Consequences of the temperature and flux dependent sticking coefficient of methyl radicals for nuclear fusion experiments

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Re-deposition of hydrocarbon compounds is a main cause for the retention of hydrogen isotopes in present day nuclear fusion devices. The formation of hydrocarbon films has been observed even in very remote regions of several fusion experiments. This observation can only be explained by the transport of neutral hydrocarbon radicals with finite sticking coefficient. Recently, it was found that methyl radicals have a sticking probability in the range of 10−4–10−5 over a wide temperature range from room temperature to 700 K. As a surprising result, it was also found that at ∼600 K the sticking coefficient becomes even negative, i.e. the room temperature growth precursor becomes an etching species. From model calculations, we extrapolate these findings from the conditions of the laboratory experiment to methyl fluxes relevant for present or future nuclear fusion devices. Consequences for possible techniques to avoid/enhance hydrocarbon radical sticking will be discussed.