Scaling Of Linear Microtearing Stability For A High Collisionality NSTX Discharge
Authors: W. Guttenfelder, J. Candy, S.M. Kaye, W.M. Nevins, R.E. Bell, G.W. Hammett, B.P. LeBlanc, and H. Yuh
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 electronion
collision frequency over mode frequency, ve/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 electron temperature gradient (ETG) instability
offering an opportunity to experimentally distinguish the two modes. Finally we note that the
kinetic ballooning mode (KBM) is found to compete with the microtearing mode at outer
locations r/a≥0.8.
__________________________________________________
Submitted to: Physics of Plasmas (October, 2011)
__________________________________________________
Download PPPL-4731 (pdf 648 KB 36 pp)
__________________________________________________