Description
With the recent ITER rebaseline decision introducing a full-tungsten first wall configuration, the efficacy of wall conditioning via boronisation has become a high priority issue in the ITER operational planning. ASDEX Upgrade (AUG), equipped with tungsten (W) plasma facing components and using deuterated diborane, has the unique capability to investigate the main issues arising for ITER: These include the influence of boronisation on plasma start up, homogeneity of the boron coating, lifetime of boronisation and tritium inventory in the boron hydride layers.
The ITER plasma start-up scenario was adapted to AUG, matching all relevant plasma parameters. First experiments without boronisation did not lead to stable plasma conditions. Applying even a partial boronisation by intentionally using only two of the installed four glow anodes immediately allowed reliable plasma start-ups and stable discharges.
Quartz microbalance sensors installed at seven different locations and a manipulator system to expose witness samples offer investigations of layers produced by the coating. The layer thickness at different locations varies only by a factor of three. Switching off one anode reduces the nearby deposition by only about 60 % as opposed to a factor ~100 reduction predicted by initial simulations carried out by ITER. The discrepancy is attributed to the sticking of borane radicals, which was measured to be ~30% while the simulations assumed 100% sticking. The in-situ measurements were complemented by 54 long term samples exposed during the entire campaign. For typical plasma edge temperatures at AUG, W is sputtered mainly by light residual impurity species. These are reduced by boronisation and by this also the W source. The deposited layers on the long-term samples were analysed by ion beam techniques to get information on the elementary composition. A typical D/B ratio of 0.3 is found. This yields a T inventory of less than 2 gr T per boronisation in ITER.