Study On Structure And Thermal Performance Of Cryostat For Vertical Test Of Superconducting Cavity

The vertical test system for superconducting cavity is used to detect the low temperature performance of the superconducting cavity,and provides a support for the safe and stable operation of the accelerator.Cryostat that is a wide-mouth liquid helium vessel is one of key equipment of the vertical test system for the superconducting cavity.Liquid helium in the cryostat vaporizes easily under external thermal load,so the insulation performance of the cryostat is very important.The insulation structure for the inner vessel is a combination insulation of high vacuum variable density multilayer(VD-MLI)and vapor cooling shield,and for the open side of the cryostat is multi-shield insulation.In this paper,the thermal performance optimization of the cryostat was studied based on the insulation structure for inner vessel and open side of the cryostat,aiming at improving the insulation performance,reducing the heat leakage of the cryostat and cutting down liquid helium consumption.The main research work and relevant conclusions are as follows.(1)Analysis of heat leakage for open side of the cryostat.A variable arrangement of radiation shields was proposed for the multi-shield insulation of open side of the cryostat,and a coupling heat transfer model between radiation shield and helium was established.The influence of distribution of radiation shield on heat leakage and optimal radiation shield arrangement were obtained by numerical calculation.The results show that the variation of heat flux is not obvious when the number of radiation shields is larger than 11,and the influence on heat leakage is small.As the number of radiation shields is 11,the method of the variable arranged radiation shield,which is to arrange 7 radiation shields in high temperature region near the external environment and 4 radiation shields in low temperature region near the liquid surface,has minimal heat leakage.In addition,by analyzing the Grashof number of helium gas between the first radiation shield(shield 1)and the liquid surface,it is found that Gr has a maximum value with the variation of the temperature of shield 1.In order to reduce the convection heat transfer of helium and heat leakage,the temperature of shield 1 should avoid the corresponding temperature when Gr has the maximum value.(2)Effect of the major structure insulation of the cryostat on heat leakage.The vapor cooling shield in the VD-MLI is connected to the inner vessel,and the connection point forms a thermal intercept on the axial heat transfer path of the inner vessel.Based on the thermal intercept,an overall heat transfer model of the cryostat was established,and the position of the thermal intercept was optimized according to Carnot's law.Meanwhile,the heat transfer models of the VD-MLI and vapor cooling shield were established,and the influences of the VD-MLI thickness and the position of the vapor cooling shield in the VD-MLI(position of the vapor cooling shield)on heat leakage were analyzed.It was found that the heat leakage of the cryostat first decreased and then increased as the position of the thermal intercept approached the liquid surface,indicating that there was an optimal thermal intercept position to minimize the heat leakage.With the rising of VD-MLI thickness,the heat leakage of the cryostat declines and the drop extent gradually becomes gentle.As the position of the vapor cooling shield moves along the VD-MLI thickness to the side of the outer vessel,the heat leakage decreases gradually.(3)Influences of the cryostat structure on heat leakage and outlet temperature of the helium gas.The upper part of the inner vessel of the cryostat is not covered by the insulation,which raised the outlet temperature of the helium gas to prevent frosting on the flange cover.Based on the overall heat transfer model of the cryostat,the effects of the uninsulated part height of the inner vessel and the liquid level on the heat leakage and the outlet temperature of the helium gas were investigated.The results show that the heat leakage of the cryostat increases as the uninsulated part height of the inner vessel goes up and is not sensitive to the change of the liquid level.The outlet temperature of the helium gas increases obviously as the uninsulated part height of the inner vessel goes down.In contrast,it does not change significantly with liquid level.(4)Thermal performance analysis of the cryostat and experiment.Based on the vertical test system of the superconducting cavity in the Institute of Modern Physics,Chinese Academy of Sciences,the experimental platform was built,and the experiments of precooling and heat leakage were carried out,which verified the rationality of the heat transfer model.Based on the optimized structure of the cryostat,the thermal performance optimization of the cryostat was studied.It was found that the evaporation of liquid helium decreased by about 6% compared with that before optimization.