Abstract
We describe experiments on the transport of heat within a cell cooled by nuclear demagnetization of copper and containing3He-B at temperatures between 0.1 and 0.4 TC. Within the bulk liquid helium, heat is transported by ballistic quasiparticles. In this case the conventional concept of boundary resistance is not applicable to the analysis of the heat exchange between refrigerant and3He. Rather, the cell geometry plays the important role. A thermal breakthrough in the cell is observed under conditions of heat flow large enough that the spin-lattice Korringa link to the copper refrigerant nuclei becomes saturated. At this point the lattice temperature rises, allowing heat to be transferred rapidly around the cell within the copper lattice, short-circuiting the liquid helium. We develop a simple model of heat transfer by ballistic quasiparticles which describes well the principal experimental features.
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