Runaway studies in disruptions with partial current relaxation

The safe operation of tokamak reactors requires a reliable modeling capability of disruptions, and in particular the spatio-temporal dynamics of associated runaway electron currents. To explore the vast parameter space characterizing off-normal events, computationally efficient reduced models are needed. Such models are necessarily of low dimensionality. Therefore, accounting for fast magnetic reconnection - driving current profile relaxation, as well as a rapid transport of heat and energetic particles - has posed a major challenge. Using the implementation of the mean field helicity transport model of [A. H. Boozer 2018 Nucl. Fusion 58 036006] in the new 1D2P disruption runaway modeling code DREAM [M Hoppe et al 2021 Comp. Phys. Comm.], we calculate the dynamics of runaway electrons in scenarios where skin currents are induced at the boundary of stochastic and intact magnetic field line regions, accompanying fast reconnection in part of the plasma. We find scenarios where significant reverse runaway skin currents are generated in the plasma edge, which may act as a seed for a regular (forward) edge-localized runaway current, when helicity transport is low in the edge.