E×B shear flows and electromagnetic gyrofluid turbulence

Abstract

Low frequency tokamak edge turbulence is modelled numerically using gyrofluid equations for electrons and ions on an equal footing. The electrons are electromagnetic, and arbitrarily strong finite gyroradius effects are included for the ions. Computations are in a globally consistent truncation of flux surface geometry arising from ideal tokamak equilibria. The turbulence is similar to that in the fluid model in steep gradient regimes, for which the electron transit frequency is comparable to that of the turbulence. The nonlinear drift wave instability is shown to be caused by E×B self-advection, and is similar for both two-and three-dimensional models. The turbulence always has drift wave mode character for the parameter regime of interest, except when the ideal ballooning threshold is reached. Turbulence interacts strongly with E×B shear flows, but does not build the flow shear to significant levels by itself. On the other hand, an imposed shear layer arising from the neoclassical equilibrium of the edge region does have the necessary properties and scaling to eventually result in a credible edge transport transition scenario.