OBSERVATION OF CHEMICAL EROSION OF CARBON-BASED WALL MATERIALS IN THETEXTOR TOKAMAK

Mass spectroscopy and optical spectroscopy have been used to measure t he formation of methane, higher hydrocarbons and of CO during the inte raction of limiters with the boundary plasma and of special carbon tar gets with the scrape-off-layer plasma (SOL) of TEXTOR. Mass spectrosco pic data are obtained by the Sniffer probe in the SOL under carbon, bo ronized and siliconized wall conditions. At target temperatures less t han 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 ma ny conditions. Their yields increase with decreasing plasma temperatur e. Siliconization of the walls reduces the methane formation only litt le but suppresses the formation of higher hydrocarbons significantly. CO formation is dominated by the actual oxygen impurity fluxes and ran ges between 0.2% up to 1.5% depending on the wall conditioning. Suppor ting data on hydrocarbon and CO formation are obtained from the outgas sing after the discharge. Optical spectroscopy has been used to determ ine methane formation yields from CH band emission in front of graphit e 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 fac tor 2-3 higher compared to those from mass spectroscopy. The CH4 forma tion 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 obse rved by molecular band emission within the range of normal plasma cond itions. They show up only for detached plasma conditions.