Simulations of sawtoothing activity with a resistive wall in the HBT-EP tokamak

The NIMROD [1] code is used to validate multiphysics models (MHD + resistive wall) for the prediction of mode structures and scrape-off-layer (SOL) currents in tokamaks using high-resolution current, magnetic, and optical diagnostics from HBT-EP [2]. NIMROD's existing thin resistive wall boundary condition is extended to include non-axisymmetric wall resistivity. Simulations of HBT-EP with a resistive wall observe periodic sawtoothing activity and motivate comparisons with experimental data [3]. Effects of varying plasma-wall separation, non-axisymmetric wall resistivity, and transport parameters on critical thresholds for sawtooth suppression are investigated. Further work on the dependence of sawtooth suppression on externally applied fields will be discussed. Applications toward better understanding the 3D structure of wall-connected currents and effects of runaway electron mitigation coil (REMC) fields will be presented. Initial validation studies of numerical models for wall-connected currents are conducted by analyzing synthetic and experimental phase differences between diagnostics on HBT-EP with the goal of improving SOL and wall models for ITER and next-step devices.