Secondary Pulse Tubes and Regenerators for Coupling to Room-Temperature Phase Shifters in Multistage Pulse Tube Cryocoolers
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Multistage pulse tube cryocoolers require separate phase shifters for each stage. For sufficiently high frequency and acoustic power, the inertance tube is commonly used for such phase shifting. For Stirling-type, multistage pulse tube cryocoolers, the warm end of the coldest pulse tube is often heat sunk to the cold end of a warmer stage rather than at room temperature to improve the figure of merit for the pulse tube and/or to achieve a larger phase shift with a cold inertance tube. The use of a secondary pulse tube or regenerator between the main pulse tube and a phase shifter allows the phase shifter to operate at room temperature where space is more readily available. It also allows for the use of commercially available pressure oscillators as expanders. The secondary regenerator amplifies the acoustic power, so a room temperature inertance tube may perform as well as a cold one. A secondary pulse tube transfers acoustic power to room temperature without amplification, so a rather small warm expander or displacer can provide the optimum phase shift even in a low-power cryocooler. In this paper, we model the behavior of these secondary pulse tubes and regenerators using REGEN3.3 and present results to assist in selecting the optimum geometry and the optimum characteristics for the expander. We show that acoustic power flows from cold to hot in such systems can be modeled with REGEN3.3 by changing the flow phase by 180 degrees.