Role of Reynolds Stress-Induced Poloidal Flow in Triggering the Transition to Improved Ohmic Confinement on the HT-6M Tokamak

Time and space resolved measurements of electrostatic Reynolds stress, radial electric field $E_r$, and plasma rotations have been performed across the transition to improved Ohmic confinement in the Hefei Tokamak-6M (HT-6M). The first experimental evidence of the correlation between the enhanced Reynolds stress gradient and the poloidal flow acceleration in the edge plasma is presented. The results indicate that the turbulence-induced Reynolds stress might be the dominant mechanism to create the sheared poloidal flow and $E_r$, which may further trigger the transition.