The linear tearing instability in three dimensional, toroidal gyrokinetic simulations

TL;DR

This paper investigates the linear tearing mode in three-dimensional toroidal gyrokinetic simulations, analyzing stability, growth rates, and mode rotation under various plasma conditions, with results benchmarked against MHD theory.

Contribution

It provides the first detailed gyrokinetic simulation analysis of the tearing mode in 3D toroidal geometry, including effects of pressure gradients and collisionality.

Findings

Identification of collisionless and semi-collisional tearing modes with a smooth transition.

Observation of a residual mode rotation frequency due to toroidal finite Larmor-radius effects.

Growth rate scaling with resistivity following a η^{1/7} law in the semi-collisional regime.

Abstract

Linear gyro-kinetic simulations of the classical tearing mode in three-dimensional toroidal geometry were performed using the global gyro kinetic turbulence code, GKW . The results were benchmarked against a cylindrical ideal MHD and analytical theory calculations. The stability, growth rate and frequency of the mode were investigated by varying the current profile, collisionality and the pressure gradients. Both collision-less and semi-collisional tearing modes were found with a smooth transition between the two. A residual, finite, rotation frequency of the mode even in the absense of a pressure gradient is observed which is attributed to toroidal finite Larmor-radius effects. When a pressure gradient is present at low collisionality, the mode rotates at the expected electron diamagnetic frequency. However the island rotation reverses direction at high collisionality. The growth rate is found to follow a $\eta^{1/7}$ scaling with collisional resistivity in the semi-collisional regime, closely following the semi-collisional scaling found by Fitzpatrick. The stability of the mode closely follows the stability using resistive MHD theory, however a modification due to toroidal coupling and pressure effects is seen.