REFRIGERATORS

Cryogenic refrigeration is a key supporting technology for both LTS and HTS applications. The significant requirements of a cryogenic refrigerator are low cost, reliability, efficiency, and small size.

Large LTS systems, such as high energy physics accelerators and Super-GM generators, use reverse Brayton cycle helium liquefiers. These are complex, expensive systems (the smallest units available cost in excess of $200,000), and are usually specifically designed for each application. Through a number of research projects conducted over the past 25 years, Japanese industry and particularly the High Energy Physics lab (KEK) have accumulated significant experience with such systems. Components such as compressors and cold boxes are available from a variety of Japanese vendors, and there are several Japanese firms capable of overall system design and integration. In general terms, Japanese technology in this area is very comparable to that available in the United States or in Europe.

Of more relevance to the present topic, however, is the progress in the past several years on smaller Gifford McMahon cycle refrigerators. These systems are widely used in the process industry as a component of vacuum pumps, but only recently have manufacturers begun to optimize their characteristics for use with superconducting magnets. The resulting devices have made possible both the conduction-cooled magnets and the "no refill" MRI systems discussed above.

4 K GM Refrigerators

GM cycle refrigerators employ a regenerator in the coldhead that transfers heat to and from the helium gas working fluid. The regenerator must have very high heat capacity and low thermal conductivity parallel to the direction of gas flow. In the 50-80 K temperature range, single-stage GM refrigerators usually use screens or pellets of copper for the regenerator. Two-stage GM refrigerators designed for use in vacuum pumps operating at 20-25 K use lead in the regenerator because of its higher heat capacity in this temperature range. The lower temperature limit of lead-based regenerators has been about 10 K because of the rapidly declining specific heat of lead below this temperature.

To produce significant refrigeration below 10 K it is necessary to have a regenerator material with a much higher specific heat than lead below 20 K. This is accomplished through use of a material with a magnetic ordering transition and a resulting peak in the specific heat in this temperature range. The materials of choice are the rare earth alloys and compounds. Both Toshiba and Mitsubishi have developed successful regenerator materials. Toshiba has worked primarily with the Er₃Ni system and MELCO with Ho_1.5Er_1.5Ru. Both companies have applied the new refrigerators to conduction-cooled magnets, as noted above. In addition, MELCO has designed a three-stage refrigerator specifically for MRI magnets. This provides cooling at 75 K and 20 K for radiation shields and at 4.2 K for reliquefication of liquid helium. The net result is that MELCO produces an MRI magnet that is initially filled with liquid helium when it is installed but requires no further helium service during its operational lifetime. MELCO has sold over 50 of these magnets to Shimadzu, which has installed them in MRI systems throughout the Pacific Rim. Although not discussed during the site visit (Appendix C), Sumitomo has also developed a successful 4.2 K refrigerator that is now incorporated in "no refill" MRI magnets sold by General Electric in the Pacific Rim.