The generation mechanisms of three-dimensional large-scale coherent structures (LSCSs) along magnetic field lines in the edge plasmas of the HL-2A tokamak are investigated using 10-tip poloidal and 8-tip radial probe arrays toroidally separated by 2100 mm. The LSCSs with radial and poloidal sizes of about (20–30)ρs (hybrid Larmor radius) ∼10–15 mm and a finite parallel wave vector are experimentally observed with high spatial and temporal resolutions. The turbulence energy from 30 to 60 kHz is found to contribute significantly to the growth of the turbulent eddies of the broad spectrum. The latter gradually increase, following an increase in turbulence energy at 30–60 kHz, and then break up into LSCSs with dipolar floating potential fluctuations at the inner side of the last closed flux surface (LCFS), where the Reynolds stress driven E × B flow is strong, the flow shearing rate has a maximum and the time scale is close to the LSCS generation time. The LSCSs are then disconnected by the E × B flow shear, move across the LCFS and enter into the scrape-off layer. The back-reaction of the LSCS on turbulence is also observed and may result in LSCS intermittent behaviours. Thus, the increasing turbulent energy and the spontaneous E × B flow shear are identified to be responsible for the generation of LSCSs, which is in agreement with the theoretical prediction and provides unambiguous experimental evidence for the LSCS generation mechanism in tokamak edge plasmas. The correlation between the sheared flow and Reynolds stress is demonstrated. The evidence for the back-reaction of LSCS on turbulence is also presented.
Edge shear flows and particle transport near the density limit of the HL-2A tokamak