Nonlinear gyrokinetic turbulence simulations of ExB shear quenching of transport

The effects of ExB velocity shear have been investigated in nonliner gyrokinetic turbulence simulations with and without kinetic electrons. The impact of ExB shear stabilization in electrostatic flux-tube simulations is well modeled by a simple quench rule with the turbulent diffusivity scaling like 1-αEγE/γmax, where γE is the ExB shear rate, γmax is maximum linear growth rate without ExB shear, and αE is a multiplier. The quench rule was originally deduced from adiabatic electron ion temperature gradient (ITG) simulations where it was found that αE≅1. The results presented in this paper show that the quench rule also applies in the presence of kinetic electrons for long-wavelength transport down to the ion gyroradius scale. Without parallel velocity shear, the electron and ion transport is quenched near γE/γmax≅2 (αE≅1/2). When the destabilizing effect of parallel velocity shear is included in the simulations, consistent with purely toroidal rotation, the transport may not be completely quenched by any level of ExB shear because the Kelvin-Helmholtz drive increases γmax faster than γE increases. Both ITG turbulence with added trapped electron drive and electron-directed and curvature-driven trapped electron mode turbulence are considered