New aspects of nonlinear effects in a focused acoustic beam in pressurized superfluid 4He

Two experiments on the nonlinear propagation of the focused acoustic pulse in superfluid ⁴He were conducted. The first involved the measurement of the reflected signal intensity as a function of input power and reflector-to-lens distance under pressure of 17-24 atm with a reflection mode acoustic microscope configuration. Reflected power vs. reflector-to-lens distance plots are becoming distorted by the increase of input power under pressure of more than 18 atm. The second involved the measurement of the reflected signal intensity with a different configuration that does not have a reflector near the focal point. The observed nonlinear phenomenon is essentially caused by superfluid ⁴He itself, and not by the fact that a reflector exists