Measurements of internal MHD activity and seed runaway electrons during MST tokamak disruptions

Disruptions in MST tokamak plasmas can be effected by rapid ramp down of the toroidal field, causing a temporary increase in the plasma current and magnetic fluctuations followed by a current quench. Fast Thomson scattering reveals an initial slow drop in the core Te followed by a final rapid drop, resulting in a flat Te profile. The equilibrium and fluctuating magnetic field components have been diagnosed with a probe inserted up to r/a = 0.45 and consisting of 20 triplets of coils separated by 1 cm. The q profile, measured before the disruption for q(a) = 2.5, Ip = 50 kA, and Te < 100 eV plasmas, exhibits a q = 1 surface inside mid-radius. With the disruption, the q profile flattens more broadly to a value just above unity. High-energy electrons are generated and rapidly lost during these disruptions, both potentially due to MHD activity. The x-ray flux from these electrons shows a strong inverse dependence on density just before start of Bt ramp down. Runaway electron and other data will also be shown from newly produced plasmas with q(a) = 1.5 - 3.5 and improved control of Ip. Work supported by US DOE.