Generation of first sound by a heater in super fluid and normal 4He

Abstract

The shape of first-sound waves was studied in both super fluid and normal 4He at pressures below and above a critical pressure P cr. The waves were generated by a film heater. The heat flux density Q and the pulse duration τ were varied in the ranges from 1 to 250 W/ cm 2 and from 0.3 to 10 μs . In the superfluid phase (He-II), the bolometer detected first-sound shock waves of compression at small pressures ( P< P cr). These waves were created at high Q due to film boiling. At high pressures ( P> P cr) the boiling is suppressed. At small Q we then observed waves of rarefaction. At higher Q the waves of rarefaction were replaced by waves of compression. These are due to overheating of the helium near the heater into the normal state. In the normal phase (He-I) the bolometer detected waves of compression at all pressures. Thus, in addition to the commonly accepted model of generation of first-sound shock waves in superfluid helium due to film boiling, one should take into account another mechanism – the thermal expansion of the fluid near the heater.