Velocimetry of edge turbulence during the dithering L-H transition with dynamic programming based time-delay estimation technique in the EAST superconducting tokamak

A dual gas puff imaging (GPI) system has been successfully assembled on the Experimental Advanced Superconducting Tokamak (EAST) and applied for the study of the dithering L-H transition in the 2012 spring campaign. A new method i.e., the dynamic programming based time-delay estimation technique, has been applied to the 64 x 64 pixels GPI video images to yield time-dependent two-dimensional velocity fields at the plasma edge on EAST. Local poloidal flow velocities up to similar to-3 km s(-1) (in the electron diamagnetic direction) and radial flow velocities up to similar to-2 km s(-1) (inward) are found inside the separatrix during a dithering burst. The radial and poloidal cross-correlation length, flow velocity and auto-correlation time inside the separatrix increase preceding the dithering burst and decrease following the dithering burst. These observations provide strong evidence for the shear flows playing an important role during the dithering L-H transition.