Critical velocities for superfluid-helium flow through a small hole

An experimental study of the flow of superfluid helium through a small hole (6-10 μ diameter) between 0.3 and 1.7 °K is reported. The hole is inside a small volume protected by a porous filter. Two distinct stable-flow regimes are observed: a fast velocity (up to 1000 cm/sec) which decreases as ${\left(\frac{{\rho }_s}{\rho T}\right)}^{\frac{1}{2}}$ between 0.5 and 1.7 °K (in this case the dissipation mechanism is probaby the thermal excitation of vortex rings), and a slow temperature-independent velocity which increases as the square root of the chemical-potential difference, for which vortex emission from a source filament pinned in the flow is given as a model.