A Gifford-McMahon cycle cryocooler below 2K

According to theory, a Gifford-McMahon (GM) cycle cryocooler with He-4 cannot cool below 2 K because of the He-4 superfluid transition near this temperature. However replacing 4 He by He-3 removes this temperature limitation. The cooling performance of a GM cryocooler with a HoCu2 magnetic regenerator material is investigated using He-3. The minimum temperature of 2.3 K with He-4 goes down to 1.65 K when the He-4 working fluid is replaced by He-3. The maximum cooling capacity at 2 K is 53.9 mW with a compressor power of about 2.5 kW, and the cooling capacity at 4.2 K is enhanced by more than 20%. The effect of a new regenerator material (NdInCu2) on the cooling performance was also investigated. The minimum temperature decreased to 1.64 K and the cooling capacity at 2 K improved to 57.1 mW with the use of this material in the bottom 40% of the regenerator.