A highly effective superfluid film breaker for high heat-lift 1 K sorption coolers

Closed-cycle single-shot 4He sorption coolers are used to provide cooling below 1 K in a number of mm-wave astronomical receivers. Helium, condensed by a 4 K stage, fills a coldhead; the vapour above the liquid is then driven off by a charcoal sorption pump to reduce the pressure, and hence temperature, of the liquid until thermal equilibrium is reached with the heat load. The coldhead then maintains this equilibrium until the liquid has completely evaporated, at which point the cooler is recycled. Below the λ-transition temperature, the mass transport of superfluid 4He via the thermomechanical effect from the cold head into the pumping tube(s) has been shown to significantly reduce the hold time and cooling power of these systems. We present modelling and experimental demonstration of a mechanically simple and highly effective superfluid film breaker developed for the 1 K sorption cooler currently being used in the QUBIC experiment.