Recent Publications

Image processing method for post-discharge plasma wall inspection in tokamaks, with application to ITER

Tarundeep Kaur Lamba, L. Moser, Martin Kocan, Martin Vincent, Yan Rong, Ding Rui, Zhang Wei, Tom Wauter, Richard A. Pitts, Walsh Michael, et al.

Stellarator island divertor shape optimization for reduced peak heat fluxes

Avigdor Veksler, Aaron Bader, Heinke Frerichs, Elizabeth Joy Paul

An automated algorithm to construct island divertors for stellarators is presented and is used to find divertors that meet heat load requirements determined by material limits. The algorithm uses just two initial conditions: two starting coordinates on the island separatrix chosen by the user. We leverage the simplicity of the algorithm to explore the divertor parameter space in a fixed magnetic equilibrium. Heat loads are approximated using the field line diffusion model implemented in the FLARE code. Divertor solutions that satisfy heat load requirements while maintaining a high power fraction captured are found using a parameter scan and a Bayesian optimization routine. The optimization finds divertors that perform the same as the parameter scan, but with a 75% reduction in computational cost. The resulting divertors satisfy heat load requirements across varying cross-field heat diffusivities. Optimization over various islands in the equilibrium shows that low-elongation islands are the easiest to find divertors that satisfy heat flux requirements. This algorithm presents a simple parameterization for island divertors and facilitates further physics and optimization studies.

Characteristics of monotonic sheaths near a wall with grazing magnetic incidence

Alessandro Geraldini, Robert J. Ewart, Stephan Brunner, Felix Parra

We consider a magnetised plasma in contact with an absorbing planar wall, where the angle α between the magnetic field and the wall is small, α ≪ 1 (in radians), and the plasma is uniform in the directions tangential to the wall. The finite ratio γ of the characteristic electron gyroradius ρ e to the Debye length λ D , γ = ρ e /λ D, is retained via a grazing-incidence (α ≪ 1) gyrokinetic treatment [1,2]. Building on a previously developed iterative scheme [2,3] to solve for the steady-state electrostatic potential in the quasineutral magnetic presheath of width ∼ ρ S , we developed a scheme that simultaneously solves for both the presheath and the non-neutral Debye sheath of width ∼ λ D in the limit λ D /ρ S → 0, provided that the electrostatic potential solution is monotonic. The code, called GYRAZE, thus provides the energy-angle distribution of ions at the wall and the velocity distributions of electrons reflected by the wall for different values of wall potential.

Multi-MA fusion accelerator performance insensitivity to electrode contamination

Anonymous

Compatibility study between CLF-1 steel and HCCB TBM Helium gas medium for engineering qualification

Leran Liu, Guoping Yang, Junshan Wan, Hongbin Liao, Xinghua Wu, Hongxiang Zhang, Zaixin Li, Fengchao Zhao, Qian Sheng, Shuqin Wu, et al.

Fracture toughness of European reference RAFM steel EUROFER97

E. Gaganidze, D. Terentyev, M. Serrano, P. Lamagnère, M. Walter, T.T.M. Nguyen, A. Zinovev, D. Bermúdez Parra, G. Pintsuk, G. Aiello, et al.

Investigation of helium ion damage on tungsten using secondary electron emission spectroscopy

Gary Williams, Randy McDaniel, Stavros Karakalos

Preparation sequence controls oxygen partitioning between boron and lithium on tungsten

Predrag S Krstic, Meral Sharkass, Swarit Dwivedi, Dalia Sayed Ahmed, Yun Kyung Shin, Adri CT van Duin

Classical reactive molecular dynamics of boron- and lithium-conditioned tungsten surfaces show that the order in which boron and oxygen are introduced - not the boron inventory - controls surface chemistry. On a thin engineered tungsten oxide, boron captures 81% of oxygen and consumes the oxide; on an engineered ~2 nm mixed WO3/WO2 film, boron captures only 20% and leaves 52% of reaction-zone tungsten in WO3-like coordination; and in a co-deposited W-B substrate later exposed to oxygen, boron captures only 28% despite a tenfold larger inventory. Adding lithium leaves the oxide-first cases unchanged but transforms the co-deposited case, where lithium becomes the dominant oxygen captor (58%) and a lithium-borate-glass-like network form. We outline the expected, divergent responses of the boron-only and boron-plus-lithium materials to deuterium irradiation.

In-situ residual hydrogen isotope analysis by laser induced breakdown spectroscopy

Rongxing Yi, Gennady Sergienko, Arkadi Kreter, Erik Wüst, Miroslaw Zlobinski, Liang Gao, Christoph Kawan, Mauricio Gago, Huace Wu, Sebastijan Brezinsek

Experimental analysis of Pt marker erosion in the outer divertor region of ASDEX Upgrade during L- and H-mode discharges

S. Saari, A. Hakola, J. Karhunen, K. Krieger, M. Rasinski, M. Balden, C. Baumann, I. Bogdanović Radović, S. Gouasmia, M. Mayer, et al.

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.

Electronic effects on sputtering of fusion materials under light ion impact

Evgeniia Ponomareva, Nima Fakhrayi Mofrad, Andrea E Sand

Understanding material behavior under ion bombardment is a crucial aspect in predicting material surface modification and the lifetime of plasma-facing materials in fusion devices. Atomistic simulations are commonly employed to calculate key ion irradiation outcomes such as sputtering and reflection yields. However, within this simulation framework, the energy dissipation of ions in the electronic system is often neglected or treated using a simple velocity-dependent friction force. Although this approximation can be adequate for slow heavy ions, light ions in the sub-keV range lose a substantial fraction of their energy to electronic excitations, requiring a more realistic energy transfer description. Here, we incorporate trajectory-resolved electronic stopping obtained from ab initio calculations into molecular dynamics simulations of hydrogen and helium irradiation of tungsten and iron. We show that density-dependent electronic stopping leads to systematic variations in sputtering yields, especially for tungsten and lower impact energies, whereas reflection yields are found to be largely insensitive to the model choice. Material-dependent features are identified and mainly attributed to different relative contributions of projectile and recoil electronic energy loss. This work provides a physically motivated approach to improve the accuracy of the sputtering data used in plasma-surface interaction modeling.

FIREWALL: A surrogate model for the rapid assessment of tokamak wall loading and melting by runaway electrons

Victor Johan Svensson, Tommaso Rizzi, Svetlana Ratynskaia, Hannes Bergström, Luca Venerando Greco, Matthias Hoelzl, Panagiotis Tolias

Runaway electron (RE) beams generated during tokamak disruptions can deposit highly localized heat loads on plasma-facing components, posing a serious risk of melting and damage. Monte Carlo particle transport simulations coupled with three-dimensional thermomechanical response modeling can quantify this damage but are too computationally demanding for device-scale assessments and extensive scenario scans. We present FIREWALL (Fast Integrated Runaway Electron WALL loads), a surrogate model that combines a database of \textsc{Geant4} volumetric energy-deposition profiles with a one-dimensional heat-diffusion solver for each wall element. FIREWALL retains the energy and incident angle distributions of impacting REs and predicts the spatiotemporal temperature evolution of detailed three-dimensional wall geometries up to the melting threshold. FIREWALL thus provides a fast physics-based framework for translating global RE simulations into global wall melting predictions, enabling large-scale screening of disruption scenarios while directing high-fidelity costly workflows to the limited wall regions where they are actually required.

Design and reliability assessment of a support structure for high heat flux testing

Zhen Wang, Yinfeng Zhu, Xuebing Peng, Wei Song

High heat flux resilience of Low-pressure plasma sprayed tungsten coatings for fusion

Gunnar Schmidtmann, Johann Riesch, Andrey Litnovsky, Marcin Rasinski, Bernd Böswirth, Daniel Dickes, Jan Willem Coenen, Sebastijan Brezinsek, Robert Vaßen, Christian Linsmeier, et al.

Erosion of hot-pressed zirconium diboride-based ceramics induced by submillisecond laser pulses

D.E. Cherepanov, M.A. Golosov, A.V. Utkin, G.A. Ryzhkov, L.N. Vyacheslavov, N.I. Baklanova, V.A. Popov, A.A. Kasatov, E.I. Kuzmin

STAR_Lite: a stellarator designed to experimentally validate non-resonant divertors

Georg Friedrich Harrer, Andrew Giuliani, Misha Padidar, Robert Davies, Shibabrat Naik, Calvin Lowe

The non-resonant divertor (NRD) offers a promising exhaust solution for stellarators, combining topological simplicity with resilience to magnetic field perturbations. To experimentally validate the robustness of NRDs in a quasi-axisymmetric (QA) configuration, we introduce STAR_Lite, a new stellarator experiment at Hampton University. This paper details the design and analysis of the first STAR_Lite coil configuration, STAR_Lite-A. The two-field-period configuration manifests an NRD through X-points with zero rotational transform, at the top and bottom of the device. The divertor legs extruding from the X-points are topologically similar to the poloidal divertors of tokamaks. To expand the experimental range, STAR_Lite-A is optimised for experimental flexibility, producing a wide range of distinct QA configurations by only varying the currents in the modular coils. The NRDs not only persist across these configurations, but numerical strike-line simulations confirm that heat exhaust remains resilient to changes in coil currents, with plasma following the divertor legs and creating a toroidal, discontinuous, strike pattern. We further examine the resilience of the NRD to magnetic perturbations caused by manufacturing errors in the modular coils. We find that quasi-symmetry and the existence of X-points are well preserved under these magnetic field changes, but the rotational transform may vary substantially and displacements of the divertor X-points may lead to one X-point having a dominant effect on edge transport. Overall, our analysis indicates that a compact, modular design can likely generate a resilient NRD structure while satisfying the practical constraints of a university-scale experiment.

Pilot-scale STARS® 14Cr ODS steel for nuclear fusion: processing, microstructural evolution and microhardness after hot rolling and short annealing

M. Roldán, E. León-Gutiérrez, N. García-Rodríguez, I. Rufilanchas, N. Ordás

Deuterium desorption from tungsten borides

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

WEST advanced wall protection achievements toward long pulse operation

Raphael Mitteau, Marie-Helene Aumeunier, Leo Dubus, Jonathan Gerardin, Valentin Gorse, Erwan Grelier, Victor Moncada, Sébastien Vives, Xavier L Litaudon, Marcin Jakubowski, et al.

Long pulse operation in magnetic fusion devices requires well controlled plasma power exhaust to the divertor & wall, and avoidance of wall hot spots that could evolve in wall damage. At WEST, 10 major plasma facing components are monitored using 10 series of temperature/power indicators, based on multiple diagnostic systems, among which the infrared viewing system is especially relevant. These indicators span from the most basic ones (temperatures, power and energy from deterministic models) to advanced processes using artificial intelligence acquired through machine learning. Some advanced processes do operate in real time, and feedback on power actuators through the plasma control system, providing active control toward remaining within the safe operational domain. Other advanced processes intervene as forensic tools post discharge to identify possible dangerous situation regarding the power loading to the wall, so that the discharge run plan is adjusted to avoid running into aggravating wall events. No critical wall power event happened during the campaigns C9 to C11 (2024-2025), totalling about 13h of plasma, that would have affected the campaign plan. While it cannot be demonstrated that the active & intelligent wall protection enabled the new plasma duration record of 1337 seconds, the wall protection system as a whole plausibly contributed to obtaining these record durations by preventing wall hot spots to become critical during the campaigns.