He II Cooling System Performance in the Ebasco Proof of Principle Experiment during Lambda Transition, Warmup, and Quench

Abstract

Temperature measurements in a 700 L bath of helium during cool down, the transition to He II, and operation in subcooled He II taken during the Ebasco SMES proof of principle experiment, POPE, are reported. Cooldown proceeds at a uniform rate throughout the bath, but the lambda transition appears to propagate from the heat exchanger to the current leads, which provide the largest heat load to the helium II region, over ten minutes. During warm-up and quench significant temperature stratification is observed with the bath consisting of boiling helium at the top and subcooled helium with decreasing temperature below. Both regions consist of a constant thickness while the helium level decreases as helium is vaporized at the top of the bath. The region of helium at the bottom of the dewar is at the lambda point, decreases in thickness with time, and exists for up to six hours after the He II heat exchanger is turned off. Calculations based on a model incorporating temperature dependent density and thermal conductivity of helium matches the measurements. This behavior is also observed during quench at an accelerated time scale.