NIMROD simulations of HIT-SI plasmas

HIT-SI (Steady Inductive Helicity Injected Torus) is a current drive experiment that uses two semi-toroidal helicity injectors driven at 5-15 kHz to generate steady inductive helicity injection (SIHI). All the plasma-facing walls of the experiment are coated with an insulating material to guarantee an inductive discharge. NIMROD is a 3-D extended MHD code that can only model toroidally-uniform geometries. The helicity injectors of the experiment are simulated as flux and voltage boundary conditions with odd toroidal symmetry. A highly resistive, thin edge-layer approximates the insulating walls. The simulations are initial-value calculations that use a zero $\beta$ resistive MHD (rMHD) model with uniform density. The Prandtl number ($Pr = 10$), and Lundquist number ($S = 5-50$) closely match the experimental values. rMHD calculations at $S\sim10$ show no growth of an $n=0$ mode and only a few kA of toroidal current whereas HIT-SI has demonstrated toroidal currents greater than 50~kA with a current amplification of 3. At higher $S (\ge20)$ the simulations exhibit significant $n=0$ magnetic energy growth and a current amplification exceeding unity: $\frac{I_{tor}}{I_{{\rm inj}}}\geq1$. While HIT-SI has shown evidence for separatrix formation, rMHD calculations indicate an entirely stochastic magnetic structure during sustainment. Results will also presented for Hall MHD, anticipated to play a crucial role in the physics of SIHI.