Preliminary MHD Validation Studies on MST

Quantitative validation of visco-resistive MHD models using MST takes advantage of its well-diagnosed, standard RFP plasmas. These plasmas are largely governed by MHD relaxation activity, so that a broad range of validation metrics can be evaluated. Nonlinear simulations using the single-fluid 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 was used previously at $S=8\times10^{4}$, which is below MST's operational lower limit of $S\geq5\times10^{5}$. The predicted $\tilde{b}$ from NIMROD was about half as large for a two-fluid case as for a single-fluid, suggesting that a two-fluid model may be necessary for quantitative agreement with experiment. Comparisons of linear and nonlinear DEBS and NIMROD runs at low $S$ are presented, focusing on how their different numerical algorithms affect their performance. Experimental equilibrium and fluctuation measurements at low $S$ from an insertable magnetic probe are compared with simulation results. Future scaling studies of $\tilde{b}$ as a function of $S$ are planned using NIMROD at low $S$ complemented by DEBS at higher $S$.