Global Linear and Nonlinear Gyrokinetic Simulations of Tearing Modes

TL;DR

This paper extends a gyrokinetic simulation code to study tearing modes in RFP plasmas, demonstrating linear stability analysis, benchmarking, and nonlinear mode interactions relevant for understanding plasma behavior.

Contribution

The paper introduces a modified gyrokinetic code with a shifted Maxwellian for global tearing mode simulation, validated through benchmarking and applied to nonlinear evolution analysis.

Findings

Good agreement with other codes and theories in benchmarking.

Unstable modes behave consistently with theoretical scaling.

Nonlinear simulations show mode coupling and energy transfer mechanisms.

Abstract

To better understand the interaction of global tearing modes and microturbulence in the Madison Symmetric Torus (MST) reversed-field pinch (RFP), the global gyrokinetic code \textsc{Gene} is modified to describe global tearing mode instability via a shifted Maxwellian distribution consistent with experimental equilibria. The implementation of the shifted Maxwellian is tested and benchmarked by comparisons with different codes and models. Good agreement is obtained in code-code and code-theory comparisons. Linear stability of tearing modes of a non-reversed MST discharge is studied. A collisionality scan is performed to the lowest order unstable modes ($n=5$, $n=6$) and shown to behave consistently with theoretical scaling. The nonlinear evolution is simulated, and saturation is found to arise from mode coupling and transfer of energy from the most unstable tearing mode to small-scale stable modes mediated by the $m=2$ tearing mode. The work described herein lays the foundation for nonlinear simulation and analysis of the interaction of tearing modes and gyroradius-scale instabilities in RFP plasmas.