Observation of chemical erosion of carbon based wall materials in the TEXTOR tokamak

Mass spectroscopy and optical spectroscopy have been used to measure the formation of methane, higher hydrocarbons and of CO during the interaction of limiters with the boundary plasma and of special carbon targets with the scrape-off-layer plasma (SOL) of TEXTOR. Mass spectroscopic data are obtained by the Sniffer probe in the SOL under carbon, boronized and siliconized wall conditions. At target temperatures less than or equal to 100 degrees C, methane yields range typically between 0.7 and 1.2%. They vary only little with changing plasma conditions. C-2-hydrocarbon formation dominates the overall carbon erosion under many conditions. Their yields increase with decreasing plasma temperature. Siliconization of the walls reduces the methane formation only little but suppresses the formation of higher hydrocarbons significantly. CO formation is dominated by the actual oxygen impurity fluxes and ranges between 0.2% up to 1.5% depending on the wall conditioning. Supporting data on hydrocarbon and CO formation are obtained from the outgassing after the discharge. Optical spectroscopy has been used to determine methane formation yields from CH band emission in front of graphite test limiters positioned at the last closed flux surface. The yields are typically in the range between 1.5 and 5% and are generally a factor 2-3 higher compared to those from mass spectroscopy. The CH4 formation is nearly constant between 200 degrees C up to 700 degrees C and decreases beyond 800-1000 degrees C. It decreases with increasing flux density. C-2 hydrocarbon emission from the limiters has not been observed by molecular band emission within the range of normal plasma conditions. They show up only for detached plasma conditions.