First-sound rarefaction and compression waves in superfluid He-II

The evolution of the form of first-sound waves, excited in superfluid He-II by a pulsed heater, with increasing power Q of the perturbing heat pulse i s investigated. In liquid compressed to 13.3 atm, a first-sound rarefaction wave (wave of heating) is observed, which transforms into a compression wa ve and then into a compression shock wave as Q increases, i.e., the change in the conditions of heat transfer at a solid-He-II interface can be judged according to the change in the form of the sound wave. It follows from our measurements that in He-II compression waves are excited at pressures P gr eater than or equal to 1 atm primarily as a result of the thermal expansion of a normal He-I liquid layer arising at the He-II-heater interface for po wer Q above a critical level. (C) 1999 American Institute of Physics. [S002 1-3640(99)00910-X].