Analysis of Exhaust Gas in JT-60U Tokamak Operation

In the operation of tokamak such as ITER, it is important to understand the behavior of hydrogen isotopes and other elements in vacuum vessel for tritium inventory control and design of the plasma exhaust process. Particularly removal of carbon-tritium codeposited layer is considered as a crucial issue for ITER, because of its suspected tritium inventory. In JT-60U, exhaust gas was analyzed in the operation of discharge cleaning and tokamak discharge experiments, and the behavior of hydrogen isotopes and carbon were investigated. In discharge cleaning experiments, the degassing rate of hydrogen isotopes in GDC (glow discharge cleaning) was much larger than that in TDC (Taylor discharge cleaning) and ECRDC (electron cyclotron resonance discharge cleaning). Exhausted hydrocarbons were far less than hydrogen. During tokamak discharge experiments, various carbon compounds such as CD₄, C₂D₂, C₂D₄, C₂D₆ and CO₂ were detected. The maximum amount of exhausted carbon was several mg for each shot. There was a strong correlation between exhausted hydrocarbon and tritium. The exhausted hydrocarbon increased with the wall temperature