Thermal efficiency study of the ALPHA-g cryostat - an analysis and heat simulation

This thesis presents a thermal study of the ALPHA-g cryostat at CERN. The thermal efficiency is characterised and thermal week points are identified. The results were obtained through analytic calculations, C++ script of programs and thermal steady-state simulations performed with ANSYS workbench 2020 R2. These models estimate heat flows and temperature distributions in components and the 2-phase systems of liquid and gaseous helium. In general, the heat loads are used to estimate the liquid helium (LHe) consumption by vaporisation. It is compared to a target consumption of 20 L h−1. Further, the gaseous helium (GHe) demand for internal cooling systems of radiation shields and vapour cooled leads (VCL) is determined. The ALPHA-g experiment studies the Baryon asymmetry of antimatter and matter by measuring the antihydrogen atomic mass. The ALPHA-g cryostat is only one subsystem of the experiment; its main purpose is to provide a cold LHe bath at 4 K to cool superconducting magnets and electrodes, in addition to cryopump an ultra-high vacuum chamber. Since LHe is present at cryogenic temperatures with a relatively low enthalpy of vaporisation (~21 kJ kg−1 at norm pressure), the cryostat contains a complex multi-layered structure to lower the incoming heat load. The thermal insulation of the LHe is achieved with an enclosing vacuum chamber with internal radiation shields and foils, as well as suitable component structures and material selection. Despite these measures to preserve the LHe, the apparatus is diathermic; some of the LHe evaporates and fills the upper section of the vessel with GHe. This creates a 2-phase system of liquid and gas in the helium-space. Most of the GHe is located in the tubular components ‘chimneys’ and ‘service tubes’ in the upper section of the cryostat. An evaluation of the thermal efficiency is carried out qualitatively in the context of ingoing heat loads and a resulting estimated LHe consumption. The total heat load entering helium-space is approximately 27 W. Due to the complex convective conditions, it could not be quantified how much of this heat load evaporates LHe and to what extend it increases the GHe enthalpy. However, a comparison of the theoretical values with measurements during a cold test allows an estimation. Approximately 18 W evaporate ~25 L h−1 of LHe. Simulations showed a thermal weak point at the chimneys with relevant heat conduction of ~8 W. The flexible bellows are identified as important thermal resistors. The temperature at the outer chimney wall is > 50 K for the most part. With a target radiation shield temperature of 50 K, the net direction of heat radiation is reversed as compared to the intended direction by design. The VCL influence this temperature the most. Analytical calculations determine that the VCL require a cooling gas mass flow of ~0.655 g s−1. This corresponds to an evaporating LHe volume flow of ~18.9 L h−1. It can be provided without additional boil-off. In comparison, the required evaporating LHe volume flow to cool the radiation shields is only ~1.3 L h−1. It corresponds to a net heat load onto the radiation shields of ~11 W. The target radiation shield temperature of 50 K can be evaluated as feasible but not necessary regarding the thermal efficiency of the cryostat. An increase up to ≤ 100 K is therefore recommended in case of cooling gas shortage. All determined heat flows take static gas inside the chimneys and service tubes into account. Analysis of this assumption showed little free convection in the upper chimney sections. In the service tubes, free convection might be relevant. It could be advised adding baffles here.