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Masayuki Yoshikawa, Naomichi Ezumi, Junko Kohagura, Mafumi Hirata, Akihiko Sugiyama, Yasunori Washo, Yoriko Shima, Yui Nagano, Takuma Okamoto, Haruhiko Himura, et al.
Tarundeep Kaur Lamba, L. Moser, Martin Kocan, Martin Vincent, Yan Rong, Ding Rui, Zhang Wei, Tom Wauter, Richard A. Pitts, Walsh Michael, et al.
Lidia Piron, Marco Gobbin, Nicolò Ferron, Matteo Baruzzo, Eric Fredrickson, Zamir Ghani, Krassimir K. K Kirov, Morten Lennholm, Roberto Piovan, P Martin
Results from the last DD and DT JET campaigns in the framework of the EUROfusion Tokamak Exploitation Work Package Activity" 2026 submitted to Nuclear FusionIn this work, we present a real-time algorithm for use in fusion-fission hybrid reactors to monitor the fast neutron production (neutron rate) generated by Deuterium-Tritium fusion reactions. The algorithm, based on insights from TFTR and JETD-Tritium operations, tracks continuously the neutrons 14.1 MeV. If this metric falls below a threshold value, due, for example, to fuel dilution, the onset of MHD instabilities or temporary actuator unavailability, the algorithm can raise an alarm. This enables plasma recovery when actuator capability permits, or ensures safe plasma termination when recovery is not feasible. The role of alpha particles is examined through test cases of the algorithm, while their confinement properties are investigated using the ORBIT code under various RFP magnetic field configurations.
DARIO BORGOGNO, Daniela Grasso, Lovepreet Singh
We numerically investigate the mutual interaction between runaway electron (RE) current and magnetic reconnection in a post-disruption plasma configuration where the plasma current is entirely carried by REs. Extending previous two-dimensional studies to full three-dimensional geometry, we show that the presence of RE current enhances the reconnection process, resulting in a reconnected area approximately 50% larger than in the absence of REs. In the 3D regime, the nonlinear interaction among multiple unstable tearing modes with different helicities drives the development of magnetic chaos, which rapidly spreads from the island separatrices to the entire reconnected region. Once global chaos is established, the chaotic magnetic field lines efficiently redistribute the RE current, leading to a progressive flattening of its radial profile. This occurs over a timescale of approximately 300τ A , corresponding to approximately 10µs for typical JET parameters, in quantitative agreement with experimental observations. These results provide a consistent physical framework for interpreting RE redistribution and loss in post-disruption plasmas, with direct implications for disruption mitigation strategies in future high-current devices such as ITER.
Simon Van Mulders, Olivier Sauter
The RAPTOR code is used to model how the time required for controlled termination of Ohmic plasmas scales from present tokamaks like TCV and JET, to reactor-grade tokamaks like ITER and DEMO. We show that ramping the plasma current Ip down to 20% of the flat-top value over a time Δt ramp−down =τLR=Li/R, with internal inductance Li and resistance R evaluated at flat-top conditions, results in an approximately self-similar peaking of the current density for these four tokamaks, indicating the adequacy of τLR as a relevant timescale for cross-machine comparison, yielding τLR= 0.033s (TCV), 2.87s (JET), 63.2s (ITER) and 166.9s (DEMO). Note that τLR is easy to evaluate, both in systems codes and on a real-time control system. For the simulated ramp-downs with Δt ramp−down =τLR, the end-of-ramp-down normalized internal inductance ℓi3 is limited below 2. An Ip ramp-down faster than τLR=Li/R requires a reversal of the boundary loop voltage and leads to the formation of a broad plasma layer carrying current in the direction opposite to the total plasma current, concomitant with ℓi3>2, a central region with low magnetic shear and strongly peaked pressure profiles. Significant reduction of plasma volume and elongation, as foreseen for ITER and DEMO, is shown to counteract the reversal of current density and the ℓi3 increase, while easing vertical stability control, potentially enabling faster Ip ramp-down scenarios. Experimental and theoretical studies should be performed to test the feasibility of such fast termination scenarios, notably with respect to vertical position control, shape control and (resistive) beta limits. An analytical model is proposed to estimate τLR based on 0D engineering parameters. For burning plasmas, ITER baseline simulations show that a termination within 150% of the L-mode τLR can avoid negative edge current density and a large ℓi3 increase for an HL transition 1/3rd into the ramp-down.
Jia Li, Dong Guo, Liu Shuo, Lombroni Riccardo, Zhixin Wang, Renyi Tao, Yumin Wang, Yuejiang Shi
Plasma current measurement is essential for tokamak equilibrium, control, and machine protection. Fiber-optic current sensors (FOCS) provide electrical isolation and immunity to electromagnetic interference, while their metallic in-vessel protective routes remain subject to electromagnetic loads and vacuum-boundary constraints. An annealed-copper FOCS protective tube in EXL-50U developed a vacuum leak, and inspection found localized flattening, rubbing, blackened surfaces, and fracture-like damage near center-column supports. Possible failure mechanisms were assessed using the assembly geometry, inspection evidence, electromagnetic estimates, and an idealized ring-compression reference. A representative normal toroidal-field (TF) ramp gives a loop current scale of approximately 1.4 kA and a distributed line load of 3.2 N/mm. A representative 500 kA vertical displacement event yields a toroidal-flux change of about 0.0639 Wb over a 0.1 ms output interval, corresponding to a finite-difference voltage scale of 639 V. An ideal impulse calculation gives currents of 1.6–12.8 kA for the assumed coupling and inductance range. The estimates show the scale of possible electromagnetic loading, but do not determine the local flattening threshold or the exact cause of failure. The observations suggest that tube motion against relatively rigid surrounding structures contributed to local deformation and possible electrical or thermal damage. The revised design uses a 316L stainless-steel tube, additional supports, polyimide insulation at support interfaces, and improved seals. We also discuss the design limitations and a ceramic electrical break as an alternative.
Thomas Wegner, Maciej Krychowiak, Laurent Krier
Real-time feedback control is essential for meeting the physics and operational demands of Wendelstein 7-X (W7-X), particularly the need to respond effectively to transient events, maintain stationary high-performance phases, and operate reliably across a wide range of magnetic configurations and plasma regimes. This work provides an overview of the feedback systems implemented and validated in recent campaigns, including the control of key plasma parameters such as density, radiation, and heating power under varying conditions. These systems rely on the W7-X segment control architecture and its millisecond-scale real-time network, which enable precise and deterministic coordination of sensors and actuators. Representative experiments highlight robust performance, including long stationary discharges and integrated multi-parameter control. Additional boundary conditions such as core electron temperature limits ensure protection against radiative collapse and other failure modes. Looking forward, planned developments include advanced actuators, neural-network-assisted control strategies, and long-term efforts toward full profile control of electron density and temperature. Overall, these developments establish real-time feedback control as an essential element of W7-X operation and a robust basis for advanced control concepts in future steady-state fusion devices.
J. F. Parisi
Building on recent ignition-access work of Delgado-Aparicio, Ono, and Menard, we show that even a single-use, gram-scale quantity of spin-polarized fuel (SPF) is useful for increasing fusion power and gain in magnetic confinement fusion machines such as tokamaks and stellarators. While fueling a fusion power plant continuously with SPF requires $\sim$kilograms per day, far beyond present capabilities, a single-use short pulse of SPF allows a fusion plasma to cross into a high-gain regime, and stay there, even after switching back to regular unpolarized fuel. With continuous polarized fueling, polarization also makes high-gain plasmas easier to control: a resonant wave that depolarizes the fuel lowers the reactivity quickly, on a much faster timescale than transport. Grams of polarized fuel can therefore improve plasma performance and open a path to scaling SPF sources and usage, from small first experiments to continuous fueling.
Lingen Huang, Long Yang, Alejandro Laso Garcia, Oliver S. Humphries, Michal Šmíd, Mikhail Mishchenko, Victorien Bouffetier, Carsten Baehtz, Erik Brambrink, Samuele D. Di Dio Cafiso, et al.
A novel diagnostic is proposed for active probing of doped inertial confinement fusion (ICF) spherical shells, enabling direct measurements of mix and burn dynamics at next-generation implosion facilities via resonant absorption imaging with an X-ray free-electron laser (XFEL). The charge-state-sensitive imaging of embedded dopants provides spatiotemporally resolved constraints on ionization, opacity, and material mix seeded by hydrodynamic instabilities in stagnated plasmas. Proof-of-principle experiments demonstrating resonant XFEL probing of hot spot in coated copper wires driven by an ultra-short relativistic laser pulse, have established the feasibility of this approach. Furthermore, atomic and radiation-hydrodynamic simulations, combined with synthetic resonant X-ray absorption imaging, extend the diagnostic concept to laser-driven direct-drive ICF shells with copper-doped ablators and radiation-driven indirect-drive shells with tungsten-doped ablators. This unique approach, combining a laser-driven implosion facility with a high-brightness XFEL, could enable precise measurements of fusion-relevant hot dense plasmas at multi-keV temperatures, provide stringent benchmarks for radiation-hydrodynamics models, and advance the realization of inertial fusion energy.
Mingdong He, Qixian Hu, Tengfei Xu, Tailin Wu, Zheng Xiong
Real-time equilibrium reconstruction is essential for tokamak plasma control. Data-driven surrogates are usually trained on high-quality labeled experimental data, but producing a large amount of labeled data sufficient for surrogate learning requires many expensive discharge shots, and few exist at the start of a new campaign or the operation of a new device. To reduce this cost, we propose to pre-train on simulated equilibria, generated at low cost by physics-based simulation, then fine-tune on limited real data. We evaluate under three settings with increasing difficulties, i.e., in-distribution, out-of-distribution (shape), and cross-campaign (temporal) splits, which mirror how reconstruction is actually deployed. Experimental results validate that synthetic pre-training is highly effective when few real labels are available: fine-tuning on only 1% of the full available real dataset cuts the per-sample normalized root-mean-square error (nRMSE) of the reconstructed poloidal flux by roughly 58% under out-of-distribution shapes and 64% under cross-campaign extrapolation, compared to a model trained from scratch on the same amount of real data. These realistic evaluations show that synthetic pre-training improves equilibrium reconstruction accuracy in the data-scarce regime that real tokamak campaigns face.
Nicola Fonnesu, Stefano Loreti, Rosaria Villari, Davide Flammini, Andrea Colangeli, Fabio Moro, Simone Noce, Alberto Previti, Axel Klix, Xavier L Litaudon
Within the framework of EUROfusion's technological exploitation of deuterium–tritium (DT) operation at JET — initiated under work package JET3 and continued under PrIO — the NEXP sub-project was conceived to exploit the substantial neutron yield expected during the DT campaigns (DTE2 and DTE3) to validate the numerical tools employed for neutron streaming and shutdown dose rate (SDDR) prediction in ITER. This validation, grounded in direct comparison between computational predictions and experimental measurements, constitutes a critical step in demonstrating the reliability of the SDDR methodology that underpins ITER's radiation safety case. As part of this activity, a dedicated dosimetry system based on spherical air-vented ionization chambers was installed in the JET Torus Hall in 2015 and has since undergone successive hardware and software upgrades to address operational challenges identified across DT experimental campaigns. This work traces the evolution of the SDDR dosimetry system and presents the analysis of the measurements acquired during the DT operation of JET, in particular the third DT campaign (DTE3) and, uniquely, throughout the ongoing final shutdown, a phase offering measurement conditions unlikely to be replicated elsewhere. SDDR predictions with the Advanced D1S tool by ENEA are described as well.
Abha Kanik, Bharat Hegde, Arun Sarma, Santosh Dubey, Laxmikanta Pradhan, Ankit Patel, Kalpesh Galodiya, Kumarpal Jadeja, Minsha Shah, Dinesh Rathod, et al.
H. R. Strauss
Tokamak disruptions caused by tearing modes can be prevented with magnetic feedback. A database of DIII-D locked mode disruptions provides criteria for major disruptions. The main criterion is that the q = 2 rational surface must be sufficiently close to the resistive wall surrounding the plasma to interact with it. This is shown in simulations of a sequence of model equilibria derived from the database. If the wall is ideally conducting, major disruptions do not occur. Magnetic feedback emulates a conducting wall, and can prevent major disruptions. Feedback coils must be designed appropriately, otherwise they are not effective in disruption prevention. Simulations of a model of MST also show the q = 2 criterion for major disruptions, as well as feedback stabilization. Feedback offers an attractive alternative to disruption mitigation by impurity injection.
Tullio Barbui, Oulfa Chellai, Luis F. Delgado-Aparicio, Alexandre Fil, Beatriz García, Kenneth W Hill, Shinichiro Kojima, Novimir Antoniuk Pablant, Brent C Stratton, D Vezinet, et al.
A new soft x-ray (SXR) synthetic tool has been developed: it is called FAST for FLYCHK-based Atomic physics and SXR Tomography. It is based on FLYCHK SXR emissivity spectra calculated for a variety of elements, from deuterium to tungsten. FAST creates 2D plasma emissivity for arbitrary plasma compositions and plasma profiles. It computes line-integrated SXR profiles based on a specific detector geometry and response. This tool is used to reproduce signals from an absolutely calibrated multi-energy SXR (ME-SXR) camera at WEST. Thanks to its energy-resolved measurement capability, the ME-SXR camera is able to separate spectral regions dominated by impurity line emission from others containing only continuum emission. At WEST, FAST and the ME-SXR camera are employed to determine impurity density profiles for tungsten and nitrogen (used as a proxy for low-Z impurities), as well as the plasma effective charge (Zeff), through SXR measurement reproduction at different energies, from 2 to 14 keV. Tungsten density is determined in two plasma scenarios at WEST: lower hybrid heating in the X-point radiator regime and lower hybrid heating with modulations of ion cyclotron resonance heating. In the first scenario the tungsten profile is found to be moderately peaked in the core with a concentration of 0.8‒1×10-4. This concentration is found to increase up to 1.4×10-4 during ion cyclotron resonance heating. Tungsten peaking factor in steady-state conditions is also retrieved and compared to theoretical predictions. It is found to be slightly lower than the combined neoclassical and turbulent transport prediction.
Wenjing Tian, Zongyu Yang, Ting Long, Shuosu Yang, Hao Wu, Deliang Yu, Xiaoxue He, Chenchong Tang, Rui Ke, Min Xu, et al.
We present a neural network-based method for real-time charge exchange recombination spectroscopy (CXRS) analysis on the HL-3 tokamak. To enhance model performance and reliability, we tailor the network architecture to the physical characteristics of CXRS spectra: the receptive field matches the Doppler-broadened line width, channel embedding encodes spatial awareness across diagnostic channels, and attention pooling focuses on the most informative spectral regions. These designs yield millisecond-level inference (0.56 ms per 32-channel) with accuracy comparable to conventional methods (R²_Tᵢ=0.93, R²_vₜ=0.98). Furthermore, model’s reliability is established through interpretability and uncertainty analysis. Interpretability analysis confirms that the model focuses on the Doppler-broadened wings of C VI for T_i and tracks line shifts for v_t, consistent with physical principles. Uncertainty estimates shows that the model appropriately expresses higher uncertainty in regions where predictions are inherently more challenging. To address data scarcity on new devices, transfer learning from HL-2A tokamak markedly boosts performance in low-data regimes while retaining a modest advantage even with full HL-3 data, provided that the spectra are properly aligned and baseline subtracted. This work establishes a real-time, reliable, and data-efficient AI framework for CXRS analysis, offering a practical path to deploy machine learning approaches in plasma control on future fusion devices.
zhongfang Guan, Erwan Grelier, Bin Zhang, Sébastien Vives, Baoguo Wang, Victor Moncada, Leo Dubus, Jinping Qian, Kedong Li, Jian Liu, et al.
This work presents an engineering-oriented data processing and model training pipeline for EAST visible and WEST infrared data, enabling a systematic study of cross-device and cross-modal electric arc detection. A lightweight convolutional neural network is adopted as a cross-device arc detection model, and joint training and transfer learning experiments are conducted on EAST visible (VIS) and WEST infrared (IR) data. Single‑modality baselines are first trained on EAST and WEST, then evaluated zero‑shot on the opposite device, followed by a comparison of frozen‑backbone and fine‑tuned transfer for VIS→IR and IR→VIS. The results show that models trained from scratch on each diagnostic channel already achieve high precision and recall for arc detection, whereas direct zero-shot cross-modal transfer drives the arc recall on the opposite domain almost to zero. In contrast, starting from the single-modality baselines and modestly fine tuning higher convolutional layers and the classification head on the target domain can significantly restore-cross device, cross-modal detection performance, with positive class precision and recall approaching or even surpassing the single-domain baselines. Class activation map analysis further indicates that, under fine-tuned and joint training configurations, the models consistently focus on the antenna, the arc, and its surrounding scattering region, supporting physically meaningful explanations and suggesting that cross-device, cross-modal training can capture arc related structures that generalize across devices while providing a lightweight, scalable route to reusing imaging-based diagnostic knowledge with reduced annotation and retuning effort.
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.
Y.S. Zhang, Y.P. Zhang, J. Zhang, W.L. Zhong, M. Isobe, K. Ogawa, D. Mazon, Q.L. Yang, Z. Gao, J.M. Gao, et al.
K. Oshima, T. Shikama, K. Hanada, T. Onchi, Y. Otsuka, K. Kuroda, A. Kuzmin, H. Idei, T. Ido, R. Ikezoe, et al.
The densities of chromium atoms and ions in coaxial helicity injection plasmas on QUEST, a spherical tokamak with a major radius of 0.68 m and a minor radius of 0.40 m, were measured using optical emission spectroscopy. Effective electron temperatures and densities were evaluated from the ratios of chord-integrated Balmer line intensities using a collisional–radiative model, yielding values of approximately 3 eV and 3.6 × 1019 m−3 near the electrodes during the early phase and 12 eV and 1.4 × 1018 m−3 in the confinement region during the closed flux surface (CFS) formation phase. Using these plasma parameters and the chord-integrated intensities of Cr I, Cr II, and C II lines, the chord-averaged densities of chromium atoms and Cr+ and C+ ions were estimated. The chromium atom density near the electrodes during the early phase was approximately 8 × 1012 m−3. The estimated ionization mean free path indicated that chromium atoms are ionized primarily near the electrodes. Cr+ ions were inferred to be transported into the confinement region by the upward plasma flow. The resulting Cr+ ion density during the CFS formation phase was approximately 1 × 1017 m−3, and the C+ ion density was of the same order of magnitude. These results provide insight into the ionization and transport of electrode-derived metallic impurities.
Woong-Ryol Lee, Taehyun Tak, Luca Zabeo, Timo Ravensbergen, Anze Zagar, Piotr Perek, Walter Van Herck, Peter De Vries, Seung-Ju Lee, Lana Abadie, et al.
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