Abstract
Sabo and Zimmermann found that the amplitude of resonant ac superfluid flow through the 10-μm orifice of a Helmholtz resonator is limited by sudden collapse of the oscillation, rather than by saturation predicted by the theory of thermal nucleation of vortices. This paper reports measurements on superimposed ac and dc flow. Saturation at a peak velocity essentially equal to the (intrinsic) dc critical velocity is observed when the dc velocity is larger than the peak ac velocity. In the opposite case collapse occurs and except near the transition the peak velocity preceding collapse is somewhat less than the dc critical velocity. The Helmholtz resonator is shown to be a sensitive device for detecting vortex generation in the saturation regime.