THE ROLE OF CONVECTION DURING THE SELF-FOCUSING OF NONLINEAR 2ND SOUND PULSES NEAR THE LAMBDA-POINT

Second sound in superfluid helium is made up of scalar temperature and vector convective velocity fields. The local sound velocity is altere d by these fields, leading to finite amplitude effects. The vector con vective field and the direction of propagation are colinear for infini te plane waves and the amplitude dependent velocity is described by th e Khalafnikov nonlinear coefficient. For finite sources of second soun d the convective velocity and the local sound velocity may not be coli near, leading to unusual nonlinear effects. We present a vector repres entation of the finite amplitude velocity of sound emitted from arbitr ary sources of second sound and apply it to a model of two point sourc es. This simple model is used to show how the linear super position of signals delayed by the scalar temperature and vector convective field s of second sound may describe a finite amplitude phenomenon: The dist ortion, due to self-focusing, of pulsed nonplanar-second sound near th e lambda point.