REDEPOSITION OF SiC - COATED GRAPHITE LIMITER IN ALCATOR C

SiC-coated graphite limiter material was eroded by a plasma during the plasma edge experiments in Alcator C, and redeposited on the molybdenum surface of the Janus probe. Surface morphology, composition and structure of the deposit was examined after a total exposure time of ~ 1000 seconds. The redeposited material typically showed a "layered" morphology. AES depth profiles revealed increased oxygen content in the redeposited material and an apparent diffusion of Si and C at the deposit-molybdenum interface. Two clearly distinguishable chemical states of silicon in the deposit, as observed by XPS, suggest that plasma ionized polycrystalline SiC is being redeposited as a mixture of SiC and SiO.. Preliminary examination of the near surface regions by SIMS indicated (a) the presence of deuterium in the deposit, (b) the presence of molybdenum in the first few layers of the deposit, (c) formation by the plasma of multiple states of Si. O, SifC,, and Si,,C.O. which do not appear in the spectrum of the unexposed SiC.