Studies of Superfluid Stirling and Pulse Tube

There is a niche for a sub-Kelvin cryogenic refrigeration unit that is simple, efficient, and does not require gravity. The superfluid Stirling refrigerator was developed in response to this demand. The refrigerator has cooled to 168 mK while exhausting heat at 383 mK, demonstrating the potential to cool to temperatures well below that attainable with a continuous 3He evaporation refrigerator. The refrigerator has also been successfully operated with mixtures of up to 36% 3He concentration with proportionately higher cooling powers, though we suspect that the flow exceeding the critical velocity poses a lower limit on the temperature attainable with high concentration mixtures. Superfluid pulse tube version of the refrigerator has been successfully operated, though the efficiency is found to be significantly lower than that of the Stirling refrigerator. Gravitational convection of fluid in the pulse tube necessitates the pulses be oriented cold side up, contrary to convention in cryostats. The gravity and the temperature gradient in the pulse tube dramatically thin down the viscous penetration depth, resulting in a slug flow in the pulse tube and hence making this an essentially ideal pulse tube. Thesis Supervisor: John G. Brisson Title: Assistant Professor of Mechanical Engineering