficial effect of multi-driver operation on homogeneity at the extraction region. A caesium conditioning procedure was developed and validated for the partial-source configuration. An anomalous amplification of the acceleration current peculiar of the vacuum-insulated source design specific to ITER sources, attributed to stray backstreaming charges and surface emission effects, was identified and mitigated by electrostatic shielding. Together with the ongoing installation of new solid-state RF generators and an improved pumping system, the results presented in this work open the way to full-source operation in the next experimental campaign and yield findings of direct relevance to the ITER injector design.

First observation of anisotropic distribution evolution of hydrogen fast ions heated by ICRF in EAST

Yang Liu, Ming Xu, Huishan Cai, Zhanhong Lin, Yihai Liao, Wei Zhang, Hailin Zhao, Jiayi Zhang, Jianjun Zhu, Ye Li, et al.

The continuous energy and pitch-angle distribution of hydrogen fast ions generated by ion cyclotron range of frequencies (ICRF) minority heating is measured in EAST using an imaging neutral particle analyzer (INPA). The measured two-dimensional fast-ion distribution exhibits a triangular structure: as energy increases, the pitch-angle distribution becomes progressively narrower, with its peak located around 0.4<|v_∥/v|<0.5. This behavior is consistent with resonance localization theory, which associates the pitch-angle peak with trapped orbits whose turning points are close to the ion cyclotron resonance layer. Through diagnostic response correction, the reconstructed spectrum shows a progressive enhancement of the high-energy minority-ion tail with rising ICRF power. A clear anisotropic evolution of the fast-ion distribution is also observed between discharges with different electron-temperature profiles. In the discharge with higher core electron temperature, the signal enhancement appears mainly for fast ions at energies above 100 keV and |v_∥/v|≥0.5, together with a broader measured pitch-angle distribution. This behavior is interpreted as the combined result of collisional pitch-angle scattering, fast-ion accumulation, and finite-orbit-width effects. The enhanced signal region corresponds to wide-orbit fast ions whose inner orbit legs extend to the hotter inner plasma region, making them more sensitive to changes in the electron-temperature profile. These results demonstrate the capability of INPA to resolve the anisotropic evolution of ICRF-heated fast ions and provide experimental insight into the optimization of ICRF heating schemes in EAST and future fusion devices.

Operational stability and beam transport constraints in long-pulse neutral beam injection on KSTAR

J.H. Jeong, S.C. Hong, B.S. Kim, J.Y. Jang, W. Cho, J.G. Kwak, S.J. Wang

Parametric instabilities of the inhomogeneous near-SOL tokamak plasma, driven by the coupled effect of the high-harmonic fast wave and of the ion and electron temperature gradients, and anomalous heating of the near-SOL ions

V. V. Mikhailenko, V. S. Mikhailenko, Hae June Lee

Electrostatic parametric instabilities in the inhomogeneous near-scrape-off layer (SOL) tokamak plasma, driven by the combined action of a high-harmonic fast wave (HHFW) with a frequency near the 30th ion-cyclotron harmonic and electron and ion temperature gradients, are investigated numerically. The results indicate the parametric decay of the HHFW into a high-harmonic ion-cyclotron (HHIC) (Bernstein) wave and HHIC quasimode. The instability exists only over a finite range of HHFW wavelengths. The development of the parametric HHIC quasimode decay instability in the near-SOL plasma leads to the onset of parametric turbulence accompanied by anisotropic ion heating, with the ion heating rate across the magnetic field significantly exceeding that along the magnetic field.

Development of a neural network-based surrogate model for neutral beam injection in KSTAR

Young-Hoon Lee, Chanyoung Lee, Hyun-Seok Kim, Minho Woo, Sang-Hee Hahn, S.S. Kim, Jae-Min Kwon

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.

Singularities of the cold plasma theory: Modeling challenges for ICRF operation in low-density edge plasma

Wouter Tierens, Chris Klepper, Raymond Diab, Guillaume Urbanczyk

Sustained ICRF operation in a fusion power plant may require low edge densities to mitigate plasma-wall interactions, a regime which was recently achieved in WEST with very little impurity sputtering. Cold plasma theory, however, predicts singular radiofrequency electric fields in this regime, both at the lower hybrid resonance and along the resonance cones, raising the question of whether standard collisional cold plasma models suffice to describe low-density edge ICRF at all. Collisions in principle remove these singularities, replacing them with finite but sharply peaked fields. We derive these peak length scales analytically and confirm them with a 2D finite-element simulation using exponential mesh refinement, achieving micrometer resolution where needed. We conclude that edge collisions in cold plasma do not remove the need to resolve length scales ordinarily associated with hot-plasma and Bernstein-wave physics.

Facilitation of explosive bursts’ prevention in reversed magnetic shear tokamak plasmas through the current condensation effect for disruption avoidance

Kun Lin Yang, Tong Liu, Zheng-Xiong Wang, Lai Wei, Jialei Wang

The reversed magnetic shear (RMS) configuration, a promising candidate for steady-state tokamak operation, facilitates high bootstrap current fractions through internal transport barriers, but also hosts multiple rational surfaces vulnerable to tearing modes. Under high bootstrap current conditions, these modes can trigger explosive bursts via strong positive feedback during magnetic island interactions, posing serious disruption risks. This study numerically investigates the suppression of such explosive phenomena in RMS plasmas using a reduced MHD model that incorporates bootstrap current, electron cyclotron current drive (ECCD) and the radio-frequency current condensation effect (CCE). Numerical findings demonstrate that the CCE fundamentally enhances ECCD efficacy, particularly for large neoclassical tearing mode islands. For moderate bootstrap fractions ( f Subscript b Baseline equals 0.3 f b = 0.3 $f_b = 0.3$ ), a counterintuitive control strategy is identified: intentionally delaying ECCD initiation until the island grows larger. This approach exploits the positive correlation between CCE effectiveness and island width, thereby achieving a shorter overall stabilisation time. In high bootstrap scenarios ( f Subscript b Baseline equals 0.6 f b = 0.6 $f_b = 0.6$ ), prone to explosive bursts, the CCE introduces a nonlinear positive feedback mechanism that strongly localises current deposition at the O-point of the islands and creates a hysteresis-like response. This enables complete island stabilisation at identical ECCD power and delaying the required onset timing of ECCD by approximately 70 percent sign % $\%$ , substantially extending the control response window. The CCE also effectively mitigates rotation-induced control degradation through enhanced current localisation. This work establishes CCE-enhanced ECCD as a robust mechanism for explosive burst prevention in high-bootstrap-current plasmas. The results provide critical insights for developing advanced stabilisation strategies in future steady-state reactors like ITER, while highlighting the need for future research on integrated control approaches that balance suppression efficiency with disruption risks.

Ripple-induced fast-ion losses modeling in the WEST tokamak

Hugo Corvoysier, J Morales, Jonathan Dufour, Guido Huijsmans, Samuele Mazzi, Remi Dumont

The confinement of fast ions is of crucial importance in present and future tokamaks. In typical ICRH-heated scenarios in WEST, fast-ion losses can represent more than 20 % of the total injected power. To investigate the origin and underlying mechanisms of these losses, we perform simulations of fast hydrogen ion trajectories in a realistic three-dimensional magnetic field. A weighting method is proposed as a post-process to study a particle distribution function with anisotropic temperatures, while preserving uniform statistics in term of energy. The influence of the initial and final particle position in phase space is studied in details. The simulation results show that ripple losses fall into two categories: deeply trapped particles moving vertically towards the baffle, and drifting bananas striking the antenna along the direction of the magnetic field. If fast particle losses to the baffle can be directly compared with infrared (IR) measurements, diagnosing particles striking the antennas is more challenging, as IR cameras detect a combination of fast-ion losses, plasma heat convection, and radiation. A net reduction of baffle strikes is observed for particles accelerated on the high field side, whereas antenna strikes, in contrast, may be enhanced. Hence, this study suggests that optimizing the ICRH absorption layer position involves a trade-off: reducing deeply trapped losses at the cost of increasing drifting banana losses.

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.

Mechanism of Ionization Avalanche in Tokamak Microwave Gas Breakdown

Jinwoo Gwak, Yeongsun Lee, Jeongwon Lee, Won Ik Jeong, Hyun-Tae Kim, Yong-Seok Hwang, Min-Gu Yoo, Yong-Su Na

Microwave breakdown driven by electron cyclotron (EC) waves provides a non-inductive route to plasma initiation in reactor-scale tokamaks. We introduce a three-dimensional Monte Carlo simulation that, for the first time, self-consistently treats nonlinear wave-particle interactions, atomic collisions, and guiding-center transport. The Monte Carlo simulation unveils the key role of parallel Brownian motion in the ionization avalanche mechanism. The predicted breakdown boundary is validated against KSTAR experiments. This work concludes that microwave gas breakdown will be successful under ITER-relevant conditions at a D$_2$ prefill pressure near 2 mPa with 1 MW of injected EC power.

Experimental investigation of ICRF antenna operation and plasma-wall interaction in the regime of propagating slow waves in front of the antenna

Raymond Diab, Laurent Colas, Seung Gyou Baek, Nicolas Fedorczak, Benoit Guillermin, James Paul Gunn, Julien Hillairet, Curtis A Johnson, Ernesto A Lerche, Guillaume Urbanczyk

Ion cyclotron range of frequencies (ICRF) antenna operation and plasma-wall interaction were investigated on the WEST tokamak in the regime where the density at the antenna limiters was sufficiently low for the slow wave (SW) to propagate in front of the antenna. Using a reciprocating emissive probe magnetically connected to the antenna, we measured for the first time the DC plasma potential, VDC, during a radial scan of the LH resonance layer across the antenna limiter. VDC peaks when the density at the antenna limiter edge approaches the LH resonance density, but never exceeds typical values of a few hundred volts. From the plasma-wall interaction standpoint, this regime is highly favorable: because particle fluxes are reduced while sheath potentials remain comparable to standard operating conditions at the same antenna voltage, local tungsten sources at the ICRF antenna and other outer-wall components become nearly undetectable. In general, tungsten sputtering from active WEST ICRF antennas is dominated by the particle flux rather than the sputtering yield; that is, it follows variations in the local density rather than the plasma potential. By contrast, sputtering in the divertor is primarily governed by the sputtering yield. Core impurity contamination is likewise significantly reduced when the antennas are positioned far from the separatrix, both with and without ICRF power, and the radiated power fraction decreases accordingly. Despite the modest coupled powers obtained at large antenna-plasma clearance, satisfactory ICRF heating is maintained as the density at the antenna limiter edge falls below the LH resonance, and no deleterious effects are observed in any key core plasma metrics when operating the antenna from this low-density region. Overall, the experimental results point toward minimal coupling to the SW by the fast wave antenna despite it being located in a region where the SW can propagate.

WEST Operation - Reliability and availability of a long pulse fusion tokamak

Valérie Lamaison, Cyril Brun, Elodie Corbel, Annika Ekedahl, Laurent Gargiulo, Sebastien Hacquin, Michael Houry, Lionel Meunier, Philippe Moreau, Lionel Toulouse

Since 2016, the WEST tokamak has demonstrated its capability to perform long plasma discharges approaching 1000 seconds in a fully metallic environment. It operates with a permanent magnetic field of up to 3.65T generated by 18 superconducting toroidal field coils cooled with helium at 1.8K produced by a cryogenic system. Since 2021, all plasma-facing components (PFCs), including the tungsten ITER-grade divertor, are actively cooled by pressurized water, making WEST representative of future superconducting fusion devices. Between 2022 and 2024, experimental campaigns achieved significant improvements in performance. The number of long-duration discharges (>100s) increased threefold thanks to the non-inductive current drive from the Lower Hybrid Current Drive (LHCD) system, culminating in a world record plasma duration of 22 minutes with 2.6 GJ injected energy. Total plasma time exceeded five hours per year, with over 70% of successful pulses. These results are enabled thanks to the availability of the WEST machine and all subsystems, higher than 70%. The study of downtimes recorded during the last three years experimental campaigns, shows four main elements/systems impacting WEST operation: the water and air leaks in vacuum vessel, the poloidal field system, the CODAC (Control, Data Access and Communication) system and the cryogenic system. Key lessons for future fusion devices to achieve plasma long pulses include a high availability of the machine based on a targeted maintenance plan to maximize reliability of all sub-systems and an effective responsiveness in incident diagnosis and repair, particularly in water leak detection on actively cooled Plasma-Facing Components (PFC).

First Experimental Evidence of Helicon Current Drive

J. B. Lestz, R. I. Pinsker, B. Van Compernolle, S. X. Tang, A. Dupuy, A. M. Garofalo, L. McAllister, C. P. Moeller, C. C. Petty, M. Porkolab, et al.

Helicon current drive is an attractive solution for driving current to sustain steady state tokamak operation in reactor conditions. Dedicated DIII-D experiments have been conducted with a MW-level helicon system and successfully demonstrated core power deposition and current drive with helicon waves launched via a traveling wave antenna. The profile of the measured electron temperature response to helicon power injection is in good agreement with time-dependent integrated modeling that incorporates ray tracing and the effects of thermal transport simultaneously. When the helicon power is injected continuously to drive co-$I_p$ current, the reconstructed safety factor profile flattens significantly faster and sawteeth are triggered earlier than in comparison shots where the helicon is replaced by a comparable amount of electron cyclotron heating. Calculation of the helicon-driven current profile yields a peaked profile in the core, consistent with the observed power deposition profile and in good agreement with ray tracing predictions. Taken together, these experimental results represent strong evidence for the first definitive observation of auxiliary current drive due to helicon waves on any device.

RF-Specific Tungsten Erosion and Global Transport in ITER under Neon Seeding

Atul Kumar, Dhyanjyoti Nath, Wouter Tierens, Jeremy D. Lore, Andrei Pshenov, Tom Wauters, Andrea Galvan, Davide Curreli, Syun'ichi Shiraiwa, Nicola Bertelli, et al.

Ion cyclotron radio-frequency heating (ICRH) is a key auxiliary heating system in ITER, but high-power RF operation can enhance plasma-material interactions through rectified RF sheath potentials on antenna structures and nearby plasma-facing components. We present the first predictive application of the STRIPE (Simulated Transport of RF Impurity Production and Emission) framework to assess RF sheath-driven tungsten (W) erosion and global impurity transport from the ITER ICRH antenna under ITER-relevant neon-seeded conditions. STRIPE couples SOLPS-ITER plasma backgrounds, full-wave RF sheath calculations, geometry-specific ion energy-angle distributions, sputtering physics, and three-dimensional impurity transport. Simulations predict RF sheath potentials of 1 to 3 kV on antenna limiter sidewalls, increasing gross W erosion by about a factor of 64 relative to thermal sheath conditions and producing a gross source of 3.34e18 W atoms per second. Erosion is governed by RF-modified ion energy-angle distributions together with local plasma flux rather than sheath voltage alone. About 10 percent of sputtered W is locally redeposited, giving a net source of 3.01e18 W atoms per second. The RF-induced antenna source remains about three orders of magnitude smaller than the thermal divertor source and more than two orders of magnitude smaller than the integrated thermal main-chamber source. After 100 ms, about 22 percent of the mobile W inventory resides within the SOLPS-covered confined-plasma region, corresponding to an annular W concentration of 1.70e-6. These results indicate that the ITER ICRH antenna is unlikely to dominate the total W source budget under the conditions considered and demonstrate the need for coupled modeling of RF waves, sheaths, sputtering, redeposition, and global impurity transport.

Electromagnetic analysis of beams for the ITER ECH Upper Launcher

P. Platania, A. Simonetto, J.M. Arroyo, E. Carbonell, N. Casal, M. Choe, D. Farina, G. Garre, M. Henderson, M. Jimenez, et al.

Per-tube parameter characterization and dual-gyrotron operation validation of the CRAFT 170 GHz ECRH test stand

Yu Wang, Zege Wu, Huaichuan Hu, Jingping She, Haoming Chang, Haibiao Zhang, Xiaojie Wang

Helium‑3 minority heating with ion cyclotron range of frequencies (ICRF) in the experimental advanced superconducting tokamak

Yongxin Zhu, Wei Zhang, Yevgen Kazakov, Jinhua Wu, Paola Mantica, Gabriele Cassella, Tao Jin, X. J. Zhang, Lunan Liu, Hua Yang, et al.

During the 2025 campaign, helium-3 ( 3 He) minority heating with waves in the ion cyclotron range of frequencies (ICRF) was investigated for the first time on the Experimental Advanced Superconducting Tokamak (EAST). With the lowest available ICRF frequency of f IC = 27 MHz, experiments were conducted at a high toroidal magnetic field of B t = 2.8 T and plasma current I p = 450 kA. To optimize 3 He minority heating, the variation of 3 He concentration was systematically explored. Real-time feedback control of the 3 He concentration was successfully implemented through spectroscopic measurement and closed-loop regulation of the 3 He gas injection, demonstrating the feasibility of the control system functions. The ICRF heating efficiency reached a maximum at a minority concentration of ∼8-9%, with the core electron temperature increasing from approximately 5.0 to 7.0 keV and the ion temperature from approximately 1.3 to 1.9 keV under 2.9 MW of ICRF power. These results are in good agreement with simulations from the two-dimensional full-wave code TORIC. Experiments further indicate that higher plasma density enhances 3He heating efficiency. We also briefly discuss the strategy for future 3 He ICRF experiments on EAST.

A trend-aware data-driven approach for short-term prediction of ICRF antenna–plasma coupling

Wentao Geng, Donghui Xia, Qihang Jiang, Yulong Deng, Junjie Wu, Lianghui Yang, Yong Hua Ding

The antenna–plasma coupling plays a critical role in the performance of ion cyclotron range of frequency (ICRF) heating systems and can vary rapidly under changing plasma conditions, posing challenges for conventional impedance matching approaches. In this work, a data-driven method is proposed for short-term prediction of ICRF antenna–plasma coupling based on experimental data from the J-TEXT tokamak. The prediction problem is formulated in a residual manner, and a trend-aware learning strategy is introduced to emphasize dominant low-frequency dynamics while suppressing high-frequency fluctuations. Causal preprocessing is applied to ensure compatibility with real-time applications. Two sequence modelling approaches, temporal convolutional networks (TCN) and long short-term memory (LSTM) networks, are employed for evaluation. Both models achieve consistent improvement over a quasi-static baseline across multiple prediction horizons in terms of averaged RMSE, with the largest improvement observed at intermediate time scales. The results indicate that prediction performance is closely related to the temporal characteristics of the coupling signal, where low-frequency-dominated dynamics are more predictable than rapidly varying perturbations. Overall, the proposed approach improves short-term prediction performance while also revealing intrinsic predictability limitations, providing a useful reference for future real-time matching and control strategies.

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.