The interplay between Reynolds stress and zonal flows: Direct numerical simulation as a bridge between theory and experiment

We describe the results of a measurement campaign on the CASTOR tokamak where the drive of flows and zonal flows by Reynolds stress was investigated by means of a dual probe head system allowing us to measure the properties of the electrostatic turbulence and the rotation velocities at the same location and at the same moment. We compare these experimental results with a turbulence model linked to a one dimensional fluid model describing the electrostatic turbulence and its influence on the background flow. The turbulence is simulated locally on the basis of the Hasegawa-Wakatani equations, completed with magnetic inhomogeneity terms. In the fluid model the toroidal geometry is correctly taken into account, while various sources and sinks like viscosity, interaction with neutrals, Reynolds stress and electric current induced by biasing are included. The good agreement of the predicted flow with the measured one demonstrates that in a pure cylindrical geometry the modelled strength of Reynolds stress acceleration of flow is overestimated.