Investigations on boronisation in the full-tungsten ASDEX UPGRADE

• Although plasma start-up without Boronisation in ITER-like divertor configuration was not successful, applying Boronisation, even with only half of the anodes active, enabled easy operation. • Reduction in tungsten erosion is not due to covering by boron layers, but due to reduction of oxygen content, which is the dominant ion responsible for sputtering tungsten. • Switching off one anode reduces the deposition by less than a factor of 2, in contrast to simulations, which predicted reduction of 2 to 3 orders of magnitude. • The amount of boron deposited depends on the kind of substrate. For relevant materials, D/B < 0.3 is found. • The deuterium content of layers is strongly reduced after exposure to air. Conditioning of the Plasma-Facing Surfaces (PFSs) in fusion devices is essential for reliable plasma operation. In ASDEX Upgrade (AUG), boronisation by a glow discharge in helium (He) with 10% of deuterated diborane is used as a standard wall conditioning technique. The recent transition of the PFSs in ITER to full tungsten has triggered considerable interest in the wall-conditioning methods employed in present-day full-metal fusion devices. In this paper first results on new investigation at AUG are reported. Although plasma start-up without boronisation in ITER-like divertor configuration was not successful, applying boronisation, even with only half of the anodes active, enabled easy operation. Spectroscopic investigations of limiters indicate that the reduction in tungsten erosion is not due to boron layer coverage, but due to the reduction of oxygen content, which is the dominant ion responsible for sputtering tungsten. Quartz microbalance instruments installed at AUG at different positions allow insitu realtimemeasurement of the deposition for each boronisation,. Switching off one anode reduces the deposition by less than a factor of two, in contrast to simulationsthat predicted strongly localized boron (B) deposition near anodes, with reduction of 2 to 3 orders of magnitude in regions between them.. This discrepancy may arise from the sticking coefficient of the precursor species, a key factor in layer homogeneity, as indicated by the cavity probe. Witness samples of different substrates exposed during coating using a manipulator allow to analyse the layers by ion beam techniques. The amount of boron deposited depends on the kind of substrate. For relevant materials, deuterium (D) to boron (B) ratio, D/B < 0.3 is found, indicating chemically active layers. Silicon and carbon substrates show higher D content specially after exposure to tokamak plasmas. The D content in the layers is strongly reduced after exposure to air.