Runaway electron dynamics in shattered pellet mitigated ITER disruptions*

Disruptions and associated runaway electron beams represent an outstanding challenge in reactor-scale tokamaks. We have systematically explored the parameter space of disruption mitigation through shattered pellet injection in ITER with a focus of runaway electron dynamics, using the disruption modeling tool DREAM. The analysis provides a rather comprehensive coverage of experimentally feasible scenarios: We consider plasmas representative of both non-activated and high-performance DT operation, use different thermal quench onset criteria and transport levels, a wide range of injected deuterium and neon quantities, as well as single-stage and two-stage injection of pellets with various characteristic shard sizes. In addition, we consider the effect of the drift of pure hydrogen pellet clouds, as well as injections that fail to reach the plasma before the thermal quench. We find that two-stage injection helps hydrogen assimilation and reduces the hot-tail generation mechanism, providing the best performing cases. It also allows a robust elimination of the runaway current in reduced plasma current scenarios. However, we find megaampere scale runaway currents in 15 MA discharges, even in non-activated scenarios. .