WAVE-FRONT DISTORTION OF ACOUSTIC BEAM IN PRESSURIZED SUPERFLUID HE-4

It has been observed that a focused ultrasonic beam in superfluid He-4 under high static pressure of 18-24 atm shows a unique nonlinear phen omenon. This phenomenon strongly depends on pressure. As a result of t he new experiment, we found that the wavefront of a spherical wave pas sing through the focal point is greatly distorted. The phase shifts di scontinuously by about pi radians in the center of the beam under 24 a tm. The main features of this effect are explained by a model that inv olves an incident component and another component whose phases differ by about pi.