Generation of the second and first sound waves by a pulsed heater in He-IIunder pressure

Evolution of the shape of the second and the first sound waves, excited in superfluid He-II by a pulsed heater, with increasing a power Q of the heat pulse has been studied. It has been found that with increasing the pressure P in He-II bath up to 25 atm, the temperature T-alpha, at which the nonlin earity coefficient ct for the second sound should reverse its sign, is decr easing from 1.88 to 1.58 K. Thus, at all pressures there exists a wide temp erature range below T-lambda where or is negative, and the temperature disc ontinuity (shock front) should be formed at the center of the propagating b ipolar pulse of the second sound. It followed from our numerical estimation s that with increasing Me pressure the ratio of amplitudes of linear waves of the first and second sounds generated by a heater at small Q should incr ease significantly. This permitted us to observe the linear wave of heating (rarefaction) of the first sound and its transformation to the wave of coo ling (compression) in He-II pressurized to 13.3 atm. Results of the measure ments made at high Q and at pressures above and below the critical pressure in He-II P-cr =2.2 atm suggest that the main reason for initiation of the first sound compression waves is the strong thermal expansion of a layer of normal fluid He-I arising at the heater He-II interface at Q higher than s ome critical value.