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. The two-stage unit consists of a commercial single-stage GM refrigerator to precool a pulse tube stage capable of reaching a minimum temperature of 2.2 K. The pressure waves for both stages are generated separately by independent 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 of the pulse tube stage was experimentally found to be lower. Experiments were carried out with and without a regenerative tube. Neodymium was used as regenerator material in the coldest section. A cooling 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 at 56 K.