Two Fluid Dynamo and Edge-Resonant m=0 Tearing Instability in Reversed Field Pinch

Current-driven tearing instabilities dominate magnetic relaxation in self-organized high temperature plasmas such as the reversed field pinch (RFP) and spheromak. In the Madison Symmetric Torus (MST) RFP experiments, they are observed in the form of magnetic field, flow velocity, and current density fluctuations that follow a temporally cyclic sawtooth behavior. During a sawtooth crash, a surge occurs in the dynamo a fluctuation-induced mean electromotive force in the generalized Ohm’s law that combines the MHD v B and j B Hall dynamos. We report new results on the physics of two-fluid dynamos as well as on the problem of spontaneous (linear) instability of edge-resonant m=0 tearing modes. The key findings are: (1) two fluid effects are critical for dynamo through their influence on the phase between the fluctuations; in cylindrical RFP, the two fluid tearing instability becomes oscillatory due to significant curvature of the field lines; (2) the two fluid version of the NIMROD code confirms analytic results during the linear stage of the instability but exhibits significant broadening of the Hall dynamo profile on the longer time scales of nonlinear evolution; (3) improved modeling of force-free RFP equilibrium predicts a wide range of RFP parameters in which m=0 tearing mode is spontaneously unstable, a result that is consistent with recent MST experimental observations.