UCLA

Recent experimental results about the edge shear flow and poloidal residual stress near the density limit of HL-2A tokamak are reported in this paper. The studies found that the plasma response passes from adiabatic to hydrodynamic response as the plasma density approaches Greenwald density, therefore the residual stress, which is the non-diffusive item in the turbulent Reynolds stress, is reduced. The plasma intrinsic poloidal torque characterized by the divergence of residual stress decreases drastically. The reduction of turbulent generation of edge poloidal flow leads to the decreased edge E×B poloidal flow shearing rate. These experimental results demonstrate the important role of reduced residual stress in the edge poloidal shear layer collapse in the proximity of tokamak density limit.