Turbulence suppression in the neighbourhood of a minimum-q surface due to zonal flow modification in reversed shear tokamaks

Zonal flow behaviour and its effect on turbulent transport in tokamak plasmas with reversed magnetic shear are investigated by global fluid simulations of electrostatic ion temperature gradient driven turbulence. It is found that for high q (safety factor) turbulent transport is high even at a minimum-q region because oscillatory zonal flows called geodesic acoustic modes (GAMs) are dominant. The turbulent transport is reduced in the neighbourhood of a minimum-q surface when q is low enough to damp the GAMs. The difference in zonal flow behaviour causes a difference in turbulent transport and may trigger the formation of ion internal transport barriers in reversed shear tokamaks