Methyl radical chemical reactions on a tungsten surface

Abstract

In this paper an interaction of the inner dome surfaces of the International Thermonuclear Experimental Reactor with a gas mixture containing hydrocarbon radicals has been modelled. To study the processes of transport and deposition of methyl radicals in a C x H y -H 2 -H (x = 1−3, y = 1−6) flow, a stream technique has been used. A radiofrequency inductive discharge initiated in methane at the inlet of a quartz tube was a source of methyl and hydrogen atoms. Methyl radicals were transported by a gas flow that passed through cylindrical tungsten inserts and were deposited on the tube wall, on silicon samples and on tungsten samples, forming polymer-like hydrogenated amorphous carbon films. It has been shown that at the tungsten insert temperature increases to 420 K, and carbon deposition rate decreased significantly. However, in the gaseous phase the secondary hydrocarbon species accumulated and condensed on the tungsten surface at a temperature lower than 340 K.

Progress in the development of deposition prevention and cleaning techniques of in-vessel optics in ITER

“…-Deposition suppression is observed for low-energy CH-radicals. In the case of contacts with ions of 30-100 eV the heated surfaces are contaminated only slightly slower than without heating [1].…”

“…Heating: Hydrocarbon deposition prevention technique utilizing the temperature of ~200 ºС, at which chemical erosion of carbon films by hydrogen atoms prevail over deposition [5] was successfully tested in modern tokamaks [6] and laboratory experiments [7]. The technique is very promising, but for a successful operation demands the following be considered: -Thin (1-10 nm) hydrocarbon-containing films [1] generated at the initial stage of the mirror operation (even without long-term hydrocarbon deposition), modify the reflective mirror characteristics. Then, the method has to be used in parallel with a reduction of partial density of hydrocarbons in the vicinity of the heated optical surfaces (see below).…”

“…-Thermal gradients along protected surface activate thermal diffusion [1] of secondary radicals (formed on the surface but not condensed) directed to the area of high temperature.…”

Laboratory Experiments on Modeling of Transfer and Deposition of Hydrocarbons in Tokamak Main Chamber Areas Shadowed From Plasma

“…Отсутствие осаждения угле-рода на кварце и кремнии имело место в прогреваемой секции трубы с температурой стенки 380-800 К. При дальнейшем повышении температуры прогреваемой секции (800-1000 К) процесс осаждения уг-лерода восстанавливался, а коэффициент s приближался к значению 10 -4 . Температурный интервал 380-800 К, в котором отсутствовало осаждение а-С:Н-плёнок, была на-звана «температурным транспортным окном» (ТТО) [20].…”

“…7). Этот факт указывал на возрастание «эффективного» коэффициента рекомбинации  метила на поверхности стали [20]. Радикалы полностью выводились из откачиваемой смеси (СН 4 /С 2 Н y /Н 2 ) за несколько сотен соуда- .…”