Recent Publications

The influence of ELM-like loading on tungsten under slow transient conditions in Magnum-PSI

T W Morgan, Martin Balden, Jos Scholte, Selanna Roccella, J H You

ITER is anticipated to operate with occasional slow transients, resulting in an increase in expected heat loads to the divertor from 10 to 20 MW m −2 and thus in the surface temperature increasing to > 2000 ◦C. At the same time mitigated ELMs may be expected to strike the surface. To investigate the consequences of this, two sets of experiments were carried out in Magnum-PSI, representing the first experiments to explore this regime. In the first set, seven ITER-like tungsten monoblocks were exposed to either hydrogen plasma, or impurity-seeded hydrogen plasma at a surface temperature of 2050 ◦C. Simultaneously five of the seven monoblocks were exposed to 10 5 ELM-like pulses using a 1 ms duration laser at an energy density of 0.1 − 0.19 MJ m −2 . Very strongly roughened and cracked laser exposed areas with localized melted regions were observed. These areas protruded several hundreds of micrometers above the original surface. In the second experiment, three tungsten plates, with either small, large or no castellations cut into the surface, were exposed to a hydrogen plasma at 2100 ◦C with a wider range of ELM-like pulse numbers (10 2 − 10 5 ) and energies (0.13 − 0.31 MJ m −2 ). The surface was found to evolve by roughening and pre-crack formation towards similar strongly protruding roughened structures as pulse number increased, while increasing the transient energy resulted in increasing the fraction of surface melting observed in the loaded region. Mass loss measurements of the plates indicated that erosion was significant, between 0.05-0.13 nm per pulse for the different plates. Extrapolating these results to ITER suggests that this mass loss rate would be relatively high and would constitute a significant tungsten source.

Deuterium desorption from tungsten borides

Maryna Bilokur, Laurent Marot, Matej Mayer, Thomas Morgan, Ernst Meyer

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.

Characterizing flux-surface shapes in tokamaks and quasi-symmetric stellarators

M. J. Gerard, M. J. Pueschel, S. D. Stewart, H. O. M. Hillebrecht, B. Geiger

Modern stellarator designs routinely attain high levels of magnetic-field quasi-symmetry through flux-surface shaping. Here, we examine different methods for characterizing stellarator flux-surface shapes in a manner analogous to flux-surface shaping in tokamaks. The methods considered use a Fourier analysis to define the shaping modes (e.g., elongation, triangularity, squareness, etc.) of equilibrium cross sections. Relative to an axisymmetric equilibrium, the additional degree of freedom in a non-axisymmetric equilibrium manifests as a rotation of each shaping mode about the magnetic axis. This analysis is performed on non-axisymmetric configurations with a high degree of quasi-symmetry and equilibria with varying quasi-symmetry quality from the quasi-symmetric Stellarator Repository database. One method in particular is shown to reduce shape complexity in quasi-symmetric equilibria by defining a set of cross sections that efficiently fill out an equilibrium volume. This is accomplished by defining a cross section as the set of points that occupy the shortest distance between the magnetic axis and an equilibrium flux surface across all quasi-symmetry contours. Using this method, we find empirically that an equilibrium geometry can be described with significantly fewer non-negligible shaping modes relative to other shape characterization methods. Moreover, the method reveals that quasi-symmetry quality is strongly correlated with equilibrium shapes that exhibit a highly constrained linear distribution of shaping modes, where an increase in shape complexity is proportional to an increase in shape rotation about the magnetic axis. It is therefore argued that this method provides a way to efficiently characterize the shape of quasi-symmetric equilibria in a manner analogous to how equilibrium shapes are described in tokamaks.

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.

High-power TCV scenario for conventional and alternative divertor studies

K. Lee, C. Theiler, M. Carpita, M. Zurita, P. Sintre, O. Février, F. Pastore, H. Reimerdes, K. Verhaegh, M. Winkel, et al.

Alternative divertor configurations (ADCs) must be evaluated under boundary plasma conditions approaching reactor-level values to be considered a reliable, physics-based solution for tokamak power exhaust. Most ADC experiments performed to date were at relatively low exhaust power. This work presents a high-power scenario on the TCV tokamak enabling the study of a wide variety of divertor magnetic shapes under an expanded SOL and power exhaust parameter space. The scenario is characterized by high power levels of electron cyclotron resonance heating ($2.5\,\text{MW}$ fully absorbed in a $\sim1\,\text{m}^{3}$ plasma) at high plasma current (edge safety factor $q_{95}\approx 2.5$), and low upstream separatrix densities ($n_{e,\text{u}}\approx1\times10^{19}\,\text{m}^{-3}$, Greenwald fraction $f_{\text{G}}\approx 0.1$). Stationary parallel heat fluxes up to $100\,\text{MW m}^{-2}$ are measured at the divertor target, an order of magnitude above previous TCV power exhaust studies. The obtained SOL collisionality and Lengyel detachment scaling metric lie within range of values expected in future reactors (SPARC, ITER, ARC).

Improved feedback control of the radiated power in Wendelstein 7-X using system identification

Anastasios Tsikouras, Felix Reimold, Gabriele Partesotti, Maciej Krychowiak, Timo Schröder, M van Berkel

The W7-X stellarator was recently upgraded with a new feedback control system for the radiated power. The radiated power, observed using bolometer cameras, is controlled using injection of impurities from the gas valve system. For designing a controller, system identification techniques were employed. These identify the response of the radiation to impurity seeding for specific plasma parameters. Using the obtained frequency response data, a controller was designed which provided accurate control of the radiated power. The system was able to reach and reliably maintain different setpoints using N 2 and Ne for the injection, both for attached and detached conditions. It enabled stable long pulse detached operation and multiple steady state radiated power levels during discharges. This allowed heat load mitigation during long discharges (> 300 s), as well as studying the plasma in different radiative regimes efficiently.

Experimental investigation of a closed vapour box module for a divertor-like configuration in Magnum-PSI

Fabio Romano, Victor Tanke, Jacob Schwartz, Robert James Goldston, Serge Brons, T W Morgan

Efficient management of extreme heat fluxes in the divertor region to extend the lifetime of the components remains a critical challenge for the realization of nuclear fusion-based power plants. Among the alternative concepts explored for the divertor region, the use of liquid metals, particularly lithium, is of interest due its ability to dissipate the incoming plasma heat flux through the vapour shielding effect (VS). In this work, we experimentally investigated a "closed" configuration of a dedicated Vapour Box Module (VBM) in the linear plasma device Magnum-PSI. The goal of the experiments is to simulate the vapour box divertor environment conditions and assess its performance in terms of power mitigation and redistribution and lithium confinement. Initial testing without Li demonstrated the efficacy of a closed VBM structure in inducing detachment via neutral gas accumulation. Apertures which enabled non-condensing gas to be effectively pumped while ensuring lithium condensed on the inner surfaces were therefore added. With a lithium capillary porous structure target used, lithium is directly vaporized by the plasma, forming a dense lithium vapour cloud that interacts with the incoming plasma. This resulted in a significant reduction of the target temperature of at least 48 %, together with a temperature locking effect, a phenomenon typically observed in the VS regime. Lithium vapour confinement within the VBM was strongly correlated with the wall temperature. Relatively cold walls promoted Li re-condensation and therefore improved Li confinement, although with the expected trade-off of increased hydrogenic retention on lithium-wetted surfaces. As the wall temperature increased, the confinement efficiency decreased, consistent with reduced Li re-condensation and thermally activated Li--H chemistry and remobilization at the walls. Diagnostic measurements through embedded thermocouples and calorimetry revealed that lithium vaporization and re-condensation processes also played significant roles in plasma power dissipation. The results advance the case for a closed divertor chamber with direct lithium evaporation from the strike-points as a viable method to manage divertor heat fluxes in future fusion reactors.

Integrated full pulse modeling for pellet injection in tokamaks: HPI2 model improvement and validation in WEST.

Alex Panera Alvarez, Florian Koechl, Jean-Francois Artaud, Eleonore Geulin, Bernard Pegourie, Enzo Vergnaud, Clarisse Bourdelle, Sven Wiesen

Reliable modeling and control of core density is essential for reactor-relevant magnetic confinement fusion operation, motivating cryogenic pellet injection as a primary fueling actuator and the need for predictive pellet source models in integrated modeling. Here we present an upgrade of the physics-based pellet code HPI2 in which the plasmoid release spatial step is determined self-consistently from ablation physics, dx var =v pel t exit (optionally rescaled to trade accuracy for computational cost), removing an ad-hoc discretization parameter and improving numerical robustness across injection conditions. The upgraded model is first validated in stand-alone against a high-field-side pellet-fueled, ohmic, WEST discharge (#58656) by comparing synthetic and measured interferometry line-integrated density increments, obtaining a mean error of ∽10%. We then perform full-radius, time-dependent integrated modeling validation by coupling the new HPI2 within the High Fidelity Pulse Simulator (HFPS) workflow (JINTRAC/IMAS), combining JETTO with SANCO for the impurity/radiation evolution and TGLF-SAT2 for the turbulent transport. The coupled simulations reproduce the main density rise and relaxation after pellet injection and the associated electron-temperature transient, while taking into account the strong influence of tungsten radiation in WEST, supporting the consistency of HPI2 as a predictive pellet particle source in integrated modeling frameworks. Ultimately, this validation study supports the use of pellet modeling tools in integrated modeling studies for larger devices such as ITER.

Cycle-Consistent and Uncertainty-Aware Neural Surrogates for Tokamak Edge Plasmas

Abdourahmane Diaw, Sebastian De Pascuale, Jae-Sun Park, Ivan Paradela Perez, Jeremy D. Lore, Stefan Dasbach

The boundary and divertor plasma govern how a tokamak exhausts power and particles, setting heat fluxes, target conditions, and the onset of detachment. Predicting these quantities is essential for operating current and future devices, but edge simulations that resolve them are too slow for parameter scans, optimization, or real-time control. Machine-learning surrogates offer a fast alternative, yet most are forward-only: they cannot recover input parameters from observations or assess the reliability of their predictions. We introduce a cycle-consistent neural surrogate for edge plasmas, combining a conditional U-Net forward model with an optimization-based inverse method built on the frozen forward network. The forward model maps five control parameters to two-dimensional plasma-state fields on the SOLPS-ITER mesh; the inverse method enforces consistency between forward and inverse predictions, a self-supervised quality check needing no ground-truth labels at inference. An ensemble of multilayer perceptrons also predicts electron temperature and density profiles at the outboard midplane and divertor targets, with uncertainty estimates that flag where more simulations are needed. The forward model achieves normalized root-mean-square errors below 2.6% and Pearson correlations above 0.95 for all fields. Cycle-consistency regularization raises the average cyclical $R^2$ from 0.59 to 0.99 without degrading forward accuracy and enables recovery of the core fueling rate; all five control parameters are recovered with Pearson $r\ge0.97$. A $k$-d tree warm start yields a database completion rate above 95%, versus roughly 30% outright failures when cold-started. With about $4\times10^6$ parameters, the model produces full 2D predictions in milliseconds, five to six orders of magnitude faster than SOLPS-ITER, enabling real-time control, parameter scans, uncertainty analysis, and digital twins.

Influence of magnetic shear on kinetic ballooning modes and electromagnetic stellarator turbulence

Paul Mulholland, K. Aleynikova, M J Pueschel, Josefine Proll, Akihiro Ishizawa

The influence of magnetic shear on kinetic ballooning modes (KBMs) and electromagnetic turbulence is reported in the Wendelstein 7-X (W7-X) and Heliotron-J (H-J) stellarators. Gyrokinetic simulations at finite normalized plasma pressure β reveal that sub-threshold KBMs (stKBMs) are present in all configurations, i.e., KBMs that are resonantly destabilized far below the iMHD limit. A reduced KBM model shows that boosting magnetic shear and weakening bad curvature stabilizes (st)KBMs in stellarator geometry. In nonlinear simulations, instead of undergoing nonlinear electromagnetic stabilization, turbulent fluxes in W7-X increase with β when stKBMs are destabilized. Lower-magnetic-shear configurations of W7-X are found to produce lower transport, due to the generation of stronger zonal flows. In H-J, turbulent fluxes reduce with β – despite hosting stKBMs – due to a boosted zonal-flow response at higher β. This highlights the importance of accounting for both linear and nonlinear dynamics when aiming to improve reactor performance.

Exhaust operational space assessment for the European Volumetric Neutron Source (EU-VNS)

Sven Wiesen, Christian Bachmann, Mattia Siccinio, Francesco Maviglia, Clarisse Bourdelle, Matti Coleman, R Neu, Jean Boscary, Gianfranco Federici

The SOLPS-ITER edge plasma numerical model is employed to predict the exhaust operational space in a small-scale tokamak device currently being investigated as the proposed European Volumetric Neutron Source (EU-VNS) with large power-to-size ratio of P/R ∼ 20 MW/m. Previous work is extended by allowing krypton seeding to dissipate and redistribute heat instead of argon. The found finite operational space is limited by both, engineering limits for the peak heat-flux expected at the divertor plasma facing components < 10 MW/m 2 , and at the same time requiring a low enough effective charge Z eff and high electron temperature T e in the core region to keep up the neutron wall load NWL ∼ T 3/2 / Z eff 2 from beam-target fusion. With krypton seeding the operational space seems to be larger than with argon and at the same time the amount of tritium throughput can be halved compared to the previous study. The required total T-throughput rate is of the order 3 − 4 · 10 22 s -1 and it is observed that pellet fuelling is deemed to be as efficient and required to fuel both the core density and the separatrix density at the same time. A positive correlation is found between the krypton enrichment in the divertor and the increase of upstream density effectively fuelled by pellets. A low level of Z eff ≈ 1.6, and the observation of the required Greenwald fraction of 50% to achieve this, is compatible with the predictions of the core plasma conditions for the current EU-VNS design point from ASTRA/METIS. Within the model assumptions made the observation of a depression of the core electron temperature below 1 keV is critical and potentially requires more ECRH power in EU-VNS than anticipated.

Upgraded Pilot-PSI: a new high flux linear plasma device for operando ion beam studies

R.H.M. Timmer, W.M. Arnoldbik, J.A.G. van Kesteren, C.J.D. Robben, R.S. Al, S. Brons, W. Melissen, M.J. van de Pol, J. Scholten, S. Alonso van der Westen, et al.

To investigate dynamic plasma surface interaction processes, we developed the linear plasma generator Upgraded Pilot-PSI (UPP). It couples high-flux plasma exposure with operando ion-beam analysis (IBA) for the first time. UPP generates steady-state hydrogen plasma up to 8.5 × 10 23 m − 2 s − 1 with electron temperatures ranging from 0.4 to 3.6 eV and densities up to 7.4 × 10 19 m − 3 . Simultaneously, up to 2.8 MeV p, 3 He, or 4 He ion beams may be used to analyse the top few μ m of the material using IBA techniques, allowing the study of hydrogen-isotope transport and exchange, dynamic retention, sputtering, and related processes. Alternatively, the ion beam can be used to induce radiation damage in the material while it is simultaneously exposed to plasma, enabling studies of defect stabilisation under plasma conditions. This paper details the design and technical specifications of UPP, focusing on the engineering challenges and solutions involved in integrating operando IBA with a high-flux plasma generator. Our first studies on tungsten exposed to deuterium plasma reveal the mechanism of D transport in the surface (first 0.4 μ m ) and the bulk (beyond 0.4 μ m up to the beam penetration depth) as well as the time to reach saturation.

Publisher's note: “Fast electrostatic microinstability evaluation in arbitrary toroidal magnetic geometry using a variational approach” [Phys. Plasmas 33 , 062504 (2026)]

M. C. L. Morren, P. Mulholland, J. H. E. Proll, M. J. Pueschel, L. Podavini, D. D. Kiszkiel, J. A. Schuurmans, A. Zocco

Code-to-code comparison of SOLPS-ITER and SONIC edge transport codes in JT-60SA tokamak plasma conditions

Giulio Rubino, Shohei Yamoto, David Coster, Gloria Falchetto, Sven Wiesen, Carlo Sozzi, Kazuo Hoshino, Yuki Homma, Tomohide Nakano, Nobuhiko Hayashi, et al.

Structural Thermal Optical Performance analysis of high-power electron cyclotron mirrors

Jorn Veenendaal, Jelle Slief, Matthijs van Berkel, Egbert Westerhof, Amritam Das, Menno Lauret, Clemens Verhoosel