Nonlocal behaviour of zonal flows in tokamak plasmas

Abstract

Oscillatory zonal flows called geodesic acoustic modes (GAMs) appearing in a high safety factor region of tokamak plasmas show nonlocal behaviour in their frequency. The frequency of the GAMs is unchanged over a certain radial region and the radial variation of the frequency is step-like. The effects of ρ* = ρi/a on the nonlocal behaviour of the GAMs are investigated by global ion temperature gradient driven turbulence simulations, where ρi is an ion Larmor radius and a is a minor radius of a torus. It is found that the radial width in which the GAMs have the same frequency is almost proportional to , while the radial wavelength of the GAMs is proportional to ρi. As for turbulent transport, the boundary between a high transport region where the GAMs are dominant and a low transport region becomes clearer for smaller ρ*.