Modeling of hydrocarbon film deposition suppression in tokamak diagnostic ports

Abstract

Because of the requirement of eliminating carbon deposition on mirrors that facilitate optical methods of plasma diagnostics in thermonuclear synthesis devices, the experimental study has been carried out of gasodynamic conditions and the relationship between the partial flows of hydrogen and methane, which are supplied to the direct current glow discharge. The formation of hydrocarbon deposits would be completely suppressed under these conditions. The processes of methane conversion in the discharge into heavier volatile hydrocarbons have been analyzed by mass spectroscopy and conversion into polymer-like a-C:H films have been analyzing by weighing and electron probe microanalysis. The degree of methane conversion was continuously decreasing upon the dilution of methane with hydrogen. The deposition was suppressed completely when methane concentration in the laminar flow of the mixture was approximately 1 mol %. The process of deposition of a-C:H films changed into the process of film erosion when methane content in the mixture was below 0.5 mol %, the Knudsen number Kn ≈ 0.1, and the Reynolds number Re ≈ 2. In order to provide the removal of plasmolysis products from the surface of the mirror located in a special diagnostic port of ITER, the specified gasodynamic conditions created by the carrier gas (hydrogen or deuterium) should be ensured, with the component of the velocity vector being directed at an angle to the surface under protection.