Improved Seal for a 4 K Gifford-McMahon Cryocooler
Gifford-McMahon refrigerators operating at ~ 4 K are currently used to cool SIS mixers on radio telescopes at the Berkeley-Illinois-Maryland array. The refrigerators are constructed by adding third stages onto standard 2-stage GM cryocoolers; Er3Ni spheres are used as the low temperature regenerators. The refrigeration capacity is 50 mW at 3.5 K.
Two refrigerators have been installed on telescopes thus far. The first has operated for 10000 hours. Its temperature fluctuates between 3.5 and 5.7 K, sometimes on time scales of minutes, because helium leaks intermittently past the seal on the third stage displacer. The temperature stability of the second refrigerator is much better. It uses a spring-energized lip seal (Bal Seal) which is mounted on the third stage cylinder, rather than on the displacer. This refrigerator has now operated on a telescope for 3000 hours. It maintains the SIS mixer at 3.0 K, stable to ±0.07 K over periods of days, even as it is tipped over an elevation angle range of 150 degrees.
The improved reproducibility afforded by the Bal Seal allows one to more reliably compare different regenerator materials. The refrigerator performance is almost identical with Er3Ni or neodymium spheres.
KeywordsRadio Telescope Stage Temperature Cycle Frequency Radiation Shield Cold Finger
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