Measurement of acoustical cavitation in liquid 4He

Abstract

A piezoelectric hemispherical transducer was used to focus high‐intensity ultrasound into a small volume of liquid helium. The transducer was gated at its resonant frequency of 566 kHz, with gate widths less than 1 ms, in order to minimize the effects of transducer heating and acoustic streaming. The onset of cavitation was detected with small‐angle scattering of laser light from the cavitation zone by microscopic bubbles. In the superfluid, the pressure amplitude at the focus was calculated based upon the acoustic power radiated into the liquid, the geometry of the transducer, and the nonlinear absorption of sound. Above the lambda point, this method is complicated by the scattering of sound by bubbles present in the liquid and on the surface of the transducer, and so a calibration curve for the diffracted light intensity at different pressure amplitudes and temperatures was obtained in the superfluid. The diffracted light intensity was then used to determine the acoustic pressure and extend the cavitati...