Energy Transfer from a Pulsed Thermal Source to He II below 0.3 K

Measurements are presented on the angular distribution of energy flux radiated from a pulsed heater immersed in He II at low temperatures. Two small carbon heaters with different surface structures are located in a relatively large volume of He II maintained at temperatures in the range of 230 mK. Heater pulse widths are varied from 50 to 1000 μsec and heater power densities $W_h$ extend from 0.1 to 20 ${\mathrm{W}/\mathrm{c}\mathrm{m}}^2$. The radiated energy flux is monitored by a number of small carbon detectors which are located at distances of 1 and 2 cm from the heaters and at angles varying from 7° to 89° with respect to the heater normals. In the arrangement used, signals free of wall reflections can be observed. For values of $W_h$ below 0.5 ${\mathrm{W}/\mathrm{c}\mathrm{m}}^2$ the energy flux is found to travel at 235 ± 10 m/sec without any appreciable dispersion; the angular dependence of the flux is characteristic of the diffuse radiation of non-interacting flux of phonons emanating from the heater surface. When $W_h$ exceeds 0.5 ${\mathrm{W}/\mathrm{c}\mathrm{m}}^2$, the angular distribution is changed. Also, at higher heater power densities a second slower component appears in the radiated flux. This component travels at 130 ± 30 m/sec and is most likely associated with the helium excitations near the roton minimum. The results are discussed briefly in terms of model calculations for the effective source of excitations. It is shown that the experimental data are consistent with formation of a layer of relatively hot helium adjacent to the heater; this layer, whose shape and extent depends on the total energy flux produced by the heater, acts as the effective source of excitations radiated into the ambient low-temperature liquid helium.