2ND SOUND VERY NEAR T(LAMBDA)

The results of an experimental investigation of the evolution of plana r, non-linear, second-sound pulses in superfluid He-4, to within 650 n K of the lambda-transition, are presented. A new method for extracting the second-sound velocity and damping is demonstrated. As predicted f rom two-fluid hydrodynamics, the pulses are well modeled by the soluti ons of Burgers' equation. The second-sound velocity (u20) and damping (D2) are extracted from fits of the model to the data. Damping data ar e obtained in this fashion to 3 x 10(-7) in reduced temperature at sat urated vapor pressure; nearly two decades closer to T(lambda) then any previous measurements. The superfluid density is extracted from the u 20 measurements and the critical exponent, zeta, is determined. A stud y of very large amplitude pulses near T(lambda) is also presented. The se pulses extend well beyond the range of validity of Burgers' equatio n. The amplitude of the shock that forms at the trailing edge of the p ulse is observed to saturate as a function of heater power and then de crease suddenly, as has been previously observed away from T(lambda). However, the pulse shapes are quite different from any previously obse rved.