Maxwell Torque Scaling Studies using NIMROD*

Locking of tearing modes is a leading cause of disruptions in tokamaks. Tearing modes are typically born rotating with the plasma; however, external magnetic perturbations exert a breaking torque, slowing the island rotation, and potentially resulting in locking. Future burning plasma experiments will have slow rotation due to their weak injected neutral beam torques relative to their moment of inertia. As such mode locking is of increased concern in these experiments. An important issue for predicting the locking is to quantify the scaling of the Maxwell torque. To address this issue extended MHD simulations, using NIMROD [C.R. Sovinec et al., JCP 2004], are used to study the torque scaling as a function of major radius. A family of similar L-mode equilibria with ITER like shaping, characteristic of the start-up conditions, are considered. A heuristic neoclassical toroidal viscosity [T.A Gianakon et al., POP 2002] is used to model the effects of poloidal flow dampening, polarization current enhancement, and bootstrap current generation. The electron dampening rate is used as a tuning parameter to normalize the saturated island width to major radius. The torque scaling in thin, medium sized, and thick islands is considered.