Nonlinear Gyrokinetic Simulations of Microtearing Modes in NSTX and NSTX-U Plasmas

Understanding and controlling turbulent transport driven by microtearing modes (MTMs) is essential for achieving sustainable fusion in spherical tokamaks. MTMs are electromagnetic instabilities that primarily drive electron heat transport and can significantly impact plasma performance. A comprehensive study of MTM-driven turbulence and transport in NSTX and NSTX-U-like plasmas is presented using nonlinear gyrokinetic simulations with CGYRO [1]. The simulations cover a wide range of plasma conditions—from low- to high-collisionality NSTX discharges—capturing diverse operational regimes and providing insight into the dependence of MTM transport on collisionality, beta, and magnetic shear. To assess predictive capabilities, simulation results are compared with experimental data. The analysis is extended to NSTX-U-like plasmas to examine the scaling of MTM-driven transport in future operational scenarios. These findings have important implications for transport modeling and the development of turbulence suppression strategies, aiding the optimization of spherical tokamak performance.

[1] J. Candy et al., J. Comput. Phys. 324, 73 (2016).