| Pulsed high magnetic field device provides a comprehensive experimental platform under extreme physical conditions,which has been an important investigation platform for many scientific researches such as condensed matter physics,materials,magnetism,chemistry,and life medical science.For scientific experiments in pulsed high magnetic field,especially observing the quantum physics phenomena,the thermal effect influence of the materials must be weakened,thus these researches need to be carried out at extremely low temperature,as low as 1.5 K in many cases.Using liquid helium-4 bath with pumping method is the most popular way to cool the sample down to 1.5 K.However,for helium-4 cryostat in which the sample is directly immersed in liquid helium,though it can provide large refrigeration capacity with simple structure,but quite a bit of liquid helium is consumed during the processes of transportation and experiment;for cryostats indirectly employing liquid helium,on the contrary,their structures usually can be more complex while the refrigeration capacities are much lower.In order to avoid the disadvantages of the above two categories of helium-4 cryostats,a helium recondensation cryogenic system for the scientific experiments in pulsed magnetic field is designed and tested in this paper,and the research is carried out from three aspects:1)Stability analysis of cryogenic system under the condition of pulsed high magnetic field.The helium recondensation cryogenic system is composed of a liquid helium cryostat and a helium liquefier,and the cryogenic system is designed for the use of scientific experiments in pulsed high magnetic field.The magnetic field environment of the helium recondensation cryogenic system and its potential influence on the system have been analyzed.Through the coupling interaction model between the key components of the cryogenic system and the magnetic field established by the COMSOL multiphysics software,the eddy current distribution and its effect on the liquid helium cryostat in the discharge process of the pulsed magnet are obtained.On the basis of the heat transfer and mechanical characteristics of the liquid helium cryostat in the helium recondensation cryogenic system,the cryostat is optimized to ensure that the low temperature system has good thermal and structural stability during the discharge process of pulsed high magnetic field.2)Design and experimental test of a helium liquefier with high performance.The small scale helium liquefier is key part of the helium recondensation cryogenic system.In this paper we use a commercial GM cryocooler with 1.5 W cooling power at 4.2K(Sumitomo model RDK415D)as the cold source,and the helium gas is cooled to liquefaction temperature by heat transfer process with the cryocooler.The core of the helium liquefaction system lies in the liquefaction process,and the precooling of helium gas before reaching the condensing component is the key part to improving the overall efficiency of the liquefier.A Seperated two-stage precooling method is adopted in the small-scale helium liquefier.The precooling process is separated to two parts by the first stage cold head of the GM cryocooler.The helium gas can be cooled to about 50 K by the first stage regenerator and first stage cold heat of the cryocooler,and then the cold helium gas pass through the surfaces of the second stage regenerator and second stage cold heat of the cryocooler.The precooling precess is carried out by direct contact heat transfer between the helium and the cryocooler equipped with copper ring heat exchangers on the surface of the cylinder.Eventually,liquid helium film is formed on the vertical surface of the condenser and get into the storage tank.The porous cylindrical condenser with many holes on the bottom has an effective condensing surface area of 0.017 m~2,and the efficiency of these hole fins is 4.092.The inner diameter of the holes is 3 mm,which can meet the requirement of free movement of helium gas and condensate liquid film.Liquefaction experiments show that it takes 7.2 hours for the liquefaction system to start producing liquid helium from room temperature,and a liquefaction rate of 96.7 g/h is obtained at 2 psi(corresponding to 19 L/day).When the operating pressure of the liquefaction system goes to 4 psi,6 psi and 8 psi,the liquefaction rate is 102.8 g/h,108.6g/h and 114.8 g/h,respectively.Through comparative analyses,we have found that the effect of pressure on the liquefaction rate is mainly due to the latent heat of helium decreases with increase of pressure,and the condensation power of the liquefier under different pressures are nearly constant at a value of 0.53 W.3)Intergartion and experimental analysis of the helium recondensation cryogenic system.The development of small-scale helium liquefier based on GM cryocooler has laid a solid foundation for the realization of helium recondensation cryogenic system for scientific experiments in pulsed high magnetic field.Through the modular design,the small-scale helium liquefier can be turned into portable helium liquefaction unit,and together with the liquid helium cryostat for scientific experiment to form a closed circular structure by adding helium transportation tubes,through which the scientific experiments can be carried out without additional filling of liquid helium.The helium liquefaction unit is also suitable for a variety of cryostats needing liquid helium,and can be widely used in fields of low temperature physics,molecular chemistry,biological and medical,etc.The helium recondensation cryogenic system for scientific experiments in pulsed magnetic field consistently provides liquid helium bath for the plug-in component of the sample,making it is convenient to carry out the low temperature tests continually.The helium gas added into the plug-in component can be liquefied easily due to the liquid helium bath.When pumping on the plug-in component,the sample area is cooled to 1.17 K with a temperature fluctuation less than 0.01 K in 30 minutes,and maintains below 1.2 K for 118minutes totally.The helium recondensation has been used to carry out the magnetoresistivity measurement of WTe2 in 60 T pulsed high magnetic field,through which the applicability and stability of the cryogenic system has been validated. |
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