A-C:H film removal in H₂ and H₂/N₂O glow and afterglow discharge

In ITER project CFC materials are proposed as divertor targets for exhaust D/T plasma. The main advantage of the CFC materials is an ability to take a high heat load and the serious shortcoming is chemical erosion susceptibility under interaction with hydrogen plasma when hydrocarbon molecules and radicals are formed. Carbon and hydrogen isotopes joint deposition may be a principal mechanism of 'dead' tritium retention. Thereupon, it is necessary to develop both methods for chemical erosion decreasing of carbon materials and techniques for T-C film removing. The aim of the work is to develop an effective gas-discharge technology for removal of hydrocarbon deposits from remote parts of divertor plasma. The experiments have been shown that the main a-C:H film gasification product is methane in the afterglow region (hydrogen discharge). In film contact directly with the discharge plasma, C₂H_x and C₃H_x hydrocarbons appeared in the erosion products. The hard a-C:H film erosion rate in the afterglow region decreased exponentially with distance from the atomic hydrogen source. The disadvantage of the hydrogen discharge is low erosion yields at 320 - 420 K (Y ∼ 0.001 at.C/at.H) in the remote parts from divertor plasma. For efficiency enhancement of hydrocarbon film removal the H₂/N₂O mixture containing oxygen component has been proposed. In the indicated medium for removal of the hard a-C:H film (1 μ m thick) located in the afterglow region, i.e., in the shadow of plasma, it needed about an hour at 420 - 500 K and N₂O partial pressures of 3 - 24 Pa. The technique suggested is one of possible variants of an effective gas-discharge technology for hydrocarbon deposit removal from the remote parts of ITER divertor plasma. However, for using of plasma cleaning (on the base of H₂/N₂O mixture) of large machines from carbon films redeposited it is necessary to carry out a systematic investigation of possible side effects connected with oxidizing and oxihydrogenating of divertor material surface layers. (author)