Long-Term Performance of 4 K Gifford-McMahon Refrigerators on the Bima Array
Eight radio telescopes in the Berkeley-Illinois-Maryland Array (BIMA) are outfitted with 4 K Gifford-McMahon refrigerators. The refrigerators cool superconducting detectors (SIS mixers) on the radio receivers; the heat load at 4 K is 10 to 20 mW. The refrigerators were constructed by welding third stages onto standard CTI model 1020 cryocoolers; Er3Ni or ErNi spheres are used as the low temperature regenerators. The refrigerators are operated almost continuously; one has been in service for 4 years. Maintenance consists of disassembling the cold heads every 1 to 2 years to clean out water and dust.
On the telescopes the refrigerators’ third stage temperatures range from 2.7 to ~5 K. Often these temperatures are stable within ± 0.1 K for weeks, even while the refrigerators are tipped from 0 to 90 degrees as the telescopes track radio sources. Sometimes, however, the temperature creeps up by > 1 K over a period of days or weeks, then jumps back to its original value following a power interruption or “defrost” cycle. Contaminants within the cold head may possibly cause this behavior. With active temperature regulation, the SIS mixer temperatures can be stabilized to ±0.01 K despite the refrigerator fluctuations.
KeywordsRadio Telescope Heater Power Stage Temperature Optimum Bias Mixer Block
Unable to display preview. Download preview PDF.
- 2.Plambeck, R., Thatte, N., and Sykes, P., A 4 K Gifford-McMahon refrigerator for radio astronomy, in “Proc. 7th International Cryocooler Conf.,” Phillips Laboratory report PL-CP-93–1001, Kirtland AFB, NM (1993), p. 401.Google Scholar
- 3.Plambeck, R.L., Improved seal for a 4 K Gifford-McMahon cryocooler, in: “Cryocoolers 8,” R.G. Ross Jr., ed., Plenum Press, New York (1995), p. 795.Google Scholar
- 4.Vespel SP-21, DuPont Engineering Polymers, Newark, DE.Google Scholar
- 5.Purchased from Toshiba America Electronics, Westboro, MA.Google Scholar
- 6.Part number R310MB-014-SP31, Bal Seal Engineering Co., Santa Ana, CA.Google Scholar