Recent Publications

Experimental Plasma Density Profiles Determined Through Measurements of the Magnetosonic Wave Speed

Cameron Kuchta, Jan Egedal, Abhishek Mhatre, Paul Gradney, Joseph Olson, Xinyu Yu, Cary B. Forest

Information on plasma density in laboratory plasmas is commonly acquired using either Langmuir probes or optical diagnostics. Here, we present an alternative approach, inferring the density profile from magnetic measurements of a plasma wave. In particular, during the process of creating a reconnecting current layer for magnetic reconnection experiments in the Big Red Ball, the reconnection drive first launches a large amplitude fast magnetosonic wave. The propagation of the wavefront is measured with high spatial and temporal resolutions by in situ magnetic diagnostics. Given a known uniform background magnetic field strength and the known dispersion relation of the wave, we here show how the characteristics of the wavefront can be applied to determine the initial plasma density profile.

A real-time data acquisition system for the magnetic diagnostic of COMPASS-U

João Oliveira, André Torres, António Joaquim Batista, Bernardo B Carvalho, Ivan Ďuran, Aleš Havránek, Tomas Markovic, Marie Vanakova, P Sladek, Jorge Sousa, et al.

A bespoke real-time data acquisition system for the magnetic diagnostic of COMPASS-U was developed. The system targets a high density of galvanicly isolated channels, each with a dedicated 2 MSPS 18-bit converter. Based on the ATCA standard, it supports up to 576 channels in a single sub-rack. The system connects the inductive magnetic sensors to the real-time tokamak control system, performing the digitization, pre-processing, and numerical integration of the magnetic signals. The modular architecture of the system is described and its advantages in handling different types of magnetic sensors are discussed. The front-end modules sit on a data acquisition main-board which reads the digital data from the converters and does the numerical integration of the signals. Since the system operates in a real-time environment where latency is one of the primary concerns, an FPGA serves as its main processing element. The FPGA is part of a System on Module (SoM) which also contains a CPU running a Linux operating system, enabling advanced remote management and supervision capabilities. A prototype of the data acquisition system was built for the purposes of validating its architecture, performance and electronic design. We have successfully demonstrated the operation of the data acquisition system and confirmed its performance meets the requirements for COMPASS-U operation.

JET neutron emissivity reconstruction using the Minimum Fisher Information method for 1 ms temporal resolution

Katarzyna Mikszuta-Michalik, Daniele Marocco, Basilio Esposito, Marco Riva, Gianluca Pucella

A tomography code based on the Minimum Fisher Regularisation method with a geometry description based on the field of view has been applied to reconstruct the neutron emissivity profiles from measurements performed in 2021 during the second JET deuterium-tritium campaign (DTE2). The Neutron Profile Monitor Upgrade installed at JET offers a unique opportunity for investigating the neutron emissivity in a full poloidal cross-section with unprecedented time resolution (~1 ms), thanks to the availability of digital data reprocessing and the high neutron emission that enables milisecond line-integrated raw data with low statistical error. Measurements of 14.1 MeV neutrons provided by the Bicron BC418 plastic scintillators installed in the JET neutron camera have been used.The analysis focuses on the effect of sawtooth crashes on the neutron emission profiles. Tomography results were compared with 1D reconstructions using two different methods and 10 ms time resolution. The use of the neutron camera data sampled at 1 ms enables the observation of hollowing of the neutron emissivity profiles during the sawtooth crash and their subsequent flattening.

Machine learning tomography for sparse-view soft X-ray diagnostics in MAST-U

Brian Steward, Marco Cecconello

Understanding magnetohydrodynamic (MHD) instabilities is crucial for advancing magnetic confinement fusion. Soft X-Ray (SXR) tomography enables reconstruction of the spatial structure of these instabilities from one dimensional line integrated measurements, however traditional tomography methods struggle with sparse diagnostic arrays. The MAST-U tokamak has a sparse camera array of 28 non-intersecting lines of sight (LoS), making traditional tomography techniques challenging. We present a machine learning (ML) tomographic method using a 50 layer residual neural network (ResNet) trained on synthetic data simulated by the transport code TRANSP. ML tomography is compared with traditional minimum Fisher information tomography, and with second derivative smoothing tomography. The ML technique shows a 78 to 94 times median improvement in the mean square error (MSE) over traditional 28 LoS reconstructions of synthetic data, at a computational speed which is 144 to 294 times greater (25 ms on GPU hardware). Notably, ML tomography with 28 LoS outperforms traditional methods even when provided with three times as much data in the form of 84 intersecting synthetic LoS. Validation on experimental data from MAST-U demonstrates accurate reconstructions of peaked and broad plasma SXR profiles, and the evolution of sawtooth instabilities are visualized with ML tomography. The sub-30 ms reconstruction time enables real-time plasma diagnostics, opening new possibilities for MHD monitoring in fusion devices.

A composite impurity-pressure index for assessing startup readiness during plasma recovery on EAST

Shuqi Yang, Yaowei Yu, Tao Zhang, Xiang Zhu, G Z Zuo, Xiang Gao

Reliable restart after vessel venting is controlled by the burn-through power balance: the plasma must ionize the residual neutrals, dissociate molecules and raise the electron temperature before ionization, charge-exchange and impurity-radiation losses exhaust the available ohmic or auxiliary power. In practice, however, operators usually do not know whether the wall has recovered until a shot has already been attempted. Six EAST plasma-recovery campaigns from 2023 to 2025 are analysed, and a pre-shot impurity-pressure index is introduced to combine the neutral pressure measured 2–3 s before breakdown with a weighted residual-gas-analyser proxy for nitrogen- and oxygen-bearing species. In this dataset, the index organises discharge duration, stable-shot probability and the loop voltage during the first 50–200 ms more clearly than either pressure or composition alone. Under the standard EAST startup condition used here, stable discharges become much more likely when the index falls to around 1 × 10-7 Pa. In the second 2025 campaign, the shot-by-shot evolution also shows threshold-like burn-through behaviour: the maximum line-averaged electron density in 0–0.2 s stays low at large index values, overshoots in a transition interval, and then settles to a more stable level as the wall recovers. Across campaigns, post-recovery values cluster much more tightly than the recovery paths themselves, indicating that the index characterizes the startup-ready wall state rather than a particular conditioning route. The metric therefore provides a practical pre-shot indicator of whether EAST has recovered sufficiently for reproducible stable startup.

Design and commissioning of a tangential Gas Puff Imaging diagnostic on the HL-3 tokamak

W.C. Wang, Z.H. Huang, Z.B. Shi, Y. Hu, J. Cheng, K.Y. Yi, W.M. Xie, Y. He, N. Wu, W. Zhao, et al.

Runaway electron control by self-excited waves

Kun Huang, Boris Breizman

Runaway-electron avalanches in tokamak plasmas can be limited by kinetic instabilities driven by the non-Maxwellian runaway distribution. We formulate a reduced model for the quasi-steady state in which the total plasma current and bulk electron temperature are prescribed, while the inductive electric field is determined self-consistently from the partition between Ohmic bulk current and runaway-electron current. Because the wave growth time is short compared with the current-decay time, we consider a marginal-stability regime, in which whistler-wave drive by the runaway electrons balances collisional damping. The resulting states separate into three regimes: a subcritical Ohmic regime without an avalanche, an avalanche regime in which runaway growth relaxes the inductive field to the avalanche threshold, and an instability-regulated regime in which self-excited whistler waves enhance momentum-space diffusion and limit the runaway current. In the instability-regulated regime, the whistler wave spectrum forms a narrow ridge, and low-energy runaway electrons carry most of the runaway current.

Neoclassical tearing mode seeding by Alfvén eigenmode coupling in MAST-U

Kitt Cameron Medley Thomas, Laszlo Bardoczi, Kieran Gibson, Juan Ruiz Ruiz, Mykola Dreval, David Anthony Ryan, Kenneth G McClements, Clive A Michael

We present evidence of tearing mode seeding by nonlinear three-wave coupling of Alfvén eigenmodes in a tokamak plasma. Here, a m,n=2,1 magnetic island appears to be seeded in a MAST-U discharge by coupling of n = 2 and n = 3 Alfvén eigenmodes, which is demonstrated by increased squared normalised bicoherence calculated from magnetic probes preceding tearing mode onset. This observation offers a new explanation for tearing modes that would be otherwise addressed as spontaneous, and indicates three-wave coupling can seed 2,1 magnetic islands in plasmas without pre-existing n ≥ 2 tearing modes. Further, this mechanism has the potential to be more prevalent in burning plasmas expected in future fusion devices.

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.

Neutron measurements with VERDI detectors in the long term irradiation station at JET tokamak during Deuterium-Tritium operation

Ion Evangelos Evangelos Stamatelatos, Marilia Savva, Theodora Vasilopoulou, K Mergia, S Messoloras, Steven Bradnam, Chantal Shand, Lee William Packer, Zamir Ghani, Paola Batistoni, et al.

The VERDI detector is a passive neutron detector using the multi-foil activation technique to measure neutron fluence in extreme fusion conditions. It features a low-activation capsule that contains a defined concentration of metallic elements. The robustness of the detector is ensured by the capsule material, while neutron detection relies on activation of the metallic elements. Post-irradiation gamma spectrometry combined with computational unfolding enables reconstruction of the neutron fluence and energy spectrum. This study reviews the development of the VERDI detector and reports the first deployment of VERDI detectors in an operational Deuterium-Tritium (D-T) fusion environment in the Long-Term Irradiation Station (LTIS) of JET tokamak during the DTE2 campaign, featuring a neutron spectrum representative of future fusion devices such as ITER and DEMO. The results provide benchmarks for validating neutronic simulations and nuclear data, supporting the design optimization and safety assessment of future fusion power plants.

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.

Design of a Pop-Up Divertor Langmuir Probe System for the HL-3 Tokamak

Jichan Xu, Shuang Xu, Qinglai Qiu, Yunfei Dai, Yang Zhang, Xiaodong Zhang, Tengfei Sun, Jinming Gao, Yuxiang Liu, Zhihui Huang, et al.

Assessing the effect of error field penetration during plasma current ramp-up in the DIII-D tokamak

C. F. B. Zimmermann, E. M. Bursch, C. Paz-Soldan, J. M. Hanson, N. Leuthold, N. C. Logan, A. O. Nelson

This work provides evidence that established error field penetration threshold scalings remain applicable during plasma current ramp-up. In dedicated DIII-D experiments with imposed $n=1$ perturbations during extended $I_p$ ramps, an apparent empirical threshold is found between $2$ and $3$~kA of applied 3D coil current, above which MHD modes are seeded. The imposed perturbation couples to the rational surfaces present during the ramp, seeding near the $q=4$ surface and penetrating as an $m/n=3/1$ mode by the end of the perturbation phase. To interpret these observations, multi-machine penetration threshold scalings are combined with equilibrium-based overlap metrics from the GPEC code, including the in-situ error fields of the device. This modeling reproduces the observed onset in the amplitude scan and classifies mode seeding across a database of 12 ramp-up discharges spanning a range of plasma currents and densities. Across this database, the seeding appears to be controlled primarily by the applied 3D coil current rather than by the plasma current or its ramp rate. Accounting for the in-situ error fields is found to be important for reliable prediction. These results are consistent with the robustness of scaling-based penetration metrics when coupled to detailed 3D field modeling under transient ramp-up conditions, and suggest the importance of accounting for in-situ error fields when assessing additional externally induced perturbations. This work is motivated by future tokamaks in which transient, non-axisymmetric error fields can arise during startup, for example from runaway electron mitigation coils.

Development of a graphical user interface for Grad–Shafranov equilibrium analysis in the Alvand-U tokamak

Yahya Sadeghi

Demonstration of mode number determination from magnetic probes at the PlasmaLab@CTU

Jana Brotánková, Jakub Dlouhý, Gergo I Pokol

Magnetic probes are the most exploited diagnostic method for investigating the spatial structure of magnetic perturbations in magnetic confinement plasmas. These structures can range from different MHD modes via magnetic islands to precursor oscillations of MHD events. Most magnetic confinement devices are equipped with sets of magnetic pick-up coils; the spatial structure of the magnetic perturbations is derived from the relative phases of the coil signals by a large variety of signal processing methods. These methods can be demonstrated on the Magnetic stand in the PlasmaLab@CTU on a new device, called Vrtichvost. It consists of a rotating assembly of wires representing plasma current perturbations and an array of eight coils resembling a poloidal array of Mirnov coils in tokamaks, plus one additional coil with exchangeable heads. The paper presents the parameters of Vrtichvost and the demonstration of the most common mode number determination methods. The significance of the measurement of the frequency transfer function of the coils for the mode number determination is also illustrated, and many further measurement possibilities are mentioned.

Numerical studies of mode coupling induced by neoclassical toroidal viscous torque in error field penetration on EAST

Cheng Ye, Youwen Sun, Hui-Hui Wang, Yueqiang Liu, Pengcheng Xie, Jian Xu, Hui Sheng, Xin-Jian Wang, T Y Xia

Mode coupling induced by neoclassical toroidal viscous (NTV) torque governs error field penetration in toroidal plasmas , causing the $2/1$ penetration threshold to deviate from linear response prediction with respect to the RMP upper-lower coil phasing ($\Delta\phi_{UL}$). This behavior originates from nonlinear modulation of toroidal momentum transport by non-resonant three-dimensional magnetic field components. In this work, the characteristics of such mode coupling is systematically investigated using the MARS-Q code. Two aspects are examined. First, the dependence of mode coupling strength on key plasma parameters is analyzed. A larger momentum diffusivity ($\chi_M$) is found to strongly enhance mode coupling and invalidate linear response criteria, whereas at low $\chi_M$ linear prediction remains applicable. A NTV torque weighting factor based on linear response is identified as a qualitative indicator of mode coupling in the nonlinear field penetration. In addition, reduced resistivity ($\eta$), higher normalized beta ($\beta_N$), and larger inverse aspect ratio ($\epsilon$) all strengthen mode coupling. Second, a critical momentum diffusivity, $\chi_{M,crit}$, is introduced to characterize the impact of boundary-induced mode coupling on the $2/1$ penetration spectrum, with smaller value indicating stronger impact. The results show that $\chi_{M,crit}$ increases with initial plasma rotation, but decreases for rotation profile with reduced flow shear. Moreover, increasing $\eta$, as well as higher $\beta_N$ and $\epsilon$ also reduce $\chi_{M,crit}$, albeit through different physical mechanisms. Across broad range of numerical scenarios, these results highlight the essential role of mode coupling in error field control for future fusion devices.

3D hot tail runaway electron generation modelling for ITER

Louis Puel, Eric Nardon, F J Artola, Di Hu

Runaway electron (RE) generation represents a major concern for future tokamaks such as ITER, where high plasma temperatures and large plasma currents significantly enhance the risk of RE formation. In particular, the hot tail mechanism, triggered during the thermal quench (TQ), occurs when a rapid drop in plasma temperature prevents high-energy electrons from thermalizing, allowing them to become REs under the accelerating effect of a strong toroidal electric field. In ITER, this mechanism is expected to be the dominant primary generation before avalanching in the case of an unmitigated/poorly mitigated disruption. The recently developed hot tail tracker framework in JOREK is used here to perform the first 3D estimates of hot tail generation during disruptions of an ITER 15 MA D–T H-mode scenario mitigated by Shattered Pellet Injection (SPI). Two situations are considered, with and without pre-disruption thermal energy degradation. The non-degraded case, more pessimistic, results in strong RE generation driven by weak stochastic losses limited by the trapped population, strong helical cooling, and the migration of hot electrons from the core toward regions of high parallel electric field. In contrast, the degraded case shows successful mitigation within the assumptions of this simulation, with suppression of the core hot tail generation and the removal of the edge seed through stochastic transport. The difference between a critical and a non critical scenario appears to be linked to localized overdensities of injected material produced by magneto-hydrodynamic (MHD) activity, which, counter-intuitively, significantly enhance local generation. This phenomenon is confirmed by a 0D two-fluid model of the bulk plasma during the TQ, coupled with a new 0D hot tail estimator based on backward tracking of a single particle in velocity space.

Mechanism behind the recombination requirement for benign termination of relativistic electron beams

George Su, Carl Friedrich Benedikt Zimmermann, Carlos Paz-Soldan, Matthias Hölzl, Pavel Aleynikov

We present a first-principles explanation of the recombination requirement for benign termination of relativistic electron (RE) beams in tokamaks. Kinetic modeling including neutrals shows that the injection of neutrals over a finite quantity window, together with recombination, increases bulk resistivity. Nonlinear MHD simulations using the JOREK code demonstrate that this preferentially amplifies edge tearing modes, producing a more stochastic edge magnetic field during RE deconfinement, resulting in a larger RE wetted area. We identify resistivity, not the free electron density, to govern access to benign termination. This provides the first broadly applicable and experimentally consistent picture of the MHD mechanisms behind the benign scenario, critical to its extrapolation to next-step devices.

Real-time tomography-based Bayesian inference from TCV bolometry data

Daniele Hamm, Christian Theiler, Luke Michael Simons, Basil P Duval, Umar Ahmed Sheikh

Radiated power information is crucial to diagnose and optimize the performance of fusion plasmas. Traditionally, at the TCV tokamak, radiated power analysis has only ever been possible following plasma discharge termination. However, recently, TCV bolometer data have become available in real-time. This offers the opportunity of integrating the radiated power information into the TCV plasma control system. In this work, we propose a novel real-time tomography-based Bayesian technique allowing estimation of the power radiated from user-defined regions of interest in the plasma. The real-time estimates are obtained as computationally cheap linear combinations of bolometer measurements, using pre-computed coefficients that are optimized for the specific discharge planned. This method is not, thus, trained on a set of synthetic or tomographically reconstructed emissivity profiles. We detail the derivation of the technique and show its equivalence to traditional tomographic estimates under suitable conditions. We then demonstrate that this technique enables accurate real-time estimation of the total, core, divertor and main chamber radiated power, by its application to a representative and heterogeneous set of TCV discharges. Finally, we discuss the robustness of the technique to faulty detectors, showing that simple precautions allow safe handling of many common issues. The computational routines implementing the described technique are provided as open-source code.