Analysis of exhaust gas composition in JT-60U discharge cleaning experiments

The composition of exhaust gas from vacuum vessel in a discharge cleaning is an essential information of the removal characteristics of hydrogen isotopes and is useful data for the design of the fuel cycle system of a fusion reactor. Especially for the hydrogen trapped in carbon layers, removal by a form such as hydrides is important process, so the analysis of chemical species and the measurement of carbon and oxygen degassing are valuable information. Thus, to establish the method of analysis of these chemical species, the concentrations and the time-dependent behaviors of chemical species such as hydrogen, hydrocarbons and other chemical species were analyzed using a gas chromatograph in JT-60U discharge cleaning experiments. In the experiments, glow discharge cleaning (GDC), Taylor discharge cleaning (TDC) and electron cyclotron resonance discharge cleaning (ECRDC) were adopted and the temperature of the first wall (room temp. -300degC) and working gas for the discharges (He, Ar and H2) were varied as the major experimental parameters. Hydrogen, hydrocarbons and other chemical species were observed in the exhaust gas. The number of chemical species increased with the first wall temperature especially in GDC. The degassing rates in GDC were much larger than in TDC and ECRDC. This tendency was similar to that of tritium degassing. Observed chemical species of hydrocarbons were methane, acetylene, ethylene and ethane. They are considered to be produced by the reaction of hydrogen with carbon material of the first wall. The productivity of hydrocarbons was significant in GDC using H2 and depended on the temperature of the first wall. From the viewpoint of application to the removal of carbon-tritium codeposited layers which is a crucial issue for ITER, consumption of carbon by the reaction was estimated. About monolayer of the carbon tile surface is expected to be removed with an hour operation of GDC. (author)