Performance of a Low-Temperature Gifford-McMahon Refrigerator Utilizing a Neodymium Disk Regenerator

Abstract

In this study, the cooling capability of a Gifford-McMahon (GM) refrigerator has been dramatically increased at temperatures near 4.5 K with no significant increase in input power. This increase in low temperature cooling power can be attributed to a change in design in the low temperature regenerative heat exchanger (regenerator). This regenerator was constructed by precisely forming flow holes through disks of neodymium (Nd) which allowed the reduction of the void fraction from about 35%, for conventionally designed regenerators that utilize spheres, to approximately 10%. Neodymium was chosen because it has a relatively high volumetric heat capacity at temperatures below 10 K and it is a pure metal that can be extruded and drawn with relative ease. The flow holes in the disks were formed using a technique similar to the one used to fabricate multi-filamentary NbTi superconducting wire. Approximately fifty disks, each separated from the next by a porous plastic cloth material, were stacked in a stainless steel tube to form the regenerator. The cooling performance of a Leybold Kelcool 4.2® refrigerator with this regenerator installed as its second stage heat exchanger is presented here and compared to its performance with the factory standard regenerator which uses Nd spheres. The new design improved cooling capacity by more than two fold at temperatures near 4.5 K.