Experience of pumping the vacuum vessel of SST-1 during the baking cycle with indigenously developed liquid nitrogen cooled sorption pump

To achieve ultra-high vacuum in the vacuum vessel of SST-1, the baking of vacuum vessel is done to remove impurities like water vapour, carbon-dioxide and hydrocarbons etc. The maximum baking temperature of the plasma-facing components (PFCs) are around 250 degrees C and the vacuum vessel is around 150 degrees C. The gas load of water vapour increases significantly during the baking operation. Therefore, to pump the water vapour during baking, a customized liquid nitrogen cooled sorption pump was designed, fabricated and installed on the radial port of the machine. It is the first time that liquid nitrogen cooled Cryopump was used on the SST-1 machine. The pumping of water vapour, nitrogen and impurity gases was observed during the 11 days of operation. Commercial Cryopumps are operating at -10 K temperature and can pump hydrogen gas however, the direct mounting of these pumps during baking results in early regeneration of the pump due to high heat load. Therefore, the experience of pumping the vacuum vessel of SST-1 with the indigenous developed sorption pump in association with the Turbo-Molecular Pump (TMP) sets is discussed systematically in the present report. The partial pressure of the water vapour using TMP sets was 1.0E-6 mbar and after incorporating the sorption cryopump with the existing vacuum system, the partial pressure reached to 1.3E-7 mbar in 48 h at 230 degrees C PFCs temperature. The pressure attained in the vacuum vessel was -1.0E-7 mbar and the partial pressure of water vapour was -1.1E-8 mbar after ramp-down the PFCs to 50 degrees C temperature.