Frequency dependence of critical velocity behavior in oscillatory flow of superfluid helium-4 through a small aperture

This thesis describes a series of experimental runs to study the critical velocity behavior of superfluid ;In these runs, the critical velocity was determined over the temperature range 0.36 K to 2.10 K. The critical velocity exhibited significant temperature dependence at all frequencies investigated. At frequencies below about 500 Hz, the critical velocity fell with increasing temperature in a near-linear fashion up to about 1.8 K, typical of "intrinsic" critical velocity behavior. At higher frequencies, the critical velocity versus temperature developed more and more downward curvature, resulting in less temperature dependence at low temperatures than at high temperatures.;At frequencies above 500 Hz, the response at temperatures below approximately 1.1 K was characterized by large fluctuations. These fluctuations corresponded to discrete dissipation events which persisted over tens of oscillator cycles and could dissipate hundreds of times the energy dissipated by a single quantum vortex evolving so as to cross all the lines of potential flow through the aperture. It is believed that the intrinsic critical velocity behavior at low frequency can be accounted for by thermally-activated nucleation of independent vortex half-rings which cross the potential flow through the aperture as they evolve. However, the large dissipation events which prevail at high frequencies below 1.1 K represent an interesting dissipation mechanism that merits further investigation.