Halo current measurements on Alcator C-Mod

An aggressive experimental campaign to measure halo currents in Alcator C-Mod is described. Arrays of magnetic probes in combination with resistive shunts in outer divertor modules measure total halo currents flowing during disruptions as well as their vertical and toroidal distributions. Toroidal rotation and toroidal asymmetries dominated by n=1 structure characterize the measurements. A vertical array of magnetic probes places an upper bound on the width of the halo region, indicating that the halo currents flow in a layer close to the plasma. Dynamic calculations of the stresses on the Alcator C-Mod vacuum vessel inner cylinder indicate enhanced stresses attributable to the asymmetries observed. Halo current measurements are presented and tentative scalings of halo current with plasma parameters are proposed.