Conceptual design of space qualified 4K pulse tube cryocooler

Pulse tube cryocoolers for space applications in general operate at high frequencies and high mean pressures in order to make the cooler system compact. However, with high operating frequencies, it is know that there is a limit to the lowest temperature a cryocooler can reach because of degradation of regenerator performance. There is evidence to show that a cryocooler in multistage configuration has to operate at 1 to 2 Hz to obtain temperatures in the 4K region. Therefore, in order to obtain a compact cryocooler producing 4K, we devised a multistage configuration in which a single stage 4K pulse tube is thermally coupled to a conventional cryocooler. Thereby, it is possible to operate the 4K pulse tube at a low frequency and the high-temperature stage at a high frequency and achieve a reasonably compact system for space use. We shall qualitatively investigate in this paper why a low frequency operation is required for achieving low temperature levels.