Recent Publications

Deuterium retention and surface modification of tungsten under exposure to deuterium-neon mixed plasmas and combined with transient heat loads

Yue Yuan, Ying Qin, Arkadi Kreter, A Terra, Sören Möller, Long Cheng, Di Hu, yuhao Li, Sijie Hao, Peng Zhang, et al.

Neon (Ne) seeding is essential in ITER to mitigate divertor heat loads, but it significantly influences plasma-surface interactions and fuel retention in tungsten (W). Moreover, transient heat loads from edge-localized modes (ELMs) further alter surface morphology and fuel retention behavior. This work investigates the combined effects of Ne seeding and ELM-like transient heat loads on surface modification and deuterium (D) retention in rolled W. Experiments were carried out in the linear plasma device PSI-2 using pure D plasma and D+10% Ne plasma at 500 K, with an ion energy of 40 eV, and a fluence of 1 × 1026 D m-2. Transient loads were applied using a pulsed laser (1 ms, 0.76 GW m-2, 1000 cycles), either successively or simultaneously with plasma exposure. For plasma-only exposures, Ne seeding caused pronounced erosion and fine cracks on blister caps, resulting in approximately 32% lower total D retention compared with pure D plasma. Transient heat loads generated temperature and stress fields, thereby causing surface roughening and dynamic recrystallization, as well as increasing D retention in both the near-surface (< 4 µm) and deeper regions. When combined with Ne seeding, the effects depended on the loading sequence. With sequential transient heat loads followed by plasma exposure, D retention decreased by about 44% in the near-surface and by 27% in total retention. In contrast, under simultaneous THL + plasma exposure, Ne seeding increased near-surface and total D retention by approximately 62% and 16%, respectively, and produced an additional broad D release peak at around 1200 K. This peak could be attributed to the formation of high-binding-energy Ne-vacancy complexes (Nei-V complexes) induced by the synergistic interaction of D-Ne and transient heat loading. These results demonstrate that both impurity seeding and transient heat loads critically determine D retention behavior, emphasizing the importance of accounting for their synergistic effects when predicting fuel retention and optimizing operation scenarios in future fusion devices.

First wall erosion induced by charge-exchange neutrals on EAST

Rui Ding, Jin Guo, Lei Mu, Guoliang Xu, Yaowei Yu, Yuming Liu, Rong Yan, Hai Xie, Dahuan Zhu, Junling Chen, et al.

Charge-exchange neutrals (CXNs), in particular of hydrogen isotopes deuterium and tritium, are expected to contribute notably to first wall erosion in future fusion reactors. To understand the CXN-induced first wall erosion under different discharge conditions in deuterium, dedicated experiments with a set of new diagnostics have been performed on EAST. Measurements of CXN energy spectrum by the low-energy neutral particle analyzer (LENPA) shows that the integrated CXN flux at the first wall positively correlated with the heating power and line-averaged electron density (n_"e" ), and increased by more than one magnitude from ohmic to high power discharges in the database. Deeper plasma fueling by supersonic molecular beam injection (SMBI) leads to a lower edge neutral pressure and thereby a ~50% lower CXN flux. The CXN flux in the intra-ELM phase is ~2 times higher than that in the inter-ELM phase. Measurements of material erosion rate by the quartz crystal microbalance (QMB) show that higher heating power can lead to stronger material erosion by CXNs. The erosion rate increases with n_"e" at first due to the higher CXN flux and then saturates due to the lower incident energy. The 3D-GAPS code is applied to model the CXN-induced erosion based on the LENPA-measured CXN energy spectrums, which shows good agreement with post-mortem analysis of exposed samples and QMB measurements.

Low deuterium retention in chemical vapor deposited tungsten with columnar grain structures under combined effects of displacement damage and helium seeding

Ting Wang, Arkadi Kreter, Peng Bi, Hanqing Wang, Y. Mao, Hao Wang, Yue Yuan, Long Cheng, Li-Qun Shi, Jun Tang, et al.

Controlling hydrogen isotope retention is a critical challenge for plasma-facing materials (PFMs) in fusion reactors. Chemical vapor deposited tungsten (CVD-W), featuring columnar grain structures, has attracted increasing attention as a candidate PFM. However, its surface blistering and deuterium (D) retention behavior under fusion-relevant, complex irradiation conditions remain insufficiently understood. In this work, CVD-W and ITER-like forged W both with grains elongated normal to the exposed surface were irradiated with pure D and mixed D+5% He plasma, with and without prior W self-ion irradiation at damage levels of 0.2 and 2 dpa. Compared with ITER-like W, CVD-W exhibits substantially enhanced tolerance to D-induced surface blister formation and markedly reduced D retention under pure D plasma exposure. Regardless of displacement damage, He seeding, or their combination, CVD-W consistently retains less D than ITER-like W across all irradiation sequences studied. This persistently low D retention in CVD-W is primarily associated with its lower defect density (e.g. fewer grain boundaries) and a blister-resistant <001> surface texture that limits blister-related D trapping. Furthermore, displacement damage and He seeding effectively suppress surface blistering, but exert opposing individual effects on D retention in both materials. Their combined effect shows a clear dependence on D fluence and material microstructure. This work highlights the advantages of CVD-W in suppressing hydrogen isotope retention under complex irradiation environments and provides valuable insights for the selection and microstructural design of advanced PFMs with improved resistance to hydrogen-induced blistering and reduced retention.

Parameter optimization of the reduced-order scrape-off-layer model DIV1D using Markov-Chain Monte Carlo sampling

Roel Rik Maria Hazelhof, Gijs Lukas Derks, Clemens Verhoosel, Stefan Dasbach, David vander Mijnsbrugge, Sven Wiesen

Accurate and efficient modeling of scrape-off layer (SOL) dynamics is essential for controlling divertor detachment in future fusion reactors. The reduced-order SOL model DIV1D provides a fast alternative to high-fidelity codes such as SOLPS-ITER, but it contains fitting parameters that are traditionally tuned manually and with limited knowledge about their posterior distribution. This paper introduces a Bayesian framework employing Markov Chain Monte Carlo (MCMC) sampling to fit DIV1D to mapped SOLPS-ITER solutions. The framework quantifies parameter uncertainties through posterior likelihood distributions, revealing parameter correlations and multi-modal behavior. A Sobol sensitivity analysis, extended with a novel adaptive formulation, provides additional insight into parameter influence and interactions. Application to SOLPS-ITER simulations of TCV shows improved fits compared to the benchmark parameter set, while results across a SOLPS-ITER density ramp on TCV highlight systematic trends and parameter correlations, suggesting the potential of density-dependent adaptive fitting. Extension of the method to SOLPS-ITER simulations of AUG demonstrates its robustness and adaptability. Overall, the Bayesian MCMC framework reduces manual workload and enables reproducible and interpretable parameter estimation, leading to improved reduced-order SOL modeling capabilities.

First wall particle fluxes and Be erosion in high-performance JET-ITER baseline plasmas

Eduardo de la Cal, Carine Giroud, Henri Aaron Kumpulainen, Juri Romazanov, Itziar Balboa, Scott Alan Silburn, Pedro Carvalho, Juuso Karhunen, Beth Thomas, Alex Tookey, et al.

In next step large fusion devices with a tungsten (W) first wall such as ITER, impurity control will be a challenge to avoid strong core radiation from highly ionized metals. Here we analyse the deuterium (D) fluxes impacting on the beryllium (Be) first wall limiters and the resulting surface erosion in high-performance neon (Ne) seeded ITER-baseline plasmas with up to 35 MW heating power in JET tokamak. Visible cameras are used to quantify the fluxes using the spectroscopic S/XB method at the regions of strongest plasma-wall interaction: the Outer Midplane (OMP) and the Upper Dump Plates (UDP). We first describe how Ne seeding, which significantly improves core plasma performance and power exhaust control, modifies the plasma flux dynamics and ELM properties at the walls. It is then shown, that the global Be erosion by sputtering is not significantly affected by Ne seeding. This is because the time-averaged fluxes are dominated by the inter-ELM phase, where the main erosion precursor is deduced to be D+. Furthermore, we observe a beneficial strong decrease of the fluxes at the OMP when slightly increasing the Separatrix-limiter clearance, indicating short far scrape-off layer (SOL) decay lengths. We also describe, how the fluxes change with average plasma density and toroidal magnetic field and plasma current. Regarding the fluxes at the UDP, we show the critical effect of the magnetic topology at the top of the chamber. The formation of a secondary Separatrix in this region enhances the parallel plasma fluxes to the UDP, which eventually becomes the main plasma-wall interaction area. Finally, we compare the experimentally estimated Be effective sputtering yields at the OMP for the inter- and intra-ELM phases with the values calculated using the SDTrim code to infer information on the mean ion impinging energies Ei and to show that the main global erosion precursor is D+. The obtained Ei values are in the range or at least compatible with those expected for the plasmas analysed: approximately 30 eV for the inter- and 1 keV for the intra-ELM periods respectively.

Integrated core-SOL modelling of JET baseline plasmas in D/D-T/T: impact of ELMs and isotopic composition on Tungsten sputtering

Rachele Cicioni, Luca Garzotti, Lidia Piron, Vito Konrad Zotta, Vassili Parail, Agata Chomiczewska, Domenico Frigione, Alexander Huber, Florian Koechl, Henri Aaron Kumpulainen, et al.

The JET DTE2 experimental campaign revealed an isotopic dependence in the baseline scenario, where the target stationary conditions achieved in deuterium plasmas could not be maintained in tritium and deuterium-tritium mixtures. This loss of stationarity was associated with a gradual increase in plasma density, reduced ELM activity, and rising core radiation. This work investigates whether isotope-dependent tungsten sputtering contributes to the loss of stationarity observed in JET DTE2 baseline plasmas. The analysis focuses on the tungsten sputtering source and on tungsten transport during inter- and intra-ELM phases, as a function of the isotopic species of the main plasma. The study is carried out using the integrated core-edge-SOL framework COCONUT/JINTRAC, which was used to evolve the plasma self-consistently across the entire domain. The simulations show a clear isotopic dependence of the tungsten sputtering source, which increases with the mass of the main plasma ion and reaches the highest levels in tritium plasmas. Despite the enhanced tungsten source, most of the sputtered tungsten remains confined to the SOL, and no substantial net increase of the tungsten content is observed in the plasma core over the ELM time window. Pedestal onditions affect tungsten transport across the separatrix and the transient tungsten response during the ELM cycle. The results also indicate that ELM dynamics, in particular the ELM frequency, can modify the transient tungsten content in the core. Overall, the simulations suggest that isotope composition and ELM activity affect the tungsten sputtering source and its short-time redistribution, but that ELM-induced sputtering alone does not explain the loss of stationarity observed experimentally.

Tungsten erosion and scrape-off layer transport modelling in L-mode helium plasma discharges in ASDEX Upgrade

Gabriele Alberti, Elena Tonello, Carlo Tuccari, Fabio Mombelli, Sebastijan Brezinsek, Timo Dittmar, Antti Hakola, Andreas Kirschner, Karl Krieger, Marcin Rasinski, et al.

Due to its unavoidable presence in thermonuclear DT plasmas and to its peculiar effects on materials, investigating the role of helium (He) in plasma-wall interaction (PWI) in current tokamaks is fundamental. In this work, PWI in L-mode He plasma discharges in ASDEX Upgrade (AUG) is modelled by exploiting simplified analytical approaches and two state-of-the-art codes. SOLPS-ITER is employed both to provide a suitable background plasma for erosion simulations and to interpret diagnostics measurements in terms of He+/2+ fraction. In particular, a 50-50% concentration of the two He ions is found in the proximity of the strike-points, while He2+ represents the dominant population farther in the scrape-off layer (SOL). The role of He ion fraction on AUG tungsten divertor erosion is first estimated by means of a simple analytical model and, afterwards, by exploiting ERO2.0, showing the major impact of He2+ in common AUG plasma temperatures. The potential influence of extrinsic impurities on divertor erosion is inferred from the preliminary comparison of ERO2.0 simulation results with experimental erosion measurements in the strike-point region. A comparison between the multi-fluid and kinetic approaches for simulating W erosion and migration reveals significant discrepancies in the predicted tungsten source and transport. In particular, he routinely adopted perfect entrainment assumption in SOLPS-ITER, i.e. setting the same velocity at the magnetic pre-sheath entrance for both main ions and impurities, is found to substantially overestimate the W source compared to ERO2.0 results. Moreover, ERO2.0 predicts a stronger tungsten transport towards the X-point than current SOLPS-ITER simulations without drifts. Comparable W influx into the core can, however, be reproduced in ERO2.0 by reducing the anomalous diffusivity.

Recent advances in physics and applications of 3D magnetic fields on the J-TEXT tokamak

Nengchao Wang, Yong Hua Ding, Zhongyong Chen, Donghui Xia, Zhoujun Yang, Zhipeng Chen, Wei Zheng, Wei Yan, Da Li, Song Zhou, et al.

This paper summarizes recent experimental and theoretical progress achieved on the J-TEXT tokamak, with an emphasis on the physics and applications of 3D magnetic fields. Key hardware upgrades, including a new ICRF system and advanced 3D magnetic coil systems (RMP, island divertor, and the external rotational transform (ERT)), enable novel investigations into MHD instabilities, disruptions, transport, and divertor solutions. A major finding is the NTM-triggered formation of electron-ITB, where magnetic island nonlinearly interacts with turbulence to suppress transport and steepen core temperature gradients, and reveals a new mechanism for confinement improvement. Furthermore, IKM-driven NTV torque is identified as a key mechanism governing intrinsic rotation. The application of ERT coils successfully creates a Tokamak-Stellarator hybrid configuration, demonstrating complete suppression of NTMs and a 20% increase in stable plasma current. For boundary control, an island divertor configuration is established, reducing peak heat loads by ~50% and enabling detachment via SMBI fuelling. In disruption physics, synergetic control using RMP and O-point aligned ECRH efficiently suppresses locked modes and disruption, while low-n MPs are capable of RE suppression. Thermal quench timescale, estimated by a unified model with stochastic magnetic field and turbulence, matches experimental observations. Additionally, AI-driven disruption prediction frameworks, incorporating adaptive anomaly detection and cross-machine domain adaptation, are developed, with interpretability analyses linking predictions to physical mechanisms. Studies on turbulence and transport elucidate the role of electrode biasing, turbulence spreading on density limit, while helium ash removal dynamics are also investigated. The unique up-down symmetric poloidal divertor configuration is measured and modelling to identify the drift effect as a key driver for asymmetry, and hence target biasing is designed and experimentally studied for control these asymmetries. These collective advances in heating, 3D field control, stability, and AI forecasting provide critical solutions for managing plasma confinement and mitigating risks in future MCF devices.

B:D layer preparation, characterization and hydrogen isotope permeability for fusion application

A. Houben, E. Warkentin, M. Rasiński, T. Dittmar, H.R. Koslowski, S. Möller, T. Wauters, B. Unterberg, Ch. Linsmeier

Towards digital twins of fusion systems

F. Jenko, J. Ball, D. Borodin, C. Bourdelle, G. Ciraolo, J.E. Cook, J.M. García-Regaña, T. Görler, M. Hoelzl, F. Imbeaux, et al.

Bridging the gap to next-step devices—while saving valuable time and resources—requires more than semi-empirical models, which struggle to predict plasma behavior in unexplored parameter regimes. Instead, validated simulation tools are essential, leveraging high-fidelity exascale computing and multi-fidelity models, including AI-based surrogates. To address these challenges, the ‘EUROfusion Theory and Advanced Simulation Coordination (E-TASC)’ initiative was launched in 2021. It includes 15 TSVV (Theory, Simulation, Verification, and Validation) projects supported by five Advanced Computing Hubs. This ‘team of teams’ has made substantial progress toward developing digital twins of fusion systems, with key scientific achievements to be presented in the paper. This includes the following topical areas: core performance in burning plasmas, magnetohydrodynamic transients, L–H transitions and ELM-free regimes, plasma exhaust, and plasma–wall interactions—with applications to both tokamaks and stellarators.

Ex situ investigation of boron, hydrogen, and oxygen distributions on W7-X divertor surfaces using picosecond-LIBS under vacuum

Huace Wu, Christoph Kawan, Erik Wüst, Sebastijan Brezinsek, Rui Ding, Rongxing Yi, Timo Dittmar, Marcin Rasinski, Gennady Sergienko

Quantitative, spatially resolved measurements of near-surface material composition on plasma-facing components (PFCs) are critical for interpreting plasma–wall interaction (PWI) driven impurity sources, assessing wall-conditioning performance, and providing model constraints for impurity migration and co-deposition in present-day magnetic confinement devices and future reactors. In this work, picosecond laser-induced breakdown spectroscopy (ps-LIBS) is applied to post-mortem graphite divertor tiles from the stellarator Wendelstein 7-X (W7-X) under device-relevant high vacuum (∼10 −7 mbar) to obtain poloidally resolved two-dimensional maps and depth profiles of carbon (C), boron (B), hydrogen (H), and oxygen (O). The ps-LIBS results are cross-validated against isotope-sensitive diagnostics (nuclear reaction analysis, NRA; laser ablation molecular isotopic spectroscopy, LAMIS) and complemented by profilometry, focused ion beam scanning electron microscopy (FIB-SEM)/energy-dispersive x-ray spectroscopy (EDS) and x-ray photoelectron spectroscopy (XPS) to relate lateral patterns to film thickness and near-surface chemistry. Pronounced poloidal heterogeneity is observed: the thickness of mixed co-deposited layers range from ∼0.3 to 0.4 μ m in erosion-dominated regions, to ∼0.9 μ m in B-rich deposition zones, and exceeds 12 μ m in 13 C-rich deposits near the outer strike line after 2.5 h of H plasma operation. Depth profiling reveals that late-phase 13 CH 4 injection forms a C–H–O enriched top layer that locally restructures the pre-existing boronized layer resulting from boronizations, including a pronounced subsurface B maximum at ∼4-5 μ m. A calibration-based approach was established to estimate B/C ratios and B inventories from ps-LIBS, enabling quantitative evaluation across the B-rich region, the 13 C-rich deposition zone, and the erosion zone. The resulting poloidal and depth-resolved observables constitute benchmark-grade constraints for validating impurity transport/co-deposition modeling and support LIBS pathways toward in-vessel, near-real-time monitoring of surface elemental evolution under varying operational conditions.

A distributed computation library for common modeling tasks in magnetic confinement fusion

A. Knieps, D. Böckenhoff, D. Boeyaert, S. Bozhenkov, M. Endler, Y. Feng, Y. Gao, J. Geiger, Y. Liang, L. Liao, et al.

Runaway plateau formation in EAST disruptions triggered by massive gases injection

T. Tang, L. Zeng, D. Chen, S. Lin, H. Zhuang, S. Zhao, H. Zhao, A. Ti, B. Zhang, Y. Sun, et al.

Runaway-electron (RE) plateaus have been observed during disruptions in experimental advanced superconducting tokamak (EAST) plasmas following the massive injection of argon into low-density ohmic discharges, which are performed with a circular plasma configuration under full-metal wall conditions. RE plateaus can maximally carry a current of 270 kA and last up to 400 ms. The formation of RE plateaus strongly depends on lower pre-disruption electron densities (0.2–0.5 × 1019 m−3) and a lower amount of argon injection (400–1000 Pa l), which correspond to an enhanced pre-disruption RE population and a reduced post-disruption electron density, respectively. Pre-disruption REs were found to survive the thermal quench, serving as extra seeds for the RE plateau. In addition, magnetohydrodynamic activity during the thermal quench was found to cause significant RE loss and affect RE plateau formation; in particular, the m/n = 3/1 and m/n = 2/1 modes were observed in sequence, leading to distinct RE loss patterns. Simulations on RE generation by a 0D model further confirmed the key roles of RE seeds to form an RE plateau in EAST.