Entry Date:
February 24, 2000

Superfluid Stirling Refrigerator

Principal Investigator John Brisson


The superfluid Stirling refrigerator (SSR) has been shown to be capable of achieving temperatures of 300 mK and has potential for
achieving much lower temperatures. Refrigeration in this temperature has application in particle physics, surface chemistry, astronomy, material science, and condensed matter physics. Compared to other existing technology, the superfluid Stirling refrigerator is more efficient, less expensive and capable of zero gravity operation.

In order to understand the SSR it is necessary to describe the standard Stirling refrigerator. The refrigerator consists of a hot and cold piston connected by a regenerator, an array of narrow channels through a high heat capacity material. Initially the working fluid, an ideal gas, is in the hot cylinder, since the cold piston is in its fully inserted position. The gas is compressed isothermally in the hot cylinder, rejecting its heat to a hot thermal reservoir. Both pistons are then moved in tandem so that the gas reversibly (and isochorically) dumps heat into the regenerator as it travels from hot to the cold cylinder. The low temperature gas is now allowed to isothermally expand in the cold cylinder, absorbing heat from a low temperature reservoir. Then by moving the pistons in tandem, the gas is forced, isochorically, back through the regenerator where it reabsorbs the heat it deposited there earlier. The refrigerator is now back in the original state and is ready to repeat the cycle.

The SSR uses a 3He-4He liquid as the working fluid. The SSR is equipped with pistons that are bypassed with superleaks, so that the
refrigerator operates only on thermodynamically active components of the mixture (the 3He, which behaves like an ideal gas, and the phonon-roton gas of excitations in the 4He). The superfluid component of the mixture acts as an inert background. The compressions and expansions of the cycle are performed only on the normal components of the 3He-4He mixture since the superfluid component freely flows through the superleaks.

There are effects at both high and low temperatures that make the working fluid"s behavior deviate from that of an ideal gas. At high temperatures the excitations in the 4He contribute, and at low temperatures the 3He becomes a Fermi gas. We hope to characterize and understand both phenomena in the SSR. We intend to build a two stage refrigerator to cool to temperatures below 150 mK from a base temperature of 1.2 K. With this refrigerator we will investigate thermodynamics of the 3He-4He mixture, and develop more efficient low temperature heat exchangers.