| dc.description |
The propagation of phonons in liquid <sup>4</sup>He is considered as a function of the number of phonons injected, the ambient pressure, the propagation distance and the ambient temperature. It is shown that there are four main groups of behaviour. The first and simplest is at high pressures, P>19 bar, where apart from some phonon-roton scattering near the heater, the propagation is ballistic and independent of distance and temperature for T<0.3 K. the second group is at low pressures and low injected phonon densities, where the behaviour is dominated by spontaneous decay and can be modelled satisfactorily. The third group is at low pressures and moderate injected phonon densities, where there is interaction within the beam between the high- and low-frequency phonons. The fourth group is at low pressures and high injected phonon densities, where interactions within the beam are so strong that there is frequency up-conversion to both medium and high phonon energies. The up-conversion process is suggested to be a combination of three- and four-phonon processes. At low pressures, the high-frequency phonons are attenuated by scattering with thermal phonons; however, the dependence of the attenuation on ambient temperature decreases as the injected phonon density increases, because the up-conversion reduces the effective distance for decay processes. |
