Phase Slip in Superfluid 4He Flow Through Microscopic Apertures

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w:\awards\awards96\*.doc 9631703 Zimmerman Experiments during the last decade on the flow of superfluid liquid 4He through micron and submicron size apertures have given new insight into the first appearance of dissipation in the flow at the critical velocity. In particular, discrete "phase-slip" events have been identified which are thought to represent the creation and evolution of single quantum vortices in the superfluid. The present work is intended to supply some of the additional information needed to test models of the creation and evolution processes. Further, it will study the transition to a different type of critical velocity behavior that is customarily observed in apertures and tubes appreciably larger than one micron in diameter. %%% This work is directed at understanding the fundamental flow properties of superfluid liquid helium. The study requires temperatures near absolute zero, where physical systems often show simpler and sometimes unusual properties. In the case of liquid helium, its purity and molecular simplicity and the appearance of a remarkable degree of order in its flow have made it a superb system for testing fundamental ideas about the properties and behavior of fluids. The present studies concern the first appearance of disorder in the flow and the accompanying dissipation of energy as the velocity is increased above a certain level, phenomena analogous to the onset of instabilities and turbulence in the flow of ordinary fluids. An integral part of this work is the training of graduate students in physics in a broad range of experimental and research skills. ***