Surface reactions of hydrocarbon radicals: suppression of the re-deposition in fusion experiments via a divertor liner

The formation of re-deposited layers consisting of hydrocarbon compounds is of major concern for the development of next-step devices, because the hydrogen bound in these layers contributes to tritium retention in a future fusion reactor. This film formation might be controlled by using a liner in the divertor pump duct to trap or transform neutral growth precursors before they deposit in remote areas of the vacuum vessel. For the understanding of film formation in such a liner the knowledge of sticking coefficients of the growth precursors is mandatory. Therefore, experiments based on the cavity technique were performed to measure surface loss probabilities of hydrocarbon radicals. In addition sticking coefficients of methyl radicals were measured directly applying well-characterized quantified radical beams. To simulate film formation in a liner, a test experiment was performed consisting of a stainless steel tube, which was exposed to a low-temperature plasma from acetylene. The variation of the film thickness along the inner surface of this tube after plasma exposure can be described by a flux comprising of different neutral hydrocarbon radical species using the previously determined sticking coefficients. Suggestions for the control of the formation of re-deposited layers are made and a possible design of such a divertor liner is discussed.