Nonlocal properties of gyrokinetic turbulence and the role of E×B flow shear

Abstract

The nonlocal physics associated with turbulent transport is investigated using global gyrokinetic simulations with realistic parameters in shaped tokamak plasmas. This study focuses on the turbulence spreading through a transport barrier characterized by an equilibrium E×B shear layer. It is found that an E×B shear layer with an experimentally relevant level of the shearing rate can significantly reduce, and sometimes even block, turbulence spreading by reducing the spreading extent and speed. This feature represents a new aspect of transport barrier dynamics. The key quantity in this process is identified as the local maximum shearing rate ∣ωEmax∣, rather than the amplitude of the radial electric field. These simulation studies also extend to radially local physics with respect to the saturation of the ion temperature gradient (ITG) instability, and show that the nonlinear toroidal couplings are the dominant k-space activity in the ITG dynamics, which cause energy transfer to longer wavelength damped modes, forming a downshifted toroidal spectrum in the fully developed turbulence regime.