Generation of the Second and First Sound Waves by a Pulsed Heater in He-II Under Pressure

Abstract

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_α, at which the nonlinearity coefficient α for the second sound should reverse its sign, is decreasing from 1.88 to 1.58 K. Thus, at all pressures there exists a wide temperature range below T_λ where α is negative, and the temperature discontinuity (shock front) should be formed at the center of the propagating bipolar pulse of the second sound. It followed from our numerical estimations that with increasing the pressure the ratio of amplitudes of linear waves of the first and second sounds generated by a heater at small Q should increase significantly. This permitted us to observe the linear wave of heating (rarefaction) of the first sound and its transformation to the wave of cooling (compression) in He-II pressurized to 13.3 atm. Results of the measurements 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 some critical value.