Flow-shear destabilization of multiscale electron turbulence

Abstract

The impact of sheared $\exb$ flow on multiscale turbulence is studied with nonlinear gyrokinetic simulations. Simulations are based on DIII-D-like, high-confinement mode (H-mode) pedestal parameters in the regime of low ion temperature gradient drive, where there is a broad spectrum of electron temperature gradient (ETG)-driven turbulence. It is found that $\exb$ shear can have a significant effect on ETG-driven electron transport, with an unexpected transition from a turbulence stabilization regime at moderate to large shearing rates $\gamma_E$ to a novel turbulence destabilization regime at low levels of $\gamma_E$. In the turbulence stabilization regime, the electron energy flux decreases monotonically with $\gamma_E$, even when $\gamma_E$ is small compared to electron mode growth rates. The stabilizing effect comes dominantly from the electron, not ion, gyrokinetic equation. In the novel destabilization regime, reduction of zonal energy results from the interaction of $\gamma_E$-modulated nonlinear drive in the zonal ion gyrokinetic equation, increasing the electron transport over a broad range of shearing rates. Neither of these effects have been observed in previous electron-scale simulations.