Experimental observation of coupling between turbulence and sheared flows during L-H transitions in a toroidal plasma

The dynamics of turbulence and plasma flows has been studied experimentally by means of Doppler reflectometry during the transition from low to high confinement mode in the stellarator TJ-II. Close to the transition threshold, gradual transitions are achieved showing an intermediate, oscillatory transient phase that facilitates the study of the mechanisms involved in the transition. A coupling between sheared flows and turbulence level is measured which reveals a characteristic predator-prey behavior consistent with the L-H transition models based on turbulence-driven flows.