Impurity analysis in the W7-AS stellarator after wall conditioning by carbonization and boronization

In 15 viewgraph pages the analysis of the effects of wall coating on impurity transport in Wendelstein 7-AS is described. In addition to the transport study the coating procedure is represented. Also, machine-specific aspects with respect to the boundary magnetic field topology and limiter action as well as the impurity radiation and deposition probe analysis are mentioned. The conclusions are as follows: (i) carbon, oxygen, iron, and titanium (the latter from TiC coated limiters) are dominant impurities; (ii) carbonization of the vessel reduces the light impurities only transiently (for a few tens of shots) while iron is more persistently suppressed; (iii) boronization durably suppresses carbon and oxygen influx due to active oxygen gettering and break of the CO desorption cycle; (iv) large titanium and iron fluxes after vessel coating are due to enhanced sputtering and ''footprints'' of localized boundary plasma fluxes after fast release of coatings; (v) impurity confinement times from aluminium laser blow-off into ECH discharges are typically one order of magnitude longer than the respective confinement times. As a consequence of this analysis the titanium carbide coated limiter tiles are being replaced by bulk boronized graphite tiles. 7 figs, 2 tabs