3D scrape-off layer currents during tokamak MHD activity and disruptions*

In tokamaks, currents transfer between the plasma edge and vessel structures by conduction through the scrape-off-layer (SOL). When these currents are not axisymmetric, they can produce large forces capable of damaging wall components. Accurate knowledge of governing principles for these currents is needed to ensure reliability of a future reactor and other large-scale tokamaks. We present measurements of asymmetric SOL and vessel currents in the HBT-EP tokamak, and analyze them in the context of models for asymmetric currents during disruptions. Currents flowing into the low-field-side wall are measured with poloidal resolution in 3 toroidal locations. Tiles can be moved radially along with their associated wall segments between discharges to study shadowing effects and radial structure of SOL currents. Electrically-isolated chamber sections allow measurements of toroidal vessel currents when sections are connected via diagnosed jumpers. Asymmetric vessel currents reach ~5% of the pre-disruption plasma current during the current quench [1]. Plans for installing diagnostic and control tiles on the high-field are also presented.

[1] J.P. Levesque et al., Nucl. Fusion 57 086035 (2017)