Resistive MHD simulations in support of SSPX

The SSPX spheromak has obtained Btor$>$0.6T, Te=350eV and t(pulse)$\approx $3ms. NIMROD simulations are used to interpret results, guide experiments, and explore upgrades. Voltage spikes during formation and sustainment are interpreted as reconnection across an n=1, negative-current layer close to the mean-field x-point. Field lines are chaotic during these events, causing rapid electron energy loss to the walls; Te$<$50eV in experiment and simulation during strong helicity injection. Sustainment occurs at a high ratio of gun current to bias flux. During slow plasma decay at low gun current, high Te results when magnetic fluctuations are low ($<$1{\%}). If q crosses low-order rational surfaces, islands form causing reduced energy confinement. Fieldlines can become chaotic (Lyapanov length $>$ 4$\pi $R); if they reach walls Te drops to $<$50eV. Changing Zeff=1 to 2.3 (SSPX value) increases ohmic heating and decreases parallel thermal conduction, affecting spheromak evolution. An experimental upgrade to allow bias field reduction following formation may allow increased efficiency operation.