Boronization with tungsten plasma-facing surfaces in ASDEX Upgrade

With the switch to tungsten for the ITER plasma facing components, wall conditioning techniques such as boronization are gaining new interest. As AUG uses tungsten plasma facing components as well as boronization, the present results are summarized and reviewed in this paper. In AUG it has been shown how to operate without boronization, but conditioning by B coating through a glow discharge (boronization) is now the standard start-up procedure. The properties of the amorphous boron hydride layers, produced by boronization, depend on the discharge conditions and the residual carbon content. At AUG, chemically active layers are required to getter oxygen and deuterium. After initial conditioning, further boronization is only required for some special scenarios. The lifetime of these subsequent coatings is about 20–30 discharges, after which a new coating or a boron powder injection is required to refresh the conditioning for these special scenarios. The main effect of boronization is to reduce tungsten sputtering from the main chamber limiter by reducing impurities. As the layers are chemically active, they react with air during venting, producing unstable whitish layers that are slowly vanishing. Specifically, they cannot be studied by scanning electron microscopy or ion beam analysis, because under vacuum conditions they are lost even faster. The remaining layers are long-term stable, but contain on average only 4.1 % of the boron injected during the boronizations. This remobilisation reduces the deuterium inventory in the layers to about 0.1 % of the input during the campaigns.