| Three nonsteady working conditions for SRF cavities's cryogenic system in BEPCⅡ(Beijing Electron-Positron Collider Upgrade) has been researched in this dissertation. These three conditions include the coldbox-stop condition, the quick-recovery condition and the shut-down condition.Liquid helium discharge process of control dewar under coldbox-stop condition has been researched. A numerical model for the process has been presented at first. The simulation for the real case was performed with the Simulink model in MATLAB. The variations of the mass flow and the pressure of the helium in the dewar with time were obtained. The method by using electric heater to control the dewar pressure was presented. The control loop for the pressure control in the dewar was designed and simulated.Vapor helium discharge process of SRF cryostat and control dewar under quick-recovery condition has been researched. Simulation of the corresponding model showed that the process is influenced by the power of electric heater and discharging helium mass flow rate. The relation between power of heater and helium flow rate has been discussed. The pressure control loop under this condition was designed, and forward control was also applied.Working condition of refrigeration system under shut-down condition has been theoretically researched. Numerical module of refrigerator's heat exchanger,J–T valve and compressor was presented. Based on the mathematical models, helium flow process of refrigeration was analyzed. Then the flow process was simulated by flow computation software. At last, operation parameter under shut-down condition was obtained and compared with parameter under normal condition.
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