The cryogenic system currently used by Wuhan National high magnetic field center includes He4 system（liquid helium immersion cryogenic system) and He3 system, which can provide low-temperature conditions for scientific measurements, and the temperature range is from 500 mK to 300 K.

He4 system

Liquid helium immersion cryogenic system has the advantages of simple structure, large cooling capacity and stable temperature, which has been widely used in scientific researches under high magnetic field. During the experiment, liquid helium was directly injected into the inner Dewar from the input port to immerse the measured sample in the liquid helium bath; The vacuum pump is used to evacuate the inner Dewar to reduce the temperature of liquid helium in the dewar. Finally, the experimental sample is cooled to a low temperature of 1.5 K. At present, it is mainly used to carry out scientific experiments with 60 T pulse magnet. The helium immersion cryogenic system uses the liquid nitrogen which cooling the pulse magnet to provide thermal radiation shield for the liquid helium cryostat and reduce the evaporation of liquid helium in the cryostat. 

Fig.1 He4 system: to 1.5K

He3 system

The most common way to obtain low temperature below 1 K is to reduce the temperature by He3. Helium three refrigeration cryogenic system is a two-stage decompression and cooling system, which adds the decompression and cooling process of helium three to the helium immersion cryogenic system. Similar to the helium immersion cryogenic system, liquid helium He4 is first injected into the liquid helium thermostat, and the liquid nitrogen soaked in the magnet provides thermal shielding for the liquid helium thermostat. Initially, the He3 cavity was in vacuum. The temperature of 4He cavity was reduced to below 1.5 K by decompressing and cooling, which met the needs of He3 liquefaction and heat shielding. During the experiment, the liquid level of He4 needs to be kept above the vacuum interlayer at the bottom of He3 cavity. When He3 gas enters the He3 cavity immersed in He4, He3 gas condenses to form liquid helium He3 under the action of peripheral low-temperature He4, and then the temperature of liquid helium He3 is reduced by a pump, which can finally reach the limit low temperature of 500 mK. 

Fig.2 He3 system: to 500 mK

DR system

The dilution refrigerator system will be updated in the Stage II project of the High magnetic field center in the near future years. According to the structural characteristics of the pulse magnet, the tail end of the dilution refrigerator is placed in the center hole of the magnet, and the liquid nitrogen soaked in the magnet provides thermal shielding for the liquid helium thermostat. In order to avoid the influence of eddy current on temperature stability, high resistivity materials and insulating materials must be used for the tail materials of the dilution refrigerator used under the strong pulsed magnetic field. The cryogenic system of dilution refrigerator has a relatively complex structure and a large consumption of liquid helium. It is generally used to provide extremely temperature condition lower than 200 mK and cooperate with pulse magnet of over 50 T to carry out special scientific research under high B/T extreme conditions.

Helium recovery system

Liquid helium is an indispensable low-temperature medium for low-temperature experiments under a strong magnetic field. At present, the High magnetic field center has independently developed two sets of helium liquefiers using 10 small GM refrigerators, with a liquid helium production efficiency of 200 L/day, and is equipped with helium recovery and purification systems to recycle helium resources. 

Fig.3 Helium recovery system

Key Contacts

Mengyu Liu

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