Experimental Study for Comparison of H2and Ar–H2Gas Mixture Glow Discharge Wall Conditioning in ADITYA Tokamak

Abstract

Low-temperature glow discharge wall conditioning (GDC) using H2 gas is effective in reduction of oxygen and carbon (low-Z) contained impurities on near surface region of the vacuum vessel wall. The high retention of hydrogen in vessel wall/components due to long operation of H2 GDC increases hydrogen outgassing during plasma operation and affects the production of high-temperature plasma in tokamak. The hydrogen retention can be reduced using inert gas GDC by sputter cleaning for short duration. But, in that case, the outgassing rate of the inert gas increases, which may impair 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 the vacuum vessel surface. In ADITYA tokamak, H2 GDC is carried out regularly after plasma operations, while the GDC with argon–hydrogen (Ar–H2) mixture has been experimentally tested to observe the reduction of the oxygen and carbon impurities along with low hydrogen retention. In Ar–H2 GDC, the formation of the ArH+ hydride ions, which have quite long life and more energy compared with 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 GDCs. We have observed a substantial reduction in oxygen and carbon impurities with a significant improvement in wall condition with Ar–H2 GDC compared with the H2 GDC. The effect of wall conditioning by Ar–H2 GDC on the performance of high-temperature plasma operation is presented in this paper.