Gyrokinetic Studies of Microinstabilities in the RFP

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.