This project consists of an experimental study of the first onset of dissipation in the flow of superfluid 4He through an orifice of micron or submicron dimensions as the rate of flow surpasses a critical threshold. Measurements will be made of the magnitude of the critical rate of flow and its dependence on temperature, pressure, and shape of orifice and of any statistical rounding of the onset as a function of temperature, in order to gain information about the fundamental processes involved, for which no satisfactory model now exists. Use will be made of a small mechanical resonator capable of resolving single-quantum-vortex events in the oscillatory flow of superfluid through an orifice that is fabricated by means of electron-beam lithography and ion- beam milling or reactive plasma etching. Studies will be made using both a single-hole configuration and a closed-loop configuration analogous to an rf-biased superconducting quantum interference device.
