The physics of E x B shear, zonal flows and streamers in tokamak edge turbulence

The role of E x B flow shear in reducing turbulent transport in the edge region of a tokamak is investigated using 3D nonlinear global simulations. Transport reduction is found to be due to both a decrease of fluctuation amplitudes as well as a change in their relative phase. A scaling of the turbulent diffusivity with E x B flow shear has been deduced from these simulations. This scaling agrees with recent experimental observations as well as analytical models. Moreover these simulations allow us to examine the dynamics of anisotropic secondary flows such as zonal flows and streamers. These flows are found to regulate intermittent transport observed in flux driven simulations. (orig.)