Study of the role of divertor neutrals and SOL turbulence in the density shoulder formation and evolution in ASDEX Upgrade

The formation of flat density profile in the low field side Scrape Off Layer (SOL), the so called shoulder formation, is characterized in the ASDEX Upgrade tokamak. The two main candidate mechanisms for its formation, i.e. modification of SOL turbulence and divertor plasma-neutrals interaction, are studied. Turbulence and blobs are characterised with midplane Langmuir probes; divertor neutrals are estimated measuring the 2D map of Da and Dg emission through the tomographic inversion of their brightness measured by two cameras. The strong interplay between upstream profiles, up-stream turbulence and divertor conditions will be highlighted.In the considered ASDEX Upgrade data set, upstream midplane density shoulder forms when increasing the electron density and it is linked with detachment. The shoulder formation is correlated with the change of edge turbulent transport, resulting in an increase of the blobs perpendicular size, and also in a change in the neutral emission, reflecting a modification in the neutral distribution in the divertor region. In particular, while the neutral D emission is localized in the inner divertor at low electron density, the emission moves towards the outer divertor target and then covers the whole lower divertor region when the upstream electron density shoulder forms.