THE AMPLITUDE DEPENDENCE OF THE VELOCITY OF NONLINEAR 2ND SOUND IN HELIUM-II NEAR T-LAMBDA

The amplitude dependence of the velocity of nonlinear second sound in Helium II, as given by the Khalatnikov nonlinear coefficient, was meas ured near T(lambda) using a single-pulse travel-time technique. Using nonplanar pulses which were amplified and focused by a spherical refle ctor, the confirmation of Khalanikov's second order two fluid model pr ediction was extended to within 7 mK of T(lambda), much closer to the critical temperature than previously reported. This study was restrict ed to single pulses of second in order to ensure that the measurement pertained only to an undisturbed, vortex-tangle-free sample of Helium II.