Laboratory study of the transport and condensation of hydrocarbon radicals and its consequences for mitigating the tritium inventory in the ITER-FEAT divertor

The surface loss probability β of CH3 radicals at a-C:H surfaces has been determined by analysis of the carbon deposition profiles along a tube flow reactor directly coupled with a methane RF discharge. β is equal to (1.0±0.2)×10−3 for a methyl to atomic hydrogen flux ratio of 10:1. β remains the same in the temperature range 300–800 K and decreases slightly between 800 and 1200 K. The deposition rate drops drastically in the range 400–800 K. Above 800 K the carbon deposition is regained. From separate experiments on magnetron sputtering of graphite by D ions, it is inferred that at 300 and 900 kinetic CHx species react at the C:D surface with the same probability. In contrast, the deposition of thermal CxHy radicals decreases by a factor of 25 with a temperature rise from 320 to 400 K, remaining at this low level up to 1000 K. The implications of these results for co-deposition in the ITER-FEAT divertor are discussed.