Interplay between turbulence, neoclassical and zonal flows during the transition from low to high confinement mode at ASDEX Upgrade

The spatio-temporal interplay between turbulence, mean and zonal flows has been investigated at the L-H and H-L transitions in the edge region of the ASDEX Upgrade tokamak. Close to both transitions, an intermediate phase (I-phase) characterized by "limit cycle like oscillations" (LCOs) is observed in which periodic bursts of turbulence correlate with E×B flow reduction and relaxation of gradients. During the I-phase, the E×B velocity is dominated by the mean flows indicating that turbulence driven flows are small. Periodic dithers between L-mode and the phases with LCOs are also observed just before the H-mode onset where the edge density and temperature profile gradients evolve on similar timescale as the flows. Thus connection between mean and E×B flows holds during the all evolution from L-mode to H-mode demonstrating the fundamental role of the neoclassical flows in the L-H transition physics.