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

The shape of first-sound waves was studied in both super fluid and normal H e-4 at pressures below and above a critical pressure P-cr. The waves were g enerated by a film heater. The heat flux density Q and the pulse duration t au were varied in the ranges from 1 to 250 W/cm(2) and from 0.3 to 10 mu s. In the superfluid phase (He-II), the bolometer detected first-sound shock waves of compression at small pressures (P < P-cr). These waves were create d at high Q due to film boiling. At high pressures (P > P-cr) the boiling i s 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 o f first-sound shock waves in superfluid helium due to film boiling, one sho uld take into account another mechanism - the thermal expansion of the flui d near the heater. (C) 2000 Elsevier Science B.V. All rights reserved.