Studies of film formation and erosion by hydrocarbon injection at the plasma edge of TJ-II

Injection of methane and ethylene at the plasma edge of the TJ-II stellarator has been performed by using an instrumented, mobile limiter. The insertion of the limiter into the plasma leads to enhanced carbon contamination due to the local deposition of hydrocarbon films. The deposition characteristics of these films, their removal by plasma erosion and the spectroscopic signatures of such erosion have been recorded. The relative erosion efficiency by locally injected hydrogen and plasma particles has been compared. The relative yields of photon emission during the deposition (CH, Hα) and erosion (CH) for the different species used is analysed and compared to pure H injection. It is found that ethylene yields only a 15% of Hα photons per injected molecular H atom compared to H2 fuelling. Also, the ratio of CH/Hα photons delivered upon molecular cracking at the edge is a factor of three higher for ethylene than for methane.