The Dynamics of Turbulence and Flow During the L-H Transition

Comprehensive 2D turbulence and flow measurements are obtained before, during and after the L-H transition. An ion gyro-radius and density scan was performed on DIII-D to investigate the physics of the L-H transition and the threshold dependence on $B_T$ and $n_e$. The amplitude of long wavelength density fluctuations scale approximately with $\rho^*$. Stronger poloidal turbulence flow shear is found at low density. The poloidal turbulence velocity spectrum changes from GAM at $\sim$300$\,$ms before the L-H transition to zonal flows near the transition. About 100$\, \mu$s before the transition, the Reynolds stress gradient, inferred from 2D BES velocimetry, increases rapidly. The poloidal velocity and shear likewise peak near the same time, consistent with the prediction that Reynolds stress drives a zonal flow that triggers the L-H. The turbulence and shear dynamics leading to a typical L-H transition will also be compared with limit-cycle-oscillations between L- and H-modes observed under certain discharge conditions.