Effects of divertor closure and X-point height on L-H transition

Divertor configuration and X-point height have a strong effect on the L-H transition power threshold (P_LH) on multiple tokamaks. In a dedicated experiment on DIII-D, the L-H transition was investigated with both Small Angle Slot (SAS) divertor and closed divertor. The P_LH are found to be below 2MW across a wide density range from 1.5-4x10¹³cm^-3, which is about 20-50% lower than the previously observed P_LH in an open divertor. At a density of ~4x10¹³cm^-3 P_LH with closed divertor is larger than the P_LH with SAS divertor, but at densities below 3x10¹³cm^-3 it is similar. It is also found that P_LH is reduced by nearly 30% when the divertor leg length (DLL) is reduced by ~5 cm, which indicates stronger geometry and neutral effects on P_LH than previously observed in an open divertor. Long wave-length density fluctuations and flow dynamics are measured by both BES and DBS diagnostics. Higher turbulence amplitude is observed with longer DLL. Turbulence characteristics and flow dynamics will be presented. The results will improve our understanding of the underlying physics of divertor configuration effects on H-mode access.