Evolution of edge turbulence and shear flow approaching the density limit in L-mode plasmas on EAST

Abstract The evolution of edge turbulence and shear flow approaching the density limit in L-mode plasmas on EAST is investigated. The relative fluctuation, cross-correlation power spectral density (CPSD), wave-number spectra, skewness and kurtosis, along with the edge poloidal flow velocity, are calculated using the Lithium Beam Emission Spectroscopy diagnostic. It is observed that there is a noticeable transition in edge turbulence with the plasma density increase. Once the plasma density (Greenwald fraction, f GW ) reaches a certain threshold, the turbulence intensity significantly increases, the turbulence propagation direction noticeably changes, the average turbulence scale significantly expands, and the turbulence intermittency enhances. These density thresholds for the turbulence transition in the pedestal and the scrape-off layer (SOL) regions are approximately f GW = 0.4 and 0.5, respectively, i.e. the transition of SOL turbulence lags that of the pedestal turbulence. The density and temperature profiles obtained through microwave reflectometry and Thomson scattering show an enhancement in outward transport as the edge turbulence undergoes a transition. The turbulence parameter α t , separatrix parameter ( T e − n e ), and pressure decay lengths are compared to those in the Separatrix Operational Space (SepOS) model, showing good agreement with the SepOS model. It suggests that the edge turbulence transition may be related to the transition from drift waves to interchange instabilities. In addition, it is observed that there is also a transition in edge shear flow during the density ramp, and the transition of the edge shear flow in the SOL also lags that in the pedestal, suggesting the evolution of edge turbulence is highly correlated with that of the edge shear flow as the density increases. Two possible mechanisms for these results are discussed: the transition and enhancement of edge turbulence may be caused by the increase of edge collisionality or the decrease of edge shear flow.