Damping of intrinsic temperature oscillations in a 4 K pulse tube cooler by means of rare earth plates

Abstract

Regenerative cryocoolers show oscillations of the refrigeration temperature that result from the periodic expansion of the working fluid (He). In case of cryogen-free operation of sensitive superconducting devices the temperature oscillations can be rather disturbing because of the associated variation of critical device parameters. The oscillations can be damped by increasing the thermal mass attached to the cold end. Here we report on the damping of temperature oscillations by making use of the high specific heat of rare-earth alloys, such as ErNi. Tests were performed on an in-house made 4 K pulse tube cooler. The damping is accomplished by ErNi plates of different thickness that were installed between the 4 K stage and the device mounting platform. With a 3.6 mm thick ErNi plate the temperature oscillation at 4 K was reduced by a factor of 19 from 275 mK to 14.5 mK, which is sufficiently low to operate an AC Josephson voltage standard. © 2008 American Institute of Physics.