Runaway electron dynamics in MST tokamak plasmas measured by a versatile multi-energy SXR camera

A versatile multi-energy soft x-ray (SXR) pinhole camera has been designed, built, and deployed for the Madison Symmetric Torus (MST) to aid the study of particle and thermal transport, as well as MHD stability physics. A novel application detecting the birth of runaway electrons (REs) measuring its time- and spatial-dependence during the linear and exponential growth phases has recently been demonstrated. The ME-SXR pin-hole camera provides unprecedented improvement in signal-to-noise-ratio, early-detection, imaging and energy discrimination at E_photon/T_e,0~20-200. REs are generated at the MST in steady tokamak plasmas with low current and toroidal magnetic field obtaining core temperatures and densities of the order of 0.1 keV and <0.1×10¹⁹m^-3, respectively. Density thresholds for both runaway electron onset and suppression are determined with simple variations in gas puffing. A detail scan of resonant magnetic perturbations intensity (RMPs, with poloidal mode number m=3) also resulted in the suppression of REs consistent with the appearance of a broad region of stochasticity. This work is supported by the DoE as a collaboration between PPPL and WiPPL.