Extended Magnetohydrodynamic Modeling Of Plasma Relaxation Dynamics In The Reversed Field Pinch

Nonlinear plasma relaxation dynamics in reversed-field pinch (RFP) conditions are investigated using extended magnetohydrodynamic modeling with the NIMROD code [Sovinec et. al. JCP 195 (2004)]. The computations display quasi-periodic relaxation events, consistent with the RFP sawtooth cycle. The first event occurs from a saturated single-helicity state and is not representative of the multi-helicity conditions in typical RFP discharges. The MHD and Hall dynamo electric fields cooperate during the initial relaxation, and the change in the plasma flow parallel to the magnetic field is in the direction of the Lorentz force density and opposite to the change in parallel current density. In subsequent events, the MHD dynamo relaxes the parallel current and is opposed by the Hall dynamo, which is smaller in magnitude. The associated Lorentz force density drives changes in flow that are in the same direction as the change in current, in agreement with experimental observations [Kuritsyn et. al. POP 055903 (2009)]. Changes in total momentum are due to viscous coupling to the wall; radial transport occurs through Maxwell stresses associated with current relaxation. The magnetic and hybrid helicity are well conserved relative to magnetic energy over the relaxation events. The helicities change by