An Experimental GM/Magnetic Refrigerator
This paper describes the results obtained with an experimental prototype of a new type of cryocooler. The cycle employed by the device (a GM/magnetic cycle) uses two thermodynamically active substances: a compressible gas (helium) and a paramagnetic solid (Gadolinium Gallium Garnet or GGG). The two substances alternately regenerate one another, thereby overcoming the low temperature thermal saturation associated with a GM cycle while producing more refrigeration than a regenerative magnetic cycle. The experimental prototype utilizes a unique regenerative upper stage device to maintain the warm reservoir temperature seen by the GM/magnetic stage at the required 10–20 K. The pressure in the apparatus is controlled using a room temperature compressor and a system of valves. The device operates at a pressure level which is on the order of 4 atmospheres and uses a pressure ratio of approximately 2. The applied field in the GM/magnetic stage is controlled with a superconducting solenoid and a power supply. The maximum applied field imposed on the paramagnetic regenerator bed is 4 tesla.
The GM/magnetic refrigerator ultimately reached a no load temperature of 4.5 K while rejecting heat at 10.9 K. The device supplied 0.36 W of refrigeration at 6 K while rejecting heat at 15.4 K. The prototype clearly shows that the GM/magnetic cycle is an effective refrigeration cycle below 15 K.
KeywordsGadolinium Gallium Garnet Expansion Valve Superconducting Solenoid Magnetic Stage Displacer Volume
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