Runaway dynamics in disruptions with current relaxation

Abstract: 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. In a disruption, instabilities can break up magnetic surfaces into chaotic field line regions, causing current profile relaxation, as well as a rapid radial transport of heat and particles. Using a mean-field helicity transport model implemented in the disruption runaway modeling framework DREAM, we calculate the dynamics of runaway electrons in the presence of current relaxation events. In scenarios where flux surfaces remain intact in parts of the plasma, a skin current is induced at the boundary of the intact magnetic field region. This skin current region becomes an important center concerning the subsequent dynamics: It may turn into a hot ohmic current channel, or a sizable radially localized runaway beam, depending on the heat transport. If the intact region is in the plasma edge, runaway generation in the counter-current direction can occur, which may develop into a sizable reverse runaway beam. Even when the current relaxation extends to the entire plasma, the final runaway current density profile can be significantly affected, as the induced electric field is reduced in the core and increased in the edge, thereby shifting the center of runaway generation towards the edge.