Split-Stirling Cryogenic Refrigerators For Detector Cooling

Abstract Unfortunately, for user and manufacturer both, the closed-cycle cryogenic cooler to date has deserved its reputation as the "weak-link" in IR systems. When the cooler requires service at intervals of a few hundred hours at best, the quality of trie system it serves is unfairly diminished.This paper addresses technological advances in the art of Stirling-cycle coolers which will increasingly cause that image of military cryocoolers to change for the better.A family of split-cycle coolers designed for long MTBF and in the final stages of development is the focus of the discussion. Their technological evolution, from multi-year-MTBF satellite system Stirling coolers developed in the U.S., and the UA 7011 cooler (tne first all-linear, military, production cooler) developed in Holland, is explained.Three new machines are discussed. Both 1/4 watt and 1 watt (nominal capacity) at 80°K linear-resonant, free-dispLacer Stirling coolers designed for thousands of hours of service-free operation are examined. The third machine is an advanced 1/4 watt at 80°K Stirling cooler incorporating the same component improvements in its free-displacer while utilizing a crankshaft-driven compressor. All three are designed to be compatible with standard U.S. 60 element and 120/180 element detector/dewars.The technologies of linear-resonant compressor and free-displacer expanders as embodied in these machines is discussed in sufficient detail that the reasons for their superior performance will be clear.Introduct ionThe Stirling refrigeration cycle, in its fifth decade of development, is by now well known to the technical community. One will recall that in its simplest embodiment, two pistons (trie compressor piston and the gas displacer) operating on a fixed charge of non-condensible gas (helium), cause the gas to be compressed, cooled, expanded and reheated repeatedly so that in a certain portion of the machine, net refrigeration at very low temperature is achieved.The ideal Stirling cycle is "Carnot-limited", that is, its efficiency for cold pro­ duction is the highest possible. It is not the intent of this paper to review the theory of operation, for that the reader is referred to the original references, Reference 1 and 2, or a number of later papers. Rather, we shall discuss new embodiments which have been developed recently from innovations introduced by the same research laboratories which "discovered" Stirling cycle technology in this century, the Philips Research Laboratories of N.V. Philips in Holland.A) Life Limiting Considerations in Stirling CoolersTne most significant life-limiting mechanisms in conventional miniature Stirling refrigerators are wear and contamination.Figure 1 is a schematic illustrating a conventional emoodiment of the Stirling refrigerator, and useful for pointing out its weaknesses. The two pistons must reciprocate with a fixed phase relationship for the refrigerator to function; as shown, this is accomplished by using a kinematic mechanism to convert rotary motion from a motor/crankshaft into the required linear motion of the pistons. The mechanism requires bearings which in turn require lubricants. Since the dynamic seal around the piston is not hermetic, lubricant vapors can migrate and be "gettered" in the low temperature region of the machine.90