Reconnections of quantized vortex rings in superfluid $^4$He at very low temperatures

TL;DR

This study investigates how quantized vortex rings in superfluid helium collide and reconnect at very low temperatures, revealing discrete flight time steps and a maximum ring density due to collisions, aligning with theoretical predictions.

Contribution

It provides experimental evidence of vortex ring reconnections, secondary loop formation, and density limitations, confirming theoretical models at near-zero temperatures.

Findings

Discrete steps in flight time distribution observed.

Maximum vortex ring density capped at 500 cm^{-2} R^{-1}.

Experimental results agree quantitatively with theory.

Abstract

Collisions in a beam of unidirectional quantized vortex rings of nearly identical radii $R$ in superfluid $^4$He in the limit of zero temperature (0.05 K) were studied using time-of-flight spectroscopy. Reconnections between two primary rings result in secondary vortex loops of both smaller and larger radii. Discrete steps in the distribution of flight times, due to the limits on the earliest possible arrival times of secondary loops created after either one or two consecutive reconnections, are observed. The density of primary rings was found to be capped at the value $500{\rm \,cm}^{-2} R^{-1}$ independent of the injected density. This is due to collisions between rings causing piling-up of many other vortex rings. Both observations are in quantitative agreement with our theory.