A high capacity completely closed-cycle 250 mK 3He refrigeration system based on a pulse tube cooler

We present the design of a completely closed-cycle refrigeration system. The system consists Of four stages; the first two, 40.5 and 2.7 K, are provided by a pulse tube cooler, the third and fourth stages by helium adsorption refrigerators. The adsorption refrigerators use He-4 and He-3 with base temperatures of 700 and 250 mK, respectively. Performance tests including some with the cooler rotated from the vertical showed that with a load of 15 muK, a temperature below 286 mK can be maintained for 72 h. (C) 2004 Elsevier Ltd. All rights reserved.