Experimental study of density gradient-driven micro-instabilities and the confinement degradation during H-mode in EAST

A broadband (BB) mode is observed by collective Thomson scattering diagnostics in repeatable shots of EAST and analyzed for the first time. This BB mode usually grows during L-H transitions, featuring a BB quasi-coherent mode with increasing frequency. During H-mode operations, it is characterized by steady-state BB in the high-frequency range (f similar to 200-2000 kHz), at the electron scale (k(theta rho s) = 1-2), mainly driven by the density gradient, and is sensitive to the value of eta(e) in the region of interest (rho = 0.4-0.8), where eta(e) = (R/L-Te)/(R/L-ne) is the ratio of the normalized electron temperature gradient and density gradient, and the region rho = 0.4-0.8 usually has a relatively low collisionality (v(eff) < 5). The frequency of BB is found to be dependent on the electron temperature and density gradient, which is a typical feature of electron-driven turbulence. A negative correlation between the energy confinement and the intensity of the BB turbulence during H-mode has been found, which indicates a strong electron thermal transport induced by the BB turbulence. The BB significantly decreases the electron temperature and causes flatter electron temperature profiles in the region of interest (rho = 0.4-0.8), thus making eta(e) decrease and the BB destabilize further. These characteristics of BB are related to the theoretical density gradient-driven trapped electron mode. It should be noted that this mode is not observed by other diagnostics in EAST, and shows very different features to the coherent modes in the edge.