Helium-II Refrigerator for 300 W at 1.8°K

Abstract

The concept of a superconducting linear accelerator promises various advantages compared to a conventional linear accelerator [1–4] and the construction of superconducting particle separators also seems to be attractive. In both cases, the main cryogenic problem consists in providing refrigeration for rf cavities at a very low temperature. The rf surface resistance of the cavities increases rapidly with increasing temperature while the costs for producing refrigeration increase with decreasing temperature. Theoretical and experimental investigations have shown that, under otherwise equal conditions, the costs of refrigeration for the cavities are overshadowed, at least down to a temperature of 1.85°K, by the advantages connected with the decreasing rf surface resistance. It might be possible that, with regard to the overall economics, even a temperature of 1.60°K or lower is more favorable than 1.85°K. However, regardless of the final temperature selected, a tolerance of about 0.02°K must be maintained by the refrigeration unit. Thus, refrigeration units are needed for rf cavities to supply the necessary refrigeration at a constant temperature which is below the λ point of helium.