Gyrokinetic Studies of Microinstabilities in the RFP

TL;DR

This study uses gyrokinetic modeling to analyze microinstabilities in RFP plasmas, revealing a transition from ITG to microtearing modes around 4.5% beta and highlighting the role of curvature drift and shear Alfven waves.

Contribution

It introduces a detailed gyrokinetic analysis of microinstabilities in RFPs, including the effects of beta and curvature drift, which was not extensively studied before.

Findings

Transition from ITG to microtearing mode at 4.5% beta

Suppression of ITG mode via shear Alfven coupling

High profile stiffness indicated by growth rate dependence

Abstract

An analytic equilibrium, the Toroidal Bessel Function Model, is used in conjunction with the gyrokinetic code GYRO to investigate the nature of microinstabilities in a reversed field pinch (RFP) plasma. The effect of the normalized electron plasma pressure ({\beta}) on the characteristics of the microinstabilities is studied. A transition between an ion temperature gradient (ITG) driven mode and a microtearing mode as the dominant instability is found to occur at a {\beta} value of approximately 4.5%. Suppression of the ITG mode occurs as in the tokamak, through coupling to shear Alfven waves, with a critical {\beta} for stability higher than its tokamak equivalent due to a shorter parallel connection length. There is a steep dependence of the microtearing growth rate on temperature gradient suggesting high profile stiffness. There is evidence for a collisionless microtearing mode. The properties of this mode are investigated, and it is found that curvature drift plays an important role in the instability.