Description
Tokamak disruptions are a major challenge for the safe operation of next-step fusion devices such as ITER. A key concern is the formation of relativistic runaway-electron (RE) beams, which can deposit highly localized heat loads and threaten plasma-facing components. In ITER, the baseline disruption mitigation strategy relies on the rapid delivery of large impurity quantities via shattered pellet injection (SPI). Predictive modelling of RE generation in SPI-mitigated disruptions is therefore essential, but the computational cost of high-fidelity approaches motivates reduced models that enable extensive parameter scans. A recent study [1] used the DREAM fluid-RE model [2] to investigate RE generation in realistic ITER disruptions, incorporating (i) RE scrape-off during plasma vertical displacement [3] and (ii) plasmoid-drift effects on impurity deposition during pellet ablation [4]. A major remaining uncertainty is the hot-tail RE generation,which is typically misestimated by fluid models. In this work, we improve hot-tail seed predictions in ITER-relevant SPI simulations with the DREAM code, by formulating and numerically solving an isotropic kinetic equation, which resolves the momentum-space dynamics neglected in fluid models. We introduce a local, dynamic transition between fluid and kinetic equation sets, triggered once the isotropic model assumptions are satisfied. In particular, since the isotropic formulation employs a collision operator linearized about a cold background, it is activated in regions where impurity deposition rapidly builds up the cold electron population. Building on [5], which found that the DREAM fluid hot-tail model yields larger seeds than the isotropic kinetic approach, we re-assess this result for ITER by extending the analysis to the SPI mitigated disruptions considered in [1].
References
[1] L. Votta et al, In preparation
[2] M. Hoppe et al, Computer Physics Communications, vol. 268, 2021
[3] O. Vallhagen et al, Journal of Plasma Physics, vol. 91, no. 3, 2025
[4] O. Vallhagen et al, Plasma Physics and Controlled Fusion, vol. 67, no. 10, 2025
[5] I. Ekmark et al, Journal of Plasma Physics, vol. 90, 2024