FIRST OBSERVATION OF SELF-FOCUSING OF NONLINEAR 2ND SOUND IN SUPERFLUID-HELIUM NEAR T-LAMBDA

Nonlinear second sound was studied in a large undisturbed sample of su perfluid helium at temperatures near T-lambda. Since the source of sec ond sound was finite sized, the second-sound pulses were expected to s how the rarefaction and amplitude attenuation characteristic of diffra cting three-dimensional pulses. It was found instead that moderately h igh amplitude pulses propagating far from reflecting walls self-focuse d into stable plane wave pulses as the temperature approached the crit ical temperature T-lambda. Self-focusing is a nonlinear phenomenon pre viously unreported for a sound wave.