IMPURITY RELEASE AND DEPOSITION PROCESSES CLOSE TO LIMITER SURFACES IN TEXTOR-94

Measurements on the formation of hydrocarbons on plasma exposed surfac es performed by mass- and optical emission spectroscopy in TEXTOR is r eported. The temperature dependence of hydrocarbon formation and the c ontribution of the hydrocarbon source to the CII ion densities near th e limiter has been observed using a graphite limiter which is external ly heatable up to 1400 K. It has been found that hydrocarbon formation occurs in a broad temperature region decreasing only for target tempe ratures above 1300 K and that hydrocarbons contribute to about 40% to the CII light. Strong methane release has been observed on copper and stainless steel limiters positioned at the LCFS while it is negligible on molybdenum and tungsten limiters under similar plasma edge conditi ons. Local transport and redeposition of molecules have been studied b y gas injection of methane and silane through holes in the limiter sur faces and by local Monte Carlo calculations. Local deposition efficien cies between 4 and 7% have been measured for injected methane and sila ne. Monte Carlo calculations show, in general, a larger redeposition p robability depending only little on local plasma parameters but signif icantly on the assumptions of the sticking and release properties of r edeposited ions and radicals on the surface. For higher surface temper atures possible carbon release by radiation enhanced sublimation (RES) has been investigated. No increase of carbon release could be observe d demonstrating that carbon release from RES is negligible under these conditions. Possible reasons for this behavior are discussed.