Experimental and Numerical Investigation of the Dynamics of Interacting Vortex Rings in Superfluid Helium

Abstract

This thesis focuses on the dynamics of the vortex rings and how they interact with each other in superfluid helium. A pulse of charged vortex rings (CVRs) is injected into the experimental cell for different pulse lengths, voltages and temperatures. It is shown that the properties of the large charged tangle near the injection tip, which releases CVRs by reconnections, present little voltage-dependence or temperature-dependence. In the zero temperature limit, the experimental time of flight agrees with the analytical calculations of an isolated CVR at low drive voltages. At drive voltages above 50 V, reconnections start to occur, which leads to the production of small rings, the wider spread of the radii of the CVRs and the change of dominant charge carriers to charged vortex tangles. At finite temperatures, when mutual friction cannot be ignored, many of the CVRs are dissipated before reaching the collector on the other side of the cell. The interactions between a pair of vortex rings, both circular and deformed, have been simulated using vortex filament method and the exact Biot-Savart law. Depending on the impact parameter, the rings can reconnect to produce one larger and one smaller rings or to merge into one large deformed loop. The interaction with a secondary deformed loop, created from previous ring collision, has a relatively high probability of generating small rings less than half of the size of the incoming circular ring, compared to the interaction between two circular rings. It is also shown that the electric field has a smoothing effect on the deformed vortex rings, which explains why the vortex rings in experiments behave like perfectly circular rings even though they should be deformed after being released by the charged tangle near the tip.

Date of Award	31 Dec 2015
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Andrei Golov (Supervisor) & Paul Walmsley (Supervisor)