Studies of the critical velocity of superfluid 4He in a 2 by 2 micron aperture in a 0.1-micron-thick foil at various frequencies

Measurements of the critical velocity behavior of oscillatory superfluid 4He flow through a 2 by 2 micron square aperture in a foil 0.1 micron thick are being made at temperatures between 0.4 and 2.0 K and at pressures 0.5 bar at various frequencies including the range from 50 to 500 Hz. The purpose of this work is to look for possible frequency-dependent deviations from the behavior seen in submicron-size apertures as aperture size is increased. A report will be given of the findings to date in relation to what is known from other experiments. Preliminary results show a dependence of critical velocity on temperature at frequencies up to 500 Hz similar to that seen in submicron apertures