Rotationally induced dissipation in superfluid helium

In an attempt to make a superfluid ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ analog of the radio-frequency superconducting quantum interference device (SQUID) that could be used as a gyroscope, we measured the critical velocity of the ac superflow through an orifice 5 μm in radius. The orifice was supported by a septum placed inside a hollow torus filled with liquid helium. The superflow through the orifice was induced by rotating the whole torus. The torus was the inertial member of a high-quality-factor torsion pendulum forced to oscillate at its resonance frequency. The occurrence of dissipation was detected by analyzing the oscillator motion. The principle on which this device works is analyzed, and results are reported of experiments in which quite reproducible critical oscillation amplitudes—well accounted for by intrinsic vortex formation—were obtained. Depending on the value of the forcing torque, the critical behavior manifested itself either as the occurrence of sudden collapses of the oscillation amplitude or as a steady excess dissipation. The energy involved in the process was estimated. Temperature dependencies of the critical amplitude up to 2.115±0.003 K were measured. The results were found to be in agreement both with the ac data obtained with the Helmholtz-resonator technique and with the data obtained with dc flow measurements.