Initial Studies of Validation of MHD Models for MST Reversed Field Pinch Plasmas

Quantitative validation of visco-resistive MHD models for RFP plasmas takes advantage of MST's advanced diagnostics. These plasmas are largely governed by MHD relaxation activity, so that a broad range of validation metrics can be evaluated. Previous nonlinear simulations using the visco-resistive MHD code DEBS at Lundquist number $S=4\times10^{6}$ produced equilibrium relaxation cycles in qualitative agreement with experiment, but magnetic fluctuation amplitudes $\tilde{b}$ were at least twice as large as in experiment. The extended-MHD code NIMROD previously suggested that a two-fluid model may be necessary to produce $\tilde{b}$ in agreement with experiment. For best comparisons with DEBS and to keep computational expense tractable, NIMROD is run in single-fluid mode at low $S$. These simulations are complemented by DEBS at higher $S$ in cylindrical geometry, which will be used to examine $\tilde{b}$ as a function of $S$. Experimental measurements are used with results from these simulations to evaluate validation metrics. Convergence tests of previous high $S$ DEBS simulations are also discussed, along with benchmarking of DEBS and NIMROD with the SPECYL and PIXIE3D codes.