The frequency dependence of critical-velocity behavior in the oscillatory flow of superfluid 4He through a 2 x 2-μm aperture in a thin foil

The critical-velocity behavior of oscillatory superfluid He-4 flow through a 2 x 2-mu m aperture in a 0.1-mu m-thick metal foil has been studied from 0.36 to 2.10 K at frequencies from less than 50 Hz up to above 1880 Hz. The pressure remained less than 0.5 bar. In early runs during which the frequency remained below 400 Hz, the critical velocity was a nearly-linearly decreasing function of increasing temperature throughout the region of temperature studied. In runs at the lowest frequencies, isolated 2 pi phase slips could be observed at the onset of dissipation. In runs with frequencies higher than 400 Hz, downward curvature was observed in the decrease of critical velocity with increasing temperature. In addition, above 500 Hz an alteration in supercritical behavior was seen at the lower temperatures, involving the appearance of large-energy-loss events. These irregular events typically lasted a few tens of half-cycles of oscillation and could involve hundreds of times more energy loss than would have occurred in a single complete 2 pi phase slip at maximum flow. The temperatures at which this altered behavior was observed rose with frequency, from similar to 0.6 K and below at 500 Hz, to similar to 1.0 K and below at 1880 Hz.