Dynamic Simulation And Experimental Study Of A 2K Superfluid Helium Refrigeration System With Cooling Capacity Of A Hundred Watt

Large-scale superconducting magnets are presently widely used in modern high-energy particle accelerators and controllable nuclear fusion experimental device, which develops superconducting magnet and superconducting RF cavity technology toward higher magnetic field and higher accelerating gradient. Meanwhile the liquid helium cryogenic systems applied on superconducting devices are developed from 4.2K temperature region toward superfluid helium temperature region.In this thesis, the thermodynamic performance of superfluid helium cryogenic system cooling SRF cavities is deeply analyzed through numerical simulation. The influence of SRF cavity operating temperature on power consumption of SRF cavity system is researched also. The dynamic simulation models of key components in both 4.5K helium cryogenic system and 2K superfluid cryogenic system are built based on lumped-parameter and distributed-parameter methods. The cooling process of both 4.5K helium cryogenic system and 2K superfluid cryogenic system are deeply analyzed using reliable dynamic simulation procedure developed in this thesis.The premise of dynamic cooling process analysis is economical and reasonable thermodynamic cycle. At first, the scheme of 2K then the refrigerating output is 100W superfluid helium cryogenic thermodynamic cycle is studied using large process analysis softare and its user-defined models. Then the thermodynamic analysis of 2K superfluid helium cryogenic is deeply studied to improve the refrigerant efficiency, system reliability, and system adaptability. Compared the refrigerant efficiencies of superfluid helium cryogenic system under different cooling schemes, it shows that the helium cryogenic system with pre-cooling heat exchanger and negative pressure heat exchanger to recover cooling capacity of helium gas has the highest refrigerant efficiency, lowest operation cost, but the system structure is more complex and the equipment cost is higher.1.3GHz SRF cavities are widely applied to high-energy accelerators. The relationship between BCS surface resistance, power consumption of refrigerator and SRF cavity operating temperature are thorough analyzed. The radio-frequency losses of SRF cavity and power consumptions of refrigerator for different operating temperatures due to BCS surface resistance of SRF cavity are researched. The result shows that the reasonable operating temperature of SRF cavity can obviously reduce the operation cost, and the optimal operating temperature of this kind of SRF cavities are determined based on the two kinds of dominate losses.The dynamic simulation study of helium system is mainly on the dynamic thermodynamic process during cooling-down, warming-up and non normal operation mode. The analysis results can provide theoretical basis for the optimum design of helium system, and practice operation.In this paper, the superfluid helium refrigerator system is divided into 2 parts: 4.5K helium refrigerator system and superfluid helium refrigerator system. Based on the thermodynamics theory of the refrigeration system, the dynamic simulation model of each cryogenic component has been built according to its working characteristic, and utilizing lumped parameter and distribution parameter technique. The different cooling-down processes of superfluid helium refrigerator have been simulated using the FORTRAN program developed by this thesis.The influences of the opening of main cryogenic governor valves and helium gas flow rate on the refrigerator system parameters during cooling-down process are considered and researched in dynamic simulation. The simulation results shows that the opening variety of compressor by-pass valve affects cooling-down time of refrigerator system directly. The helium gas flow rate entering refrigerator system is limited by the negotiabilityof turbine expander and throttle valve, and must be controlled during cooling-down process by adjusting compressor by-pass valve.Through dynamic simulation the complex temperature distribution regularity along helium flow direction is obtained, and the temperature variety tendency on temperature-entropy figure of temperature measuring points along helium flow direction in helium refrigerator is presented in this thesis.The dynamic simulation of cooling down process shows that utilizing cryogenic heat-exchanger the time when the superfluid appears in the superfluid helium refrigeration system is earlier than time that the saturation liquid helium appears in the system.The dynamic simulation result also indicates that the trial-run scheme of helium refrigerator can be optimized and the debugging cost and time can be saved though dynamic simulation.