Simulating Global Sawtooth Magnetic Reconnection Events in the MST RFP

Advances in computational power have now made possible nonlinear resistive MHD simulations of the reversed field pinch (RFP) at experimentally relevant parameters. In particular, global magnetic reconnection events known as sawtooth crashes have now been simulated at parameters matching those of 400kA discharges in MST (S$\sim $4x10$^{6})$ with the D\textsc{ebs} code. At these parameters, the simulated sawtooth event is not only similar in character, but also in duration to the events observed in MST. This implies that a single fluid MHD model is able to reproduce the dynamics leading to reconnection times that are significantly faster than Sweet-Parker. One possible mechanism for reducing the reconnection time is a plasmoid-like structure forming along the reversal surface during a sawtooth crash. To explore this, several simulations with an artificially truncated m=0 mode spectrum were performed. As the number of allowed toroidal mode harmonics is reduced, the sawtooth crash duration increases and the magnitude of the dynamo electric field decreases. Interestingly, while it has long been known that the m=0, n=1 mode plays a critical role in the sawtooth dynamics in MST, without the higher n, m=0 modes the simulations do not produce well defined sawteeth. The effects of limiting the number of m=0 modes on the character and duration of the sawtooth crash as well as initial work using the magnetic field information from these simulations to model the evolution of both the thermal and beam generated fast ion populations will be presented. This work supported by the US DOE and NSF.