Investigation on phase shifting for a 4 K Stirling pulse tube cryocooler with He-3 as working fluid

• A three-stage Stirling pulse tube cryocooler with He-3 as working fluid was optimized theoretically and experimentally.
• The phase shifting capability of different working fluids at liquid helium temperatures was revealed.
• A no-load temperature of 4.03 K was obtained, which is the lowest temperature achieved by a three-stage SPTC with He-3.

He-3 is generally recognized for its ability to provide more excellent thermophysical performance than He-4, especially in the 4 K temperature range. However, this was not always the case in our preliminary experiments on a three-stage Stirling-type pulse tube cryocooler (SPTC). Our ongoing studies, as reported in this paper, demonstrate that the different working fluids also affect the performance through their phase shifting capability. This feature has been passed over in large part by researchers considering refrigerant substitution. Unlike previous theoretical analyses that focus primarily on regenerator losses, this report investigates the effects of the working fluid on the phase angle at the cold end in order to quantitatively reveal the relationship between the lowest attainable temperature and the cooling capacity. The analysis agrees well with our experimental results on a three-stage SPTC. While running with the operating parameters optimized for He-3, the lowest temperature of the SPTC decreased from 5.4 K down to 4.03 K. This is the lowest refrigeration temperature ever achieved with a three-stage SPTC.