Impact of divertor closure and X‑point height on the L‑H transition in DIII‑D

The L‑H transition power threshold (P_LH) in a partially closed divertor and with the small angle slot divertor (SAS) on DIII‑D is found to be about half of the P_LH in open divertor plasmas at densities below 2×10¹³ cm^‑3, and 30% lower at higher densities with otherwise similar parameters. P_LH also reduces as the X‑point moves closer to the target plate, i.e. at shorter divertor leg length (DLL). Comprehensive 2D turbulence and flow measurements in the plasma edge reveal that turbulence characteristics, flow dynamics and EΧB drift near divertor are closely related to the P_LH dependencies. As the DLL is reduced, P_LH is observed to reduce by another 30%, while turbulence amplitude correspondingly increases, potentially providing a higher Reynolds stress drive for shear flow. The turbulence poloidal advection velocity from BES measurements increases across the separatrix into the SOL prior to the transition with shorter DLL. Ion saturation current (J_sat) measured by Langmuir probes inside the divertor region shows a significant reduction when the DLL is reduced, suggesting that EΧB drifts near the divertor region may play an important role.