Thermal Hysteresis at 4 K with a GM Cryocooler

Abstract

There has been observed a phenomenon of thermal hysteresis with pneumatically-driven two-stage Gifford-McMahon (GM) cycle cryocoolersoperating at 4 K. The magnitude of the hysteresis was measured to be a maximum difference of 0.2 K at the minimum-load temperature of the second stage. The effect occurs after a cooldown from room ambient, when the asymptotic minimum-load temperature can be decreased another 0.1 K after subsequently applying a temporary additional heat load. The decreased minimum-load temperature is not transient. The effect occurs after either short or long post-cooldown stabilization periods. However, the effect is not cumulative; additional post-cooldown temporary heat loads do not continually decrease the minimum-load temperature. The effect was observed on all of several expanders of the same design, which were constructed over the course of a year. These expanders have a reciprocating displacer with internal coaxial regenerators, and the second stage regenerator contains layers of spheres of lead alloy and the popular rare-earth composite materials. The effect occurs regardless of all basic operational variables (power, frequency, pressure, stroke, etc.). This hysteresis is an irreversibility; the changes in the refrigerant helium properties are dependent and not causal. The many possible physical, thermal, and pneumatic dynamic conditions that could cause or combine to cause this effect were investigated. The initial hypotheses were: vaporization of a contaminant layer inside the heat station, and/or heat transfer hysteresis due to pressure oscillations in the expansion volume, and/or a shift in the fluid expansion volume due to gas spring hysteresis. A shift in the heat flux in this region of compressed liquid forced convection boiling is considered to be the primary cause.