Analyses On CPMU Sub-cooled Liquid Nitrogen System & Key Equipments And Experimental Study On Pressure Stability

The Cryogenic Permanent Magnet Undulator(CPMU)is one of the development trends of the insertion technology.The CPMU,made of NdFeB or PrFeB materials,has a better magnetic field performance at low temperature than that at room temperature.The operating temperature of the CPMU is in the range of 50K-150 K up to the magnetic material to be adopted.The CPMU cooling system mainly consists of the CPMU sub-cooled liquid nitrogen cooling system and the CPMU magnet array cooling loop.Since 2013,Shanghai Institute of Applied Physics has developed has been study on the CPMU and its key technology.The first CPMU prototype was built in 2014.In order to meet the need of the off-line magnetic field measurement and online test of CPMU,we developed the CPMU sub-cooled cooling system.This paper is based on the development of the CPMU sub-cooled cooling system.The flow diagram and design parameters of the CPMU sub-cooled cooling system were introduced.The heat loads of the cooling system were calculated and analyzed.The relationship among the cooling capacity of the cooling system,mass flow rate and physical properties of the circulating liquid nitrogen.was analyzed.The sub-cooled heat exchanger is one of the key components of the cooling system.The sub-cooled heat exchanger was designed as a coiled type.The heat transfer surface and structure parameters of the heat exchanger were calculated.The liquid nitrogen pump is another key component of the cooling system;it compensated the pressure drop of the circulating liquid nitrogen,and provided circulating power for liquid nitrogen.As the base of the selection of the liquid nitrogen pump,the flow resistance of the sub-cooled liquid nitrogen going through the whole CPMU cooling system was calculated.The pipeline characteristic curve is obtained,which provides basis for the selection of the liquid nitrogen pump.The first CPMU and its cooling system were installed in SSRF since August,2016.A series of tests were implemented.Cooling capacity of the cooling system was tested and meets the cooling needs well.The pressure control technology of the CPMU sub-cooled cooling system was tested.The stability of the pressure in the pressure control cylinder does not meets the design parameter at the beginning of the test.There were periodic pressure fluctuations far beyond the design parameter.The problem was solved perfectly in the later tests.The pressure fluctuation in the pressure control cylinder was less than 2kPa while the volume of liquid nitrogen in the pressure cylinder decreased to less than 40 percent of the volume of the whole pressure control cylinder.The stability of the CPMU sub-cooled cooling system reaches the design requirements.