Analysis of vertical displacement events and halo currents in COMPASS-D

Vertical displacement events (VDEs) and their attendant `halo currents' flowing in the plasma facing components pose a potentially serious hazard to operation in large fusion devices. Experiments on the COMPASS-D tokamak indicate that, on this machine, peak halo currents I? exhibit a scaling I? Ip0/q95, with all data falling within the boundary I? ? 1.2 Ip0/q95. Toroidally asymmetric halo currents peak along with the symmetric halo currents, and are nearly fixed in toroidal phase, according to magnetic coil and resistive shunt data. The toroidal peaking factor (peak to average ratio) of the halo currents decreases with increasing I?/Ip0, but exhibits significant scatter. Here, the experimental arrangement is described, and the above results are discussed. A model for the observed boundary in the peak halo current data is introduced, employing near perfect imaging of the plasma current by a halo that becomes highly conducting during a VDE. The model reproduces both the above limit observed on COMPASS-D and also the higher boundary observed on Alcator?C-Mod. In addition, the model results are consistent with reports from other machines of the effectiveness of schemes to reduce halo currents such as gas puffing and killer pellet injection.