The Physics of Zonal Flow-Drift Wave Turbulence Interactions: A Synthesis of Time-domain, Fourier Domain, and Direct Visualization Studies

The nonlinear interaction between drift turbulence and zonal flows plays a key role in determining the turbulence amplitude, spatiotemporal scale and subsequent transport and is therefore of significant interest in magnetic fusion research. We summarize multipoint time-domain probe studies, frequency-domain studies, and direct visualization studies of this important process. The fluctuation-induced Reynolds stress is shown to be consistent with the observed shear flow; spatially resolved direct measurement of nonlinear exchange of energy between drift fluctuations and zonal flows and direct imaging show how the drift instability generates isotropic vortex structures which then propagate outwards towards the zonal flow shear layer, where they are stretched and thinned, and then incorporated into the shear layer, thereby maintaining it against damping mechanisms. The drift fluctuation-zonal flow nonlinear interactions, shearing rate, and cross-field transport are consistent with a critical gradient transport model.