Nonlinear gyrokinetic simulations of microtearing modes in NSTX and NSTX-U-like plasmas

Abstract

A systematic nonlinear gyrokinetic analysis of microtearing modes (MTMs) in National Spherical Tokamak eXperiment (NSTX) discharges spanning high-, moderate-, and low-collisionality regimes around mid-radius is presented using the fully electromagnetic code CGYRO. The simulations retain the complete set of perturbed fields, impurity species, equilibrium E→×B→ flow shear, and inter-species collisions, yielding electron heat flux predictions in improved agreement with experimental measurements. In the high-collisionality regime, impurity effects are found to play a critical role in reproducing experimentally observed electron heat flux levels. At moderate-collisionality, enhanced zonal flows and fields modify turbulent structures and reduce transport. In the low-collisionality regime, linear analysis reveals coexistence of MTMs and a hybrid ballooning-parity mode; however, nonlinear simulations demonstrate that strong E→×B→ shear suppresses hybrid-mode-driven transport, leaving ion heat fluxes near neoclassical levels, consistent with experimental observations. A collisionality scan of the high-collisionality case is further performed to assess NSTX-U-relevant conditions, revealing a transition at lower collisionality in which enhanced zonal flows and fields significantly suppress MTM-driven turbulence and electron heat transport. Sensitivity studies with respect to equilibrium flow shear, temperature and density gradients, electron β, safety factor, and magnetic shear indicate that, at the collisionalities anticipated for NSTX-U, MTMs are expected to contribute to electron heat transport without imposing a dominant confinement constraint. These results provide a nonlinear basis for interpreting collisionality trends in spherical tokamaks and for guiding transport predictions in NSTX-U regimes.