Transfer and Deposition of Carbon- and Nitrogen-containing Radicals and Ions Produced by Glow Discharge in Hydrogen/Methane/Nitrogen Mixtures

摘要

Transfer and deposition of the decomposition products of a dc glow discharge with hollow cathode in a hydrogen/methane flow are discussed. Hydrocarbon (a-C:H) films were deposited at inner surfaces of the hollow cathode and quartz tube bounded the discharge. The carbon deposition rate in the hollow cathode was 3-10 times higher than in the positive column and in afterglow range. Acetylene, ethylene, and ethane prevailed in the plasma-reaction gaseous products. When molecular nitrogen was introduced to the near-cathode plasma at a flow rate equal to or higher than that of methane, the carbon deposition rate at the cathode inner surfaces dropped down by 1.5 to 2 orders of magnitude. The carbon that has been consumed in the a-C:H films formation in the cathode is now transformed mainly to hydrogen cyanide (HCN). The carbon deposition rate in the positive column and in afterglow range (T = 300 K) were comparable with those in the H_2/CH_4 mixture. In the a-C/N:H deposits, the nitrogen concentration was 15 to 25 at percent. The experiments showed that nitrogen can be used as a component of the gas flow specially introduced near the mirror in protecting the optical surfaces in diagnostic channels of the ITER tokamak against a-C:H film deposition.