Overall Performance of SST-1 Tokamak Vacuum System

Vacuum system of steady-state superconducting tokamak (SST-1) is comprised of 23 m³ ultrahigh vacuum (UHV) chamber with baking channels embedded on the vacuum vessel (VV) being heated with a dedicated nitrogen gas baking facility, gas puffing system, wall conditioning system and 39 m³ high vacuum cryostat (CST) chamber housing the superconducting magnets, cold structures, 80-K thermal shields and associated auxiliary components. The pumping system for the SST-1 VV consists of two turbo-molecular pumps each of 5500 l/s (N₂ gas) capacity providing a total effective pumping speed of 4675 l/s and the CST consists of three such pumps with a total effective pumping speed of 3250 l/s. To achieve UHV inside the VV, it was baked at 135 °C for a longer duration of 100 h, while four numbers of graphite based limiters were baked at 250 °C for 50 h. An ultimate pressure of 4.6 × 10^-8 mbar was achieved inside VV while an ultimate vacuum achieved inside the CST was 1.1 × 10^-5 mbar. The gas puffing system has performed satisfactorily during the plasma trials assisted with electron cyclotron resonance heating preionizations. This paper describes our overall experience during the entire campaign of successful operation of SST-1 during first plasma.