Scaling of linear microtearing stability for a high collisionality National Spherical Torus Experiment discharge

Abstract

Linear gyrokinetic simulations are performed based on a high collisionality NSTX discharge that is part of dimensionless confinement scaling studies. In this discharge, the microtearing mode is predicted to be unstable over a significant region of the plasma (r/a = 0.5–0.8), motivating comprehensive tests to verify the nature of the mode and how it scales with physical parameters. The mode is found to be destabilized with sufficient electron temperature gradient, collisionality, and beta, consistent with previous findings and simple theoretical expectations. Consistent with early slab theories, growth rates peak at a finite ratio of electron-ion collision frequency over mode frequency, νe/i/ω ∼ 1–6. Below this peak, the mode growth rate decreases with reduced collisionality, qualitatively consistent with global confinement observations. Also, in this region, increased effective ionic charge (Zeff) is found to be destabilizing. Experimental electron beta and temperature gradients are two to three times larger than the inferred linear thresholds. Increasing magnetic shear (s) and decreasing safety factor (q) are both destabilizing for ratios around the experimental values s/q = 0.6–1.3. Both the Zeff and s/q scaling are opposite to those expected for the ETG instability offering an opportunity to experimentally distinguish the two modes. Finally, we note that the kinetic ballooning mode is found to compete with the microtearing mode at outer locations r/a ≥ 0.8.