Vacuum system of SST-1 Tokamak

Abstract

Vacuum chambers of Steady State Superconducting (SST-1) Tokamak comprises of the vacuum vessel and the cryostat. The plasma will be confined inside the vacuum vessel while the cryostat houses the superconducting magnet systems (TF and PF coils), LN2 cooled thermal shields and hydraulics for these circuits. The vacuum vessel is an ultra-high (UHV) vacuum chamber while the cryostat is a high-vacuum (HV) chamber. In order to achieve UHV inside the vacuum vessel, it would be baked at 150°C for longer duration. For this purpose, U-shaped baking channels are welded inside the vacuum vessel. The baking will be carried out by flowing hot nitrogen gas through these channels at 250°C at 4.5bar gauge pressure. During plasma operation, the pressure inside the vacuum vessel will be raised between 1.0×10−4mbar and 1.0×10−5mbar using piezoelectric valves and control system. An ultimate pressure of 4.78×10−6mbar is achieved inside the vacuum vessel after 100h of pumping. The limitation is due to the development of few leaks of the order of 10−5mbarl/s at the critical locations of the vacuum vessel during baking which was confirmed with the presence of nitrogen gas and oxygen gas with the ratio of ∼3.81:1 indicating air leak. Similarly an ultimate vacuum of 2.24×10−5mbar is achieved inside the cryostat. Baking of the vacuum vessel up to 110°C with ±10°C deviation was achieved with a net mass flow rate of 0.8kg/s at 1.5bar gauge inlet pressure and supply temperature of 230°C at the heater end. Also during gas feed system installation, the pressure inside the VV was raised from 3.01×10−5mbar to 1.72×10−4mbar by triggering a pulse of lower amplitude of 25 voltage direct current (VDC) for 100s to piezoelectric valve. This paper describes in detail the design and implementation of the various vacuum subsystems including relevant experimental results.