Preliminary Test Results for a 25 K Sorption Cryocooler Designed for the UCSB Long Duration Balloon Cosmic Microwave Background Radiation Experiment

Abstract

A continuous operation, vibration-free, long-life 25 K sorption cryocooler has been built and is now in final integration and performance testing. This cooler will be flown on the University of California at Santa Barbara Long Duration Balloon Cosmic Microwave Background Radiation experiment in Antarctica in December 1997. The cooler will refrigerate a focal plane composed of eight microwave feed horns, two working at 30 GHz and six at 42 GHz, with InP High Electron Mobility Transistor amplifiers. This will be the first hydride sorption cooler used to support an astrophysics experiment. As such, it is an important milestone in the development of vibration-free coolers for astrophysics applications.

The cooler uses hydrogen as the refrigerant and LaNi_4.8Sn_0.2 as the hydride sorbent. The materials, components, design margins, and assembly procedures are entirely consistent with space flight qualification requirements. Several features have been incorporated into the cooler design for long term reliability and temperature stability. A high pressure tank and low pressure sorbent bed are used to stabilize the cold end temperature to better than 1 mK/sec. Small ZrNi compressors are utilized to activate the compressor element gas-gap thermal switches without valves. To greatly enhance contamination tolerance, commercially available porous metal flow restrictors are used as the Joule-Thomson plug. Passive check valves direct the refrigerant flow, simplifying cooler operation enormously. A design description and preliminary test results are presented. Also presented are the results of flow tests conducted to determine the relationship between pressure drop and hydrogen mass flow rate as a function of temperature for a range of commercially available flow restrictors.