  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.

Plasma Physics