Flow-shear destabilization of multiscale electron turbulence
The impact of sheared ${\mathbf{E} \hskip -1pt \times \hskip -1pt \mathbf{B}}$ 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 ${\mathbf{E} \hskip -1pt \times \hskip -1pt \mathbf{B}}$ 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 $γ_E$ to a novel turbulence destabilization regime at low levels of $γ_E$. In the turbulence stabilization regime, the electron energy flux decreases monotonically with $γ_E$, even when $γ_E$ 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 $γ_E$-modulated nonlinear drive in the zonal ion gyrokinetic equation, increasing the electron transport over a broad range of shearing rates. Finally, neither of these effects has been observed in previous electron-scale simulations.