DISRUPTIONS AND HALO CURRENTS IN ALCATOR C-MOD

Disruptions in Alcator C-Mod can generate large eddy currents in the h ighly conducting vacuum vessel and internal structures, including a si gnificant poloidal component due to halo currents. In order to underst and better the stresses arising from the resulting JxB forces, Alcator C-Mod has been fitted with a comprehensive set of sensors to measure the spatial distribution and temporal behaviour of the halo currents. It is found that they are toroidally asymmetric, with a typical peakin g factor of 2. The asymmetric pattern usually rotates toroidally at a few kilohertz, thus ruling out first wall non-uniformities as the caus e of the asymmetry. Analysis of the information compiled in the C-Mod disruption database indicates that the maximum halo current during a d isruption scales roughly as either I-p(2)/B-phi or I-p/q(95), but that there is a large amount of variation that is not yet understood.