Evolution and cleaning of the deposit layers on the lower divertor of WEST fully equipped with ITER grade components

Abstract

After 3 h of accumulated time from repeated plasma shots during C7 campaign performed in 2023, a deposited layer appeared on the ITER-grade W-monoblock of the lower divertor of WEST, mostly on the high field side. The growth of the deposit was observed during the campaign using infrared cameras, showing a large increase of the area covered by the deposit (x4) in the last two hours of cumulated plasma time. The deposit becomes problematic for the operation as it generates flakes which provoke radiative collapse when entering the plasma. A cleaning of the lower divertor is mandatory. A first cleaning was done using adhesive tape to remove all weakly adhered parts of the deposit. This method was chosen because it was easy to implement and did not generate dusts inside the tokamak. The cleaning enables partial removal of the more lightly adhered deposits but a large fraction remains stuck on the monoblock. A second cleaning was tried during 2024 operation by using the plasma as cleaner. A scenario was developed to put the inner strike line directly on the deposit to heat it and try to remove it by thermal stress. The deposit reaches temperature up to 1560°C but was not removed. The impurities generated were higher than normal operation and decreased during the cleaning session (−50% of light impurities observed at the end of the cleaning discharge session), showing an effect of cleaning by removing impurities from the deposit.