Comparative study of glow discharge wall conditioning using H₂ and Ar-H₂ gas mixture of ADITYA Tokamak vacuum vessel

Low temperature glow discharge wall conditioning (GDC) using H₂ gas is effective in reduction of oxygen and carbon (low-Z) contain impurities on near surface region of vessel wall. The high retention of hydrogen in vessel wall/components due to long operation of H₂ GDC increases hydrogen out-gassing during tokamak operation and affects the production of high temperature plasma. The hydrogen retention can be reduced using inert gas GDC by sputter cleaning for short duration. But in that case the out-gassing rate of inert gas increases, that again impairs the plasma performance. To overcome above problems, the GDC with hydrogen-inert gas mixture can be used for better removal of C and O surface contaminants and low hydrogen retention in surface. In ADITYA tokamak, H₂-GDC is carried out regularly after plasma operation, while the GDC with argon-hydrogen (Ar-H₂) mixture has been experimentally tested to observe the reduction of oxygen and carbon impurities along with low hydrogen retention. In Ar-H₂ GDC, the reason being the formation of ArH⁺ hydride ions, which has quite long life and more energy compared to H₂⁺ ions formed in H₂ GDC for breaking the bond of wall molecules. A systematic comparative study of H₂ GDC and Ar-H₂ Mixture GDC by changing the mixture ratio has been carried out in ADITYA tokamak. The relative levels of oxygen and carbon contain impurities have been measured using residual gas analyzer in both GDC's. We have observed a substantial reduction in oxygen and carbon impurities with a significant improvement in wall condition with Ar-H₂ GDC compared to the H₂ GDC. The effect of wall conditioning by Ar-H₂ GDC on the performance of high temperature plasma operation will be presented in this paper. (author)