Numerical simulations of dynamo suppression during pulsed poloidal current drive in the reversed field pinch

Laboratory experiments with Pulsed Poloidal Current Drive (PPCD) show the reduction of the resistive magnetohydrodynamic (MHD) fluctuations that introduce stochasticity to magnetic field line trajectories and lead to heat and particle transport in the Reversed Field Pinch (RFP). We investigate the suppression of the MHD instabilities during PPCD by analyzing the application of an inductive poloidal electric field pulse to saturated RFP states using fully nonlinear numerical MHD simulations. Since the most effective technique in PPCD experiments applies pulses of the poloidal electric field while simultaneously decreasing the loop voltage, our study also analyzes the effects of reducing the applied toroidal electric field during the PPCD pulse application.;Studies of the effect of the inductive poloidal pulse on single saturated kink instabilities show that the initial linear stability is enhanced by the addition of current driven parallel to the magnetic field in the plasma edge. The pulse application removes the source of free energy to sustain saturated instabilities. In simplified saturated systems where the coupling between fluctuations is limited, the pulse driven current in the edge is linearly stabilizing to key modes that transfer power between the equilibrium and nonlinearly driven fluctuations.;When only the inductive poloidal pulse is applied to the saturated RFP system, the overall fluctuation levels rapidly decrease. The linear stability of the core-resonant and edge-resonant fluctuations is initially enhanced by the increase in parallel edge current. The reduction of power transfer to fluctuations that absorb power from the equilibrium and transfer power through modal coupling alters the normal nonlinear power balance of the system. Later in the pulse, the increase of the pinch velocity leads to pinched profiles and the destabilization of the innermost resonant fluctuations. The initial dynamo suppression by the applied poloidal pulse is almost identical in simulations where the loop voltage is simultaneously reduced, but reducing the loop voltage keeps the mean profiles from evolving to more pinched and unstable configurations later in the pulse.