Regenerators for Liquid Hydrogen Cryocoolers

Future NASA exloration, planetary and astrophysics missions will require various enhancements in multi-stage cryocoolers. These include increased efficiency, reduced vibration and reductions in overall system mass and power consumption. For the small coolers required, pulse tube and Stirling coolers offer the best opportunities. At present, the efficiency of these coolers is limited by the effectiveness of low-temperature-stage regenerators. Below about 60 K, two factors play key roles in reducing the effectiveness of regenerators. One is that the heat capacity of most materials falls rapidly with decreasing temperature, thereby, severely limiting the number of useful materials to a few in common use. A second factor is that these commonly used materials are only available in powder form, a form known to raise reliability issues. In this effort, we will address both factors. We will use newly developed materials with high heat capacities at temperatures below 80 K, higher than that of commonly used materials. Further, we will develop novel low-temperature regenerator matrix configurations that will address both the aspect of high-efficiency and regenerator durability. Both the void fracton and the ratio of surface area to solid fracton of the regenerator matrix will be varied to achieve high efficiency.

There are a number of commercial applications which require cryocoolers that will benefit from high efficiency low-temperature regenerators. A partial list includes: - Superconducting electronics - Superconducting magnets for MRI systems - Superconducting magnets for power generation and energy storage - SQUID magnetometers for heart and brain studies - HTS filters for the communication industry - Liquefaction of industrial gases - Cryopumps for semiconductor manufacturing.

NASA is considering missions to the outer planets carrying significant amounts of propellant. Cryogenicly stored propellants offer the highest specific impulse of any chemical system. Zero boil-off (ZBO) propellant storage can directly impact these long-term exploration missions. It minimizes the launch mass such missions require. For ZBO missions high efficiency cryocoolers capable of providing cooling at 20 K are required. Other coolers being developed by NASA for use in astrophysics studies and radio-astronomy, will also greatly benefit from the londurable high efficiency low-temperature regenerators.

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