3D hot tail runaway electron generation modeling for ITER

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Abstract
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.

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