Modeling and experiments on fast cooldown of a 120 Hz pulse tube cryocooler

High frequency operation of a pulse tube cryocooler leads to reduced regenerator volume, which results in a reduced heat capacity and a faster cooldown time. A pulse tube cryocooler operating at a frequency of 120 Hz and an average pressure of 3.5 MPa achieved a no-load temperature of 50 K. The cooling power at 80 K was about 3.35 W with a cooldown time from 285 K to 80 K of about 5.5 minutes, even though the additional thermal mass at the cold end due to flanges, screws, heater, and thermometer was 4.2 times that of the regenerator. This fast cooldown is about two to four times faster than that of typical pulse tube cryocoolers and is very attractive to many applications. In this study we measure the cooldown time to 80 K for different cold-end masses and extrapolate to zero cold-end mass. We also present an analytical model for the cooldown time for different cold-end masses and compare the results with the experiments. The model and the extrapolated experimental results indicate that with zero cold-end mass the cooldown time to 80 K with this 120 Hz pulse tube cryocooler would be about 32 s.