Regenerative cryocoolers widely used for advanced applications, such as sensor cooling, superconducting systems and cryopumps, consist mainly of a compressor, regenerator and expander. A working fluid (generally helium gas) flows periodically in these main parts. The regenerator that is located between the compressor and expander is operated in a wide, lowtemperature range from 77 to 4 K, and a wide frequency range from one to a few hundred hertz. The regenerator is one of the most essential parts because its efficiency is directly linked to the performance of the cryocoolers. Therefore, understanding the properties of regenerators (including regenerator material) and helium are important. In this paper, the thermodynamic efficiency of a regenerator, and the temperature dependence of the specific heat of regenerator materials and pressurized helium are presented. Furthermore, several types of simulated temperature and specific heat (heat capacity) distributions within a 4 K regenerator are shown to clarify the intriguing effect of magnetic regenerator materials.
