Nonlinear gyrokinetic turbulence simulations of E×B shear quenching of transport

Abstract

The effects of E×B velocity shear have been investigated in nonliner gyrokinetic turbulence simulations with and without kinetic electrons. The impact of E×B 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 E×B shear rate, γmax is maximum linear growth rate without E×B 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 E×B 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.