Gyrokinetic Simulations of Tearing Mode Evolution Towards Multi-Scale Interactions

The nonlinear evolution of global tearing modes (TMs) occurs in the reversed field pinch (RFP) and interacts with microturbulence, affecting fluctuation amplitudes and transport. Earlier work used an ad-hoc tearing-type perturbation to model the impact of TMs on microturbulence in local simulations with the gyrokinetic code GENE. Here, the global version of GENE is modified to allow a current-gradient drive by implementing a shifted Maxwellian into the background distribution of the Vlasov equation and coupling it with field equations based on the shifted Maxwellian to reproduce TMs self-consistently. The code is then benchmarked for different computational scenarios, including against the gyrokinetic PIC code ORB5. Various plasma parameters are scanned, including mass ratio, plasma beta, and collisionality. Linear growth rates are in good agreement, and scalings are consistent with theory. The code is then employed to study linear and nonlinear TMs of a non-reversed RFP discharge. Analysis of nonlinear TM saturation is presented. Interactions with trapped- electron modes, effects on zonal flows, and back-reactions on the TMs will be studied by adding small scales in saturation.