Development Of 2 K Cryogenic System In CiADS Superconducting Testing Center

China Initiative Accelerator Driven System(CiADS)is an Accelerator Driven sub-critical System(ADS)device for accelerator driven transmutation research,contracted by institute of Modern Physics,Chinese Academy of Sciences.The project was started in Huizhou,Guangdong province in 2018.The device can carry out the strategy research on the stability,reliability,long-term operation of the superconducting linear accelerator,high power spallation target,and subcritical reactor.In order to achieve higher energy,higher quality and higher stability,CiADS superconducting linear accelerator requires higher performance of RF superconducting cavity.Therefore,all superconducting cavities of the accelerator are designed to operate in the 2 K superfluid helium temperature region.The superconducting cavity must be tested before running on-line to obtain its performance parameters to ensure that the cavity performance meets the design requirements.After a thorough survey on the research progress of 2 K cryogenic system worldwidely,based on the engineering requirements of CiADS,a research protocal of 2K cryogenic system was proposed for the vertical test of superconducting cavities.In the present study,cryogenic system flow organization and design,cryogenic thermal analysis and calculation,design and type selection of main equipments,study and development of sub-atmosphere heat exchanger,and system operation and test were studied systematically.The 2 K cryogenic system adopted some new designs.The two tests Dewar can be freely switched,so the superconducting cavity test can be carried out continuously,reducing the cooling and temperature return process and shortening the waiting time of performance test.It can meet the requirements of cavity performance test during the project construction of CiADS.At the same time,this cryogenic system also has other cryogenic test functions,such as sub-atmospheric heat exchanger test and cryogenic thermoacoustic oscillation test.The system has some unique design.For the first time,low temperature valve box,sub-atmospheric heat exchanger and other equipment are placed in the upper side of the Dewar,saving the radial space of the Dewar.In order to obtain higher accuracy of temperature measurement in the temperature region of 2 K,a new type of internal and external tube temperature measurement structure was designed,and a new measuring instrument and method are used.After the construction of the cryogenic system,first cooling down experiment research on superconducting cavity was successfully completed.It took 30 hours for continuous cooling down and liquid helium accumulation from 300 K to 4.5 K,and 2.5hours for acquiring superfluid helium from 4.5 K to 2 K,which met the design requirements of the project.Up to now,the system has been successfully delivered to use,and more than 20 different superconducting cavities have been tested,such as Nb₃Sn cavities,Nb plated copper cavities,and pure Nb cavities.The system operates stably.Liquid helium pressure is controled steadily within 3000±10 Pa and the liquid helium level is controlled within 5%of the set point.The successful development of the cryogenic system of CiADS Superconducting Test Center not only provides reliable guarantee for the cryogenic test of superconducting cavity,but also accumulates extensive experience for the design and construction of the large 2 K cryogenic system of superconducting linear accelerators such as HIAF and CiADS for the Twelfth Five-year major scientific projects.