Dynamics of oscillatory plasma flows prior to the H-mode in the HL-2A tokamak

Abstract

This paper discusses edge oscillatory plasma flows, geodesic acoustic mode (GAM) and limit cycle oscillations (LCOs), which have been measured by Doppler reflectometry prior to the high confinement mode (H-mode) in the HL-2A tokamak. The complex relations between the flows and background turbulence have been analyzed. It was observed that the GAM and LCO coexist, and these two flows and turbulence have strong nonlinear interactions during the intermediate confinement phase (I-phase). Dynamics of the shear flows and turbulence prior to the H-mode shows that the oscillatory flows quench the turbulence along with the increase of the mean E × B flow at the early stage of the I-phase, then the oscillatory flows are damped and the further increased mean flow takes over the role in turbulence suppression. The reduced turbulent transport results in the formation of a steep edge transport barrier. It suggests that the oscillatory flows can initiate the L–H transition through providing a positive feedback for the increase of the mean E × B flow strength.