The Dynamics of Turbulence and Shear Flow Approaching the L-H Transition

Comprehensive 2D turbulence and turbulent flow measurements have been obtained before, during, and after the L-H transition during an ion gyro-radius scan in DIII-D to understand if the strong threshold dependence on $B_T$ is connected to turbulence behavior. Other non-dimensional parameters ($\nu^*$, $q_{95}$, $\beta$) were kept nearly constant at the pedestal top. The amplitude of long wavelength density fluctuations, measured with the 2D BES array, is found to scale approximately with $\rho^*$. A mean shear flow layer is observed near $r/a \sim 0.92$ with a shearing rate exceeding the local turbulence decorrelation rate. Velocimetry shows that the GAM, which peaks near $r/a\sim 0.9$, appears a few hundred ms before the L-H transition, and decays in amplitude approaching the transition, while a lower-frequency flow structure increases in amplitude during this period. New measurements of the density dependence of the turbulence-zonal flow system will also be presented.