50 mK多级绝热去磁制冷机的实验研究

Experimental research on a 50 mK multi-stage adiabatic demagnetization refrigerator

随着近年来凝聚态物理、空间观测与量子技术的不断发展,极低温制冷需求也日益增多.绝热去磁制冷具有不依赖重力、结构紧凑、成本较低等特点,可同时满足空间和地面应用.本文设计并研制了一台50 mK多级绝热去磁制冷机.该机为三级串联架构,由一台GM型脉管制冷机提供4 K预冷;采用钆镓石榴石和十二水合硫酸铬钾作为制冷工质;研制的主动式/被动式气隙热开关,用以控制级间传热;针对磁屏蔽、恒温控制等构建了多个数值模型进行辅助设计.目前该机获得的最低制冷温度为38 mK,100 mK时制冷量约71 mJ,温度波动10.6μK.本研究为后续开展50 mK连续绝热去磁制冷奠定了重要基础.

With the development of condensed matter physics,space observation,and quantum technology in recent years,the demand for ultra-low temperature refrigeration has increased.The adiabatic demagnetization refrigerator(ADR)has the advantages of being unaffected by gravity,compact structure,and relatively low cost,which can meet the needs of space and ground applications.In this paper,a 50 mK multi-stage ADR is designed and developed which comprises a GM-type pulse tube cryocooler for precooling and three ADR stages connected in series.For easy installation and maintenance,the three ADR stages are placed on a separate cold plate which is connected to the 4 K cold plate via copper columns.The Dy_(3)Ga_(5)O_(12)(GGG)is employed as the refrigerant in the first stage,whereas CrK(SO_(4))_(2)⋅12H_(2)O(CPA)is utilized for the second stage and third stage.To control heat transfer between stages and the 4 K heat sink,active gas-gap heat switch and passive gas-gap heat switch are developed,with the latter having a switching ratio over 1000.The 4 T,2 T and 1 T superconducting magnets are utilized in the 1st,2nd and 3rd stage,respectively,and a numerical model is used to optimize the design of magnetic shielding.In addition,an ADR simulation model with a proportional-integral-derivative(PID)controller is constructed to assist in tuning the controller parameters in the experiment.The lowest temperature achieved in the experiment is 38 mK,with a temperature fluctuation of 10.6μK.The durations of different cooling power(1,2 and 3μW)at 100 mK are also measured.It is calculated that the no-load maintenance time is about 4.3 h,the leakage heat power is about 4.5μW,and the total cooling capacity is about 71 mJ.This refrigerator is the first Chinese multi-stage ADR that can reach a temperature under 50 mK,which lays an important foundation for subsequent research on continuous adiabatic demagnetization refrigeration.