Validation of MHD Models using MST RFP Plasmas

Rigorous validation of computational models used in fusion energy sciences over a large parameter space and across multiple magnetic configurations can increase confidence in their ability to predict the performance of future devices. MST is a well diagnosed reversed-field pinch (RFP) capable of operation with plasma current ranging from 60~kA to 500~kA. The resulting Lundquist number $S$, a key parameter in resistive magnetohydrodynamics (MHD), ranges from $4\times10^4$ to $8\times10^6$ for standard RFP plasmas and provides substantial overlap with MHD RFP simulations. MST RFP plasmas are simulated using both DEBS, a nonlinear single-fluid visco-resistive MHD code, and NIMROD, a nonlinear extended MHD code, with $S$ ranging from $10^4$ to $10^5$ for single-fluid runs, and the magnetic Prandtl number $Pm=1$. Validation metric comparisons are presented, focusing on how normalized magnetic fluctuations at the edge $\tilde{b}$ scale with $S$. Preliminary results for the dominant $n=6$ mode are $\tilde{b}\sim S^{-0.20\pm0.02}$ for single-fluid NIMROD, $\tilde{b}\sim S^{-0.25\pm0.05}$ for DEBS, and $\tilde{b}\sim S^{-0.20\pm0.02}$ for experimental measurements, however there is a significant discrepancy in mode amplitudes. Preliminary two-fluid NIMROD results are also presented.