Low-frequency cryocoolers are compressed inefficiently

Millikelvin refrigerators are enabling tools for many fields of modern science, including quantum information processing. They are typically precooled to 4 K by low-frequency pulse-tube refrigerators, which perform a thermodynamic cycle based on compressing and expanding helium gas. Although the efficiency of these refrigerators is known to be poor, only the loss mechanisms that occur near the cold end have been sufficiently studied and characterized. Here, we combine thermoacoustic calculations with measurements from a commercial pulse-tube refrigerator to determine the efficiency of the generation and transmission of acoustic power, which the refrigerator uses to provide cooling. We find that electrical power consumed by the compressor is converted to acoustic power at a rate of only 0.24, while acoustic power is transmitted through the refrigerator at a comparatively high efficiency of 0.66. The compression process therefore dominates the portion of the system efficiency that is not limited by fluid and material properties. To show that more efficient compression is possible, we also performed measurements on a new metal-bellows compressor that converts electricity to acoustic power at factors up to 0.43. Our results show that low temperatures may be achievable with two or perhaps even three times less electricity.