Disruption Halo Currents on HBT-EP

Four poloidal arrays of grounded tiles mounted on the high-field side (HFS) serve as diagnostics for the halo current flowing between the plasma and the wall during the current quench phase of disruptions on HBT-EP. We present three observations using the tile arrays. First, we present the fact that electron current, especially during high frequency rotation of the halo currents, exceeds the measured and calculated ion saturation current by 2-4 times during axisymmetric halo currents and up to 10-20 times during rotating halo currents. This observation exceeds the empirical current-density limitation suggested by results from a study on COMPASS in [1] and [2]. Details and implications of this observation are presented. Second, the operation of the Runaway Electron Mitigation Coil (REMC) affects the spatial distribution of halo currents, leading to an n=1 toroidal asymmetry in the halo current distribution. Filament reconstructions show that this asymmetry is due to the REMC’s effect on plasma shape. Finally, cathode spots, or arcing, are frequently observed on the tiles during the disruption. Results of a statistical study of the poloidal distribution of the tiles arcs are presented.

[1] J. Adamek et al 2022 Nucl. Fusion 62 086034 [2] F J Artola et al 2021 Plasma Phys. Control. Fusion 63 064004