Thermally actuated 3He pulse tube cooler

An isotope of Helium gas, He-3, is an attractive working gas for cryocoolers that strive to efficiently provide cooling temperatures below 4 K. The drawback of this gas is that it is extremely expensive. To minimize the total amount of gaseous 3He, a hybrid cycle has been proposed. A single-stage pulse tube cooler using He-4 as the working gas and either a Stirling-type or GM-type compressor system can provide a cooling temperature around 40 K, starting from room temperature, A secondary cycle using He-3 can then be thermally attached to this cold head to produce a secondary cooling temperature below 4 K. The operating frequency of the secondary cycle should be lower than 2 Hz to prevent degradation of the performance of the lower-stage regenerator. In this study, a thermally actuated pressure wave generator driven between the temperatures of 40 K and 300 K was selected; it has no difficulty in generating a pressure wave below a frequency of 2 Hz. Workflow analysis calculations indicate that the total amount of He-3 working gas required for this secondary cycle may be minimized by the use of a warm expander type of phase control.