PINNED VORTEX DENSITY IN HE-4 FILMS PRODUCED AND DETECTED BY 3RD-SOUND RESONANCES

Resonant frequencies in a circular third sound resonator are shifted b y DC persistent current flow fields present in the resonator. Each mod e is shifted differently according its ''overlap'' with the flow. Meas urements of these shifts consequently reflect the flow field, and henc e the distribution of vortices. In addition, large amplitude circularl y polarized third sound modes are used to create and destroy the flow states. Several conclusions can be drawn from the experiments: 1) Vort ex densities of hundreds of thousands per square centimeter exist in q uasi-equilibrium; 2) Vortex distributions include large densities of b oth circulation senses polarized to different radii within the cell; 3 ) Pinning strengths are consistent with the bulk pinning model of Schw arz1 applied to a film; and 4) Third sound amplitudes with AC flows ap proaching the Feynman critical velocity create new vorticity. These re sults offer validity to the hypothesis that pinned vortices are a perv asive feature of superfluid films at low temperatures.