Generation and suppression of runaway electrons in MST tokamak plasmas

This paper explores the behavior of runaway electrons in tokamak plasmas at low electron density, plasma current, and magnetic field using experimental data from the Madison Symmetric Torus (MST) and computational data from the NIMROD nonlinear resistive 3D MHD code. Density thresholds for the onset and suppression of runaway electrons are determined experimentally in steady tokamak plasmas, and in plasmas with a population of runaway electrons, resonant magnetic perturbations with different poloidal mode numbers are applied. Poloidal mode number m  =  3 perturbations suppress the runaway electrons, while perturbations with m  =  1 have little effect. This difference is consistent with the difference in computed magnetic topologies. The m  =  1 RMP has little effect on the topology, while the m  =  3 RMP produces a broad region of stochasticity, which can allow for rapid loss of the runaway electrons.