A. Von Keudell et al., Surface reactions of hydrocarbon radicals: suppression of the re-deposition in fusion experiments via a divertor liner, J NUCL MAT, 290, 2001, pp. 231-237
The formation of re-deposited layers consisting of hydrocarbon compounds is
of major concern for the development of next-step devices, because the hyd
rogen bound in these layers contributes to tritium retention in a future fu
sion reactor. This film formation might be controlled by using a liner in t
he divertor pump duct to trap or transform neutral growth precursors before
they deposit in remote areas of the vacuum vessel. For the understanding o
f film formation in such a liner the knowledge of sticking coefficients of
the growth precursors is mandatory. Therefore, experiments based on the cav
ity technique were performed to measure surface loss probabilities of hydro
carbon 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 consist
ing of a stainless steel tube, which was exposed to a low-temperature plasm
a from acetylene. The Variation of the film thickness along the inner surfa
ce of this tube after plasma exposure can be described by a flux comprising
of different neutral hydrocarbon radical species using the previously dete
rmined sticking coefficients. Suggestions for the control of the formation
of re-deposited layers are made and a possible design of such a divertor li
ner is discussed. (C) 2001 Elsevier Science B.V. All rights reserved.