Developments are being carried out to extend the practical use of turbomachine-based reverse Brayton cycle cryocooler technology to lower capacity applications. These developments focus primarily on cooling loads in the range of 0.35 W to 2 W at temperatures from about 35 K to 70 K in a single stage cycle with neon. The technology is readily adaptable to multistage loads with either neon or helium as the working fluid. A multistage helium cycle could provide cooling at loads of approximately 100 mW at temperatures as low as JO K. The goal is to produce systems incorporating vibration-free turbomachines with performance characteristics comparable to those that have been demonstrated for larger cycles -i.e., the 5 W 65 K single stage reverse Brayton cycle cryocooler. This paper reports recent progress on this effort, which should result in highly reliable turbomachine based cryocoolers with input powers less than 150 W.The critical components for a low capacity reverse Brayton cycle cryocooler include the expansion turbine, the compressor and the recuperator. Advanced designs presently under development for the two turbomachines use high speed gas bearings and high energy permanent magnets for electromechanical energy conversion. An alternator operating at cryogenic temperatures converts the turbine power from the expansion of the gas to electric power that is dissipated through a resistive load at room temperature. The miniature centrifugal compressor is driven by a high efficiency three-phase AC motor using features that are similar to those of the turboalternator. Recent developments on an advanced parallel plate recuperator concept promise a substantial weight and size reduction in comparison to present slotted plate heat exchangers. The paper describes test results for these components and presents performance projections for several low capacity cryocooler cycles.
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