EVOLUTION OF IMPLODING CYLINDRICAL 2ND SOUND-WAVES IN SUPERFLUID HE-4

The use of cylindrical imploding pulses allows to produce locally larg er temperature and vortex line densities and provides also information on the interplay between thermodynamic and geometric factors. Above a certain fairly low level of temperature perturbation second sound sho ck waves followed by a superfluid vorticity field are generated. To un derstand the flow and heat transfer it is therefore of great interest to measure both the spatial and the temporal evolution of the temperat ure and of the vortex line density. The temperature was measured using superconducting bolometers and a newly developed method to deduce the vortex line density from the amplitude variation of short test pulses was used. The observed influence of the intensity and duration of the heat input and initial vortex line density level on the flow characte ristics will be presented. The experimental observations will be compa red with our numerical calculations and also in the case of very small heat pulses (acoustic case) with analytical calculations.