Implication of a shock front in the X-point region of ASDEX Upgrade on the evolution of divertor detachment

Existing numerical fluid transport code packages containing an as complete as possible model of our current understanding of the volumetric processes in the divertor are not able to qualitatively reproduce the experimental observations in the high field side, HFS, divertor of ASDEX Upgrade during detachment. In contrast to numerical models it is observed experimentally that as the ion flux density to the inner divertor drops a region of high density $is formed in the X-point region of the far Scrape-Off Layer, SOL, of the HFS divertor extending away from the separatrix. In the X-point region supersonic flows are theoretically predicted and measured experimentally. In a Gedankenexperiment we compare the critical conditions for the onset and offset of a shock front and its implications for the spatial change of the recycling along the divertor target. If the poloidal temperature gradient was of the order of the neutrals mean free path then a shock front could form close to the X-point. We compare the implications to the experimental dependencies under which the high density in the far SOL of the HFS X-point region appears.