Enhancement of the gradient in the Reynolds stress due to applied shear flow on HBT-EP

A detailed understanding of the interaction between shear flow, which suppresses turbulence, and the generation of flow via the Reynolds stress is critical for elucidating the L-H transition dynamics on magnetic confinement devices. To this end, a systematic analysis of the edge turbulence on HBT-EP reveals that when shear flow is applied via a biasing electrode, the gradient in the Reynolds stress at the last closed flux surface (LCFS) is enhanced. The measurements indicate that under biasing, the Reynolds stress increases in a radially varying manner inside of the LCFS, while the Reynolds stress is reduced outside the LCFS. This reduction stems from the strong suppression of turbulence by the E×B shear flow in the scrape-off layer. The resulting Reynolds force is found to be comparable to the J×B force from the biasing electrode current. The connection between the Reynolds stress enhancement inside the LCFS and the anisotropization of the turbulent eddies due to shear flow will be discussed. The detailed edge turbulence measurements in this study are facilitated by the deployment of a novel rake probe array, which allows for the simultaneous acquisition of spatially and temporally resolved Reynolds stress information.