Scrape-off-layer currents during disruptions in HBT-EP

Non-axisymmetric halo currents that occur during tokamak disruptions can result in significant asymmetric forces on the vessel, potentially causing damage to components. Understanding the generation and rotation of these currents is important for establishing design requirements for devices to tolerate disruptions. A new set of halo current diagnostic and control tiles has been installed on the high-field-side (HFS) of the HBT-EP tokamak, in regions where there is strong plasma-wall contact during disruptions. The tiles can be switched between having passive low resistance connections to the vessel, or active biasing with respect to the vessel or other tiles. Driving current during active biasing is expected to be severely constrained due to the low density of the plasma edge and scrape-off layer; possible methods to increase the current limit for control purposes are discussed. This first stage of HFS implementation uses tiles at 2 toroidal locations separated by Δφ=90°, integrating over 6 poloidal regions with Δθ∼13° resolution. We present initial measurements from these tiles during disruptions and non-disruptive mode activity. Plans to expand the system to a set of four equally-spaced poloidal arrays are described.