Experimental modeling of transport and deposition of hydrocarbon radicals on ITER-FEAT cold trap

The surface loss probability, β, of methyl (CH3) radicals and atomic (H) hydrogen on a growing film surface was measured at lowered temperatures. The loss of reactive species was investigated by analysis of the deposition profiles along a quartz tube reactor directly coupled with a methane RF discharge (so-called ‘stream’ technique). The experiments were performed using special external containers with dry ice (∼200 K) and liquid nitrogen (77 K). It has been found that β(CH3) on the growing film surfaces increases from 1.6×10−4 at 300 K to 6.2×10−4 at 200 K. Based on the data analysis, β for H can be deduced. The β(H) values were 3×10−4, 1×10−3 and at least 10−2 at 300 K, ∼200 K and 77 K, respectively. Film deposition from hydrocarbon radicals generated in a magnetron discharge is studied at temperatures from 300 K down to 120 K. Transition from formation of film-like to drop-like deposit is observed with cooling the substrate from 220 to 120 K. The application of the results to the understanding of transport and deposition of neutral reactive species on ITER-FEAT cold trap have been given.