Computations of Macroscale Dynamics and Transport in the Ultra-low Safety Factor Regime

Nonlinear resistive-MHD computation is applied to examine the interaction of global-scale fluctuations with a self-consistently evolving profile in the regime where 0 < q(a) < 1. Relatively poorly understood fluctuation phenomena without disruptive kink modes are observed when achieving these ultra-low-q (ULq) conditions in the Madison Symmetric Torus (MST). This regime is possible without disruptive kink modes due to the device’s close-fitting shell [N. C. Hurst, et al., POP 29, 080704]. MST also utilizes feedback-controlled programmable power supplies (PPS) to drive current, allowing a wide range of q(a) to be maintained. Our NIMROD simulations provide information on fluctuations that complements the analysis of experimental data [A. Keyhani, et al., poster]. Computations of ULq conditions start from vacuum field and applied loop voltage with proportional-differential control that targets a desired plasma current value, roughly modeling PPS. They model MHD dynamics with Ohmic heating and energy transport from parallel conduction along the magnetic fluctuations. Results show regular MHD activity for q(a) values above 1.6 and more irregular fluctuations for positive q(a) values below 1. This is qualitatively consistent with experimental observations, where fluctuations become less regular as q(a) is decreased below 2 with exceptions for rational q(a) values below 1 [N. C. Hurst, et al., poster], which have not yet been simulated.