MOLECULAR IMPURITIES IN ASDEX UPGRADE PLASMA DISCHARGES

The introduction presents a historical review of the role of molecules in tokamak research starting from the first installations at the Kurchatov Institute. Molecular impurities were mostly considered as a transient conditioning problem, but with the use of carbon for wall and limiter elements, it was perpetuated. New results about the elementary processes involved in hydrogenic carbon erosion are reported and the existing data base is briefly discussed. Results from mass spectrometry are presented as well. as data from optical spectroscopy including determination of CD4 and CD fluxes from molecular band intensities. A typical yield of about 5% for hydrogenic chemical erosion is obtained. In combination, all these results show the impact of hydrogenic carbon erosion. They strongly suggest that with boronized walls it remains as the dominating process for the carbon fluxes in the SOL and even dominates the carbon concentration in the central plasma in spite of a high SOL screening action for hydrocarbons.