Using a cylindrical piezoelectric transducer to focus ultrasound in superfluid helium

Ultrasound in bulk liquid and superfluid helium has been used for multiple studies, including generation of quantum turbulence, and inducing homogenous or heterogenous cavitation. Both hemispherical and planar ultrasonic transducers have been used in the past, which optimize the focusing volume and the degree of focusing. In this paper, we demonstrate the application of a cylindrical piezoelectric transducer to achieve a linear focusing configuration. We have developed a new method to calibrate the pressure generated by the ultrasonic transducer, where we measured the threshold ultrasound drive at which mist was observed. The results were compared with those from a hemispherical geometry. The linear focusing configuration was further demonstrated to observe the cavitation of single electron bubbles arranged in a linear array. Our experiments are relevant to studies in liquid helium that require large pressure oscillations in controlled volumes.