Development of a thermal switch for faster cool-down by two-stage cryocooler

Abstract

A gas-gap type thermal switch is developed to reduce the initial cool-down time of cryogen free magnets. The switch is a closed cylinder that contains several pairs of axial fins in staggered array and is filled with a gas. The switch connects the first and the second stages of a two-stage cryocooler (ON state) to take advantage of the large refrigeration capacity at the first stage. At cryogenic temperatures, however, the thermal isolation (OFF state) can be achieved without any external actuation, because the gas is frosted and the corresponding vapor pressure is significantly decreased. A detailed heat transfer analysis that takes into account the phase change, the gas convection in continuum or molecular state, and the axial conduction in the fins is presented. Based upon the analysis, a new configuration for the switch is proposed the transfers more heat during ON state and less heat during OFF state compared to the conventional structure. The two switches are fabricated and tested with a two-stage Gifford–McMahon cooler. The experiment shows that the newly developed gas-gap switch achieves a superior thermal performance.