Role of Edge Turbulence and Flows in the Density Dependence of the L-H Transition Power Threshold on DIII-D

The trigger mechanism of the L- to H-mode transition is not currently fully understood. Empirical scaling studies of the L-H transition power threshold have discovered global plasma parameter dependences, including a strong density dependence. The current work investigates the potential role of edge turbulence and flows in this density dependence by performing detailed measurements during a density scan experiment on DIII-D. Preliminary analysis indicates that the signatures of geodesic acoustic modes (GAMs) exist in both the perpendicular flow and electron temperature fluctuations ($\tilde{T}_e$) prior to the L-H transition. Both $\tilde{T}_e/T_e$ at the GAM frequency and $\tilde{T}_e/T_e$ of broadband fluctuations are observed to decrease with increasing density. Measurements of density turbulence, $E\times B$ flow, together with linear stability analysis will also be reported.