Impact of closure on the deeply detached divertor in DIII-D

Experiments on DIII-D have shown that divertor closure has only a weak impact on the deeply detached state where the divertor ion flux is reduced to a small level, including the associated core performance. While closure has been shown to impact detachment onset conditions, it is unclear that it will have a strong effect under strong detachment as required for wall armor integrity in magnetic fusion devices beyond ITER. This has been tested in DIII-D, where intrinsic carbon is the primary radiator, through fueling scans in a) an open divertor geometry, b) a moderately closed geometry with nearby baffling, and c) a tightly baffled geometry. As in previous studies, closure is found to impact the onset of detachment, with changes of ~25-35% in the line-averaged density at which roll-over of the divertor ion saturation current is observed. However, when the divertor is pushed into deep detachment at high density, all configurations behave similarly, with strong radiation localized near the X-point, highly reduced divertor ion flux profiles, high divertor neutral pressure, and degraded global confinement. The more closed divertors show an increased ratio of divertor to midplane neutral pressure, indicating that closure can aid pumping even in highly detached conditions. In all cases high divertor neutral compression was maintained with deep detachment, with a transition out of H-mode providing the practical limit to detachment. The pedestal pressure is degraded as the density increases towards deep detachment regardless of closure, consistent with a ballooning-limited pedestal as indicated by stability calculations. All configurations show a reduction in core confinement (~20%) when deeply detached, although confinement is higher prior to detachment with more closed divertors.