Connecting Microscopic L-H Transition Physics to the Power Threshold

A physics-based model of the L-H transition power threshold $P_{th}$ is needed to confidently extrapolate auxiliary heating requirements for burning plasmas. The goal of this work is to link differences in the microscopic turbulence-flow interaction to the macroscopic power threshold density and isotope scaling. The turbulence-driven poloidal ion flow is found decisive for initial turbulence suppression, with a Reynolds stress gradient (evaluated from BES data) sufficiently large to account for the measured poloidal flow acceleration. The turbulence-flow energy transfer rate $P_E=< \tilde{v}_r\tilde{v}_q>\partial (v_q + v_{dia})$ / $\partial r$ depends on the L-mode seed flow shear, which shows a similar density dependence than $P_{th}$ (increasing below the density $n_{min}$ where $P_{th}$ has a minimum, and increasing above $n_{min}$). Differences in turbulence properties, and lower L-mode diamagnetic seed flow shear are found in hydrogen plasmas compared to deuterium plasmas, supporting the experimentally observed $P_{th}$ isotope scaling.