A Gifford-McMahon Cycle Cryocooler below 2K

According to theory, a Gifford-McMahon (GM) cycle cryocooler with 4He cannot cool below 2 K because of the 4He superfluid transition near this temperature. However replacing 4He by 3He removes this temperature limitation. The cooling performance of a GM cryocooler with a HoCu 2 magnetic regenerator material is investigated using 3He. The minimum temperature of 2.3 K with 4He goes down to 1.65 K when the 4He working fluid is replaced by 3He. 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%.