Suppression of runaway electrons with a resonant magnetic perturbation in MST tokamak plasmas.

Runaway electrons generated in MST tokamak plasmas are now being probed with resonant magnetic perturbations (RMP's). An RMP with m$=$3 strongly suppresses the runaway electrons. Initial modeling of these plasmas with NIMROD shows the degradation of flux surfaces with an m$=$3 RMP, which may account for the runaway electron suppression. These MST tokamak plasmas have Bt$=$0.14 T, Ip $=$50kA, and q(a)$=$2.2, with a bulk electron density and temperature of 5x1017 m-3 and 150 eV. Runaway electrons are detected via x-ray emission. The RMP is produced by a poloidal array of 32 saddle coils at the narrow vertical insulated cut in MST's thick conducting shell. Each RMP has a single m but a broad n spectrum. A sufficiently strong m$=$3 RMP completely suppresses the runaway electrons, while a comparable m$=$1 RMP has little effect. The impact of the RMP's on the magnetic topology of these plasmas is being studied with the nonlinear MHD code, NIMROD. With an m$=$3 RMP, stochasticity is introduced in the outer third of the plasma. No such change is observed with the m$=$1 RMP. NIMROD also predicts regularly occurring sawtooth oscillations with a period comparable to MHD activity observed in the experiment.