Cycle design for the ISABELLE helium refrigerator

The superconducting magnets for the ISABELLE storage ring/accelerator are designed to be operated at 3.8 K using a forced-flow supercritical helium cooling system. The ISABELLE refrigerator has been designed subject to these special requirements. The design output is 13.65 KW of refrigeration below 4.2 K (for cooling the magnet and distribution system), 55 KW at 55 K (to cool heat shields for the whole system) and 100 g/s of liquefaction (for magnets power leads cooling). The system incorporates a sub-cooler section that produces liquid helium at 5.3 atm and 2.6 K and circulates it through the loads, and a Claude-type main refrigerator section. The main refrigerator section has five stages of cooling, with four of them below liquid nitrogen temperature. Liquid nitrogen precooling is not used. With 60% isothermal compressors the efficiency of the refrigerator system will be about 26% of Carnot.