Post mortem analysis of a tungsten coated tile from the outer divertor strike point region of ASDEX upgrade

In the present study, the structure and the composition of co-deposited layers developed at the outer divertor strike point tile 1 in ASDEX Upgrade during three campaigns from 2009 to 2013 were examined. The samples were cut from representative locations which differed in received flux: private flux, strike point, the highest plasma fluence and “moderate" flux regions. High resolution scanning (SEM) and transmission electron microscopy (HRSTEM) combined with energy-dispersive X-ray spectroscopy (EDS) and optical profilometry have been used to identify deposits composition and morphology as well as to assess coating degradation. The aim of the post-mortem analyses was the evaluation of materials mixing and evidence of plasma-induced damage. Our results confirm that the outer divertor of AUG is generally a net erosion region for tungsten, however, the strong deposition of eroded tungsten takes place close to the outer strike point (private flux region). The observations on cross-sections of the tungsten coatings revealed micro-cracks around the strike point region. These cracks are caused by thermal stresses due to periodic heating and cooling of the layer during ELMs.