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-theta exhibit a scaling I-theta proportional to I-p0/q(95), with all data falling within the boundary I-theta less than or equal to 1.2I(p0)/q(95). 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 P (peak to average ratio) of the halo currents decreases with increasing I-theta/I-p0, 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.