Neutral density estimation in the ASDEX upgrade divertor from deuterium emissivity measurements during detachment and shoulder formation

Neutral density distribution in the divertor region of ASDEX Upgrade is estimated in different discharge conditions. The 2D divertor emission of D-alpha and D-gamma is obtained with a tomographic inversion of their brightness measured by two cameras. From these emissions, 2D maps of neutrals density n(0) can be obtained by a simple model that takes into account both excitation and recombination processes populating deuterium n = 3 and n = 5 levels. In attached discharges, the neutral density n(0) is larger in the inner divertor, and when increasing core n(e) its maximum moves toward the dome and the outer divertor region. In particular, the modification of the n(0) distribution is studied in plasmas where the mean electron density in the core increases, moving from an attached to a detached phase. In these discharges, the outboard midplane scrape off layer electron density increases and its profile flattens, with the formation of the so called density shoulder. The modification of the neutral density is followed during this evolution, showing that both the shape and the absolute value of the n(0) distribution change in time, suggesting that they play a role in the two phenomena of shoulder formation and detachment. Moreover, in the detached phase, also the recombination contributes to the neutral deuterium emission.