Gyrokinetic analysis of thermal transport scaling in NSTX and MAST

It remains unclear how thermal energy confinement will scale when extrapolating from present-day STs to CTF conditions at higher plasma current and toroidal field (lower collisionality). To address this theoretically we present linear gyrokinetic simulations investigating microstability in the outer half-radius of NSTX and MAST discharges that vary I$_{p}$ and B$_{T}$. In high collisionality discharges (low I$_{p}$ {\&} B$_{T})$ microtearing modes are often predicted to be unstable. These modes are weakened when artificially reducing electron collisionality, consistent with experimental scaling trends at higher I$_{p}$ {\&} B$_{T}$. Whether other modes (ITG/TEM, ETG) arise depends on additional parameters such as profile gradients, effective ionic charge, beta, and flux surface shaping. We also discuss the numerical complications in non-linear microtearing mode simulations that include electromagnetic perturbations, collisions and toroidal flow and flow shear. This work is supported by US DOE contract DE-AC02-09CH11466.