Thermo-structural analysis of SST-1 cryopump

Abstract

A liquid nitrogen-based cryopump for Steady State Superconducting Tokamak (SST-1) is designed for pumping of water vapour from vacuum vessel during baking and under heavy gas load conditions. During baking, the load of the water vapour molecules increases while the resultant speed of available Turbo-Molecular Pumps is less for water vapour which increases the pressure of the vacuum vessel. Cryopump with the same opening area as of TMPs has more pumping speed for water vapour molecules. Hence, the said cryopump will pump water vapour molecules with higher pumping speed. In principle, water vapour molecules get condensed at a surface temperature of <120 K which corresponds to the saturation pressure of <10−10 mbar. The pump is based on the cryosorption principle where nitrogen gas and water vapour get adsorbed on to the cryosufaces at a temperature of <80 K. The amount of gas pumped and accumulated on cryosurfaces depends on many factors such as the temperature of a gas, microscopic surface roughness of cryosurface, its temperature and thermal conductivity, etc. [1]. Various components of this cryopump are thermal shield, cryopanels coated with suitable sorbent, baffles and vacuum chamber. For reliable and long duration operation of this pump, detailed thermal analysis is done to study the temperature distribution and thermal stresses generated for the efficient operation of the pump. Thermo-structural analysis of the developed pump is carried out for failure protection. The design by analysis method is followed to realize the concept of cryopump for SST-1 and the same is described in this paper.