Impact of divertor closure on the path to complete detachment

Recent experiments on DIII-D have shown that divertor closure has a weak impact on the deeply detached state, where the divertor ion flux is reduced to a small level. Tests used a) a flat, open divertor geometry, b) a flat geometry with nearby baffling, and c) a tightly baffled geometry. The degree of closure impacts the onset of detachment, with changes of ~25-35% in the line-averaged density at which roll-over of the divertor ion saturation current (I_sat) is observed, consistent with previous results. When the divertor is pushed into deep detachment, the different divertor configurations are observed to behave similarly, with strong radiation localized near the X-point, highly reduced divertor ion flux profiles, and high divertor neutral pressure. All configurations also show a reduction in confinement (~20%) when deeply detached, although confinement is higher prior to detachment with more closed divertors. The more closed divertors show increased ratio of divertor to midplane to neutral pressure, indicating that closure can aid pumping even in highly detached conditions. The deeply detached state was not observed when the toroidal magnetic field was reversed, with back-transitions out of H-mode observed prior to strong I_sat reduction or clearly localized X-point radiation.