Detailed Study off Nonlinear Wave Front Distortion of Focused Sound in Superfluid ~4He

摘要

We have investigated a nonlinear phenomenon which appears in a focused sound in superfluid 4He under pressure higher than 18 atm. Wave front dis-tortion of the focused ultrasound by nonlinear effect was obtained by the Fourier transform of the transducer output as a function of the defocusing length. The wave was found to suffer discontinuous wave front distortion for the input power above a certain value. This distortion is well represented by the picture that a second wave whose phase is shifted by approx. n develops, and interferes with the original wave. The amplitude of thfs second wave decreases suddenly as the pressure is lowered below 18 atm and the nonlinear wave front distortion also disappears. The possible mechanism of this second wave generation are discussed.