At temperatures below about 0.3T
c
, heat is transported through 3He by ballistic quasiparticles. This suggests an interpretation of the exp
(-Δ/k
T) variation of the effective boundary resistance between 3He-B and the coolant in our experiments. While most quasiparticles generated by a heat source are reflected back by collisions with the walls, a small fraction will disappear into the interstices between the sheets of sinter-coated refrigerant and are reabsorbed. Hence the equilibrium quasiparticle density near the heater should be simply proportional to the heat applied, in approximate agreement with experiment. These ideas are confirmed in experiments with a new cell. Quasiparticles generated at the far end of the sinter channels do not penetrate into the experimental volume, and no thermometer response is observed until the lattice of the refrigerant is heated sufficiently for heat to be transported by the alternative channel of electronic conduction.
Ballistic quasiparticles are produced by means of a supercritically driven wire resonator in 3He-B at temperatures below 0.25 T
C
. At 0 bar pressure the observed critical velocity of the wire is about 9 mm/s relative to the container (or about 18 mm/s relative to the maximum superfluid back flow). This velocity scales with (Δ/p
F
) as a function of pressure, confirming that pair-breaking of the superfluid is the dissipation mechanism. At higher temperatures, the scattering of existing quasiparticles leads to dissipation at much lower velocities. The ballistic propagation of a pulse of quasiparticles through the superfluid is demonstrated by the mechanical force it applies to a second wire resonator at a distance of up to 6 mm. The detector is incorporated into a SQUID circuit to yield an ultra-sensitive force detector. We observe strong signals at the detector at the same frequency as the power dissipation in the generating wire, and of a magnitude proportional to the power dissipation. The order of magnitude of these signals agrees with a simple theoretical model of ballistic quasiparticle propagation. Comparison of the received forces from a number of different driving wires suggests that the quasiparticles are emitted with almost cylindrical symmetry, with a low flux emitted parallel to the wire axis. The phase of the received signals at 0 bar indicates that the propagation is rapid. The small reduction in signal as the temperature is raised shows that the attenuation of the quasiparticle beam is low. Future experiments are proposed on the influence of Andreev scattering of quasiparticles from a super flow field.
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