The BIMA cryocooler is a 3-stage Gifford-McMahon cryocooler developed
at U.C. Berkeley to cool SIS (Superconductor-Insulator-Superconductor)
mixers on radio telescopes. The cryocooler is basically a CTI model
1020 cold head with a third stage welded onto the end.
Spheres of Er3Ni or HoCu2 (both obtained from Toshiba America Electronics) are used
as the regenerator material in the third stage. The cooling capacity
is roughly 50 mW at 4.2 K.
Papers describing the cryocooler:
"A 4 K Gifford-McMahon Refrigerator for Radio Astronomy," R. Plambeck,
N. Thatte, and P. Sykes, 1993, Proceedings of the 7th International
Cryocoolers Conference (Air Force Philips Lab publication PL-CP-93-1001),
p. 401. scanned pdf
"Improved Seal for a 4 K Gifford-McMahon Cryocooler," R.L. Plambeck
1995, in Cryocoolers-8 (R.G. Ross, Jr., ed.), New York: Plenum,
p. 795. pdf
"Long Term Performance of 4 K Gifford-McMahon Refrigerators on the
BIMA Array," R.L. Plambeck 1998, in 'Advances in Cryogenic Engineering,'
vol 43 (P. Kittel, ed.), New York: Plenum, pp. 1815-1821. (presented at
the Cryogenic Engineering Conference, Portland, Oregon, 1997 Jul)