Vertical displacement events (VDEs) and their attendant `halo currents' flowing inthe plasma facing components pose a potentially serious hazard to operation inlarge fusion devices. Experiments on the COMPASS-D tokamak indicatethat, on this machine, peak halo currents Iθ exhibit a scalingIθ ∝ Ip0/q95, with all data falling within theboundary Iθ ⩽ 1.2 Ip0/q95. Toroidally asymmetric halocurrents peak along with the symmetric halo currents, and are nearly fixed intoroidal phase, according to magnetic coil and resistive shunt data. Thetoroidal peaking factor (peak to average ratio) of the halocurrents decreases with increasing Iθ/Ip0, but exhibitssignificant scatter. Here, the experimental arrangement is described, and theabove results are discussed. A model for the observed boundary in the peakhalo current data is introduced, employing near perfect imaging of the plasmacurrent by a halo that becomes highly conducting during a VDE. The modelreproduces both the above limit observed on COMPASS-D and also thehigher boundary observed on Alcator C-Mod. In addition, the model results areconsistent with reports from other machines of the effectiveness of schemes toreduce halo currents such as gas puffing and killer pellet injection.
On the magnitude and distribution of halo currents during disruptions on MAST