(TH/P3-18) Two Fluid Dynamo and Edge-Resonant m=0 Tearing Instability in Reversed Field Pinch

Abstract.  Current-driven tearing instabilities are believed to 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, tearing instabilities 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. The dynamo modifies parallel electric field and plasma current profiles. This ultimately leads to current flattening in the core and current sustainment in the plasma edge. We report new analytic and numerical 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 m=0 mode is of a special importance for RFPs because of its impact on mode coupling, ion heating, momentum and energy transport. The key findings are: (1) two fluid effects are critically important for dynamo through their influence on the phase between the fluctuations; two-fluid theory yields a non-zero flux surface averaged Hall dynamo, absent in resistive MHD; (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.

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