Post-disruption reconnection event driven by a runaway current

Abstract

The role of a runaway current in a post-disruption plasma is investigated through numerical simulations in an asymmetric magnetic reconnection event. We first reproduce the known linear results on the growth rate, the rotation frequency, and the formation of a microlayer smaller than the resistive one as found in Liu et al. [Physics of Plasmas 27, 092507 (2020)] and then focus on the nonlinear regime where are our main findings. We find that while the resistive layer controls the transition of the island from the linear to the nonlinear stage, the microlayer width controls the transition of the runaway current from the linear to the nonlinear phase. This latter transition is accompanied by a redistribution of runaways according to a spiral-like structure within the island. The same structure is also found in the thermal electron distribution when the electron inertia effects into the Ohm's law are taken into account. Finally, nonlinear simulations show that the island rotation frequency tends toward zero when the saturation is reached.