A three-stage Stirling pulse tube cryocooler operating below the critical point of helium-4

Precooled phase shifters can significantly enhance the phase shift effect and further improve the performance of pulse tube cryocoolers. A separate three-stage Stirling pulse tube cryocooler (SPTC) with a cold inertance tube was designed and fabricated. Helium-4 instead of the rare helium-3 was used as the working fluid. The cryocooler reached a bottom temperature of 4.97 K with a net cooling power of 25 mW at 6.0 K. The operating frequency was 29.9 Hz and the charging pressure was 0.91 MPa. It is the first time a refrigeration temperature below the critical point of helium-4 was obtained in a three-stage Stirling pulse tube cryocooler.

► A separate three-stage Stirling type pulse tube cooler (SPTC) was investigated.
► A cold inertance tube was introduced into the third stage as a good phase shifter.
► The cooler reached a bottom temperature of 4.97 K using only helium-4 not helium-3.
► It is the first time a temperature below 5 K in a 3-stage SPTC with frequency at 30 Hz.