Characterizing the transition from high recycling to partial detachment

Experiments at DIII-D have explored the transition from the high recycling to the partially detached divertor condition in L- and H-mode with an unprecedented level of detail. Improved divertor and core Thomson scattering diagnostics were coupled with high resolution spectroscopic studies of molecular and neutral emissions. 2-D $T_{e}$ and $n_{e}$ profiles of the outer leg reveal the earliest indications of formation of the detachment front at the target plate, reducing local $T_{e}$ at the outer strike point from 8-10 eV to 2-3 eV with a marginal ($<10\%$) increase in $$ upstream. These data help guide and expose any missing physics in simulations of detachment onset using state-of-the-art boundary codes, and in predictions for operation with a partial detached divertor in future devices.\par