Performance of a single-stage 4 K pulse tube cooler with neodymium regenerator precooled with a single-stage GM refrigerator

A study on the experimental performance of a hybrid cooler is presented. Th e two-stage unit consists of a commercial single-stage GM refrigerator to p recool a pulse tube stage capable of reaching a minimum temperature of 2.2 K. The pressure waves for both stages are generated separately by independe nt rotary valves using the same 6 kW helium compressor. This allows one to run each stage at its optimum frequency. The GM refrigerator operates at a fixed frequency of 2 Hz while the optimum frequency for the pulse tube stag e was experimentally found to be lower. Experiments were carried out with a nd without a regenerative tube. Neodymium was used as regenerator material in the coldest section. A cooling power of 0.26 W at 4.2 K was achieved at the pulse tube stage with an additional heat load of 50 W at the GM stage a t 56 K. (C) 1999 Elsevier Science Ltd. All rights reserved.