Volume recombination in divertor I of ASDEX Upgrade

High density operation in the ASDEX Upgrade divertor I with horizontal target plates is reported. Density rampup experiments were carried out to characterize detached plasma conditions in the divertor. During the detached phases, hydrogen continua and spectral line emission from high-n shells were observed in the divertor due to the volume recombination. The spectroscopic measurements provide a consistent picture of the evolution of the divertor plasma parameters during the density ramp. By means of the ADAS atomic physics program package, the rate of volume recombination was evaluated, including the effect of opacity. The relative importance of volume recombination in comparison with the target plasma sink is discussed. Observations indicating differences in volume recombination between the two divertor legs are presented, and the connection of volume recombination to divertor detachment is addressed.