Microscopic dynamics of quantized vortices in turbulent superfluid in the T=0 limit

Lead Research Organisation: Lancaster University
Department Name: Physics

Abstract

Turbulence is ubiquitous in nature and affects almost every aspect of our daily lives. Despite its overwhelming importance, turbulence is poorly understood, mainly because of the complexity of turbulent motion over a very wide range of length scales. Turbulence in superfluid helium, known as quantum turbulence, is special, because quantum mechanics restricts all vortices to have a single fixed value of circulation. Thus we are dealing with a dynamic tangle of vortex lines, all of the same strength. Turbulence, including its quantum variant, is an inherently non-equilibrium phenomenon: remove the driving force, and the turbulence decays.
Our goal is to confront the two remaining, mutually interconnected, challenges of quantum turbulence in the T=0 limit: (i) to observe and investigate the elementary processes occurring with individual vortex lines inside bulk tangles; (ii) explore the interaction, and its consequences, of vortex lines with solid boundaries.

(i) Below 0.5K damping of the motion of vortex lines effectively vanishes. While it is expected that vortex reconnection and deformation on a broad range of length scales are the main ingredients of their dynamics, no direct observations of these at low temperatures have been made so far. The programme will produce sequences of 2D and 3D images of vortex lines, their bundles and tangles - in different types of turbulent flow, visualized through fluorescence of either He2* excimers or dyed nanoparticles as tracers. Hence, we will obtain information on different aspects of quantum turbulence, and its distinction from classical turbulence. This new technique could revolutionize the study of quantum turbulence. As quantum turbulence mimics classical turbulence on large length scales, our direct visualization of the structure and dynamics of the region of concentrated vorticity might also make an important contribution to the understanding of intermittency in classical turbulence when coherent structures cause rare events of large amplitude.

(ii) The understanding of the dynamics of vortex tangles near solid walls is another outstanding fundamental question. The creation of quantum turbulence seems to be "seeded" by remanent vortices pre-existing in the superfluid. It was suggested that the evolution to fully-developed quantum turbulence as the amplitude of an oscillating structure increases may occur via a 2-stage process. First, shaking of the lines sloughs off a gas of small vortex rings, which reconnect to form a random tangle. This tangle itself behaves like a fluid of small viscosity undergoing laminar flow. Then at a higher velocity there is a second transition when the flow turns turbulent. We propose to test this picture experimentally.

All earlier experiments on the generation of quantum turbulence by oscillating structures have used objects with convex surfaces; the flow round them is classically unstable at a low velocity, so that the two supposed transitions are not clearly separated. In contrast, we propose experiments where the helium is inside a pill-box that oscillates about its axis, thus eliminating all flow over convex surfaces. The two transitions should then be well separated and identifiable as characteristic increases in damping. We will also illuminate the fundamental properties of the remanent vortices themselves, by investigating their pinning to microscopic protuberance. Recent measurements indicate that vortex pinning get weaker at low temperatures, perhaps through reconnections with lines of the mesh of remanent vortices. To test these results, we propose experiments in a spherical cell, a geometry in which pinned vortex loops are inherently unstable, as well as visualization of remanent vortices, both away from and near boundaries.

Publications

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Description We have demonstrated the feasibility of creating a low-frequency oscillator that is likely to be useful in the investigation of dissipation and the creation of quantum vorticies in superfluid He-4.
Exploitation Route Other people working in the same area are likely to find our new oscillator design useful.
Sectors Aerospace, Defence and Marine,Energy,Other

 
Title High amplitude cryogenic oscillator 
Description We are developing a novel kind of low-frequency torsion oscillator. It incorporates a Be-Cu torsion rod and a body made of Araldite. Because of being very light, the resonant frequency is relatively low at around 74 Hz and therefore suitable for our experiments on vortex creation in superfluid He-4. Unusually, the driving electrodes are circular, enabling the oscillator to be driven to amplitudes almost an order of magnitude higher than conventional torsion oscillators without being short-circuited by direct touches. The prototype is working well, and has been tested: at room temperature, in air and in vacuum; at 77K in vacuum; and down to about 50 mK in vacuum and in superfluid He-4. Some nonlinearity is evident at the largest amplitudes. This will be measured systematically in vacuum to provide a comparison and calibration for the measurements of vortex creation in the superfluid. 
Type Of Material Improvements to research infrastructure 
Year Produced 2019 
Provided To Others? No  
Impact None yet. 
 
Title Torsional oscillator 
Description We are developing a novel kind of low-frequency torsion oscillator. It incorporates a Be-Cu torsion rod and a body made of Araldite. Despite being very light as a result, the resonant frequency is relatively low at around 74 Hz. Unusually, the driving electrodes are circular, enabling the oscillator to be driven to amplitudes almost an order of magnitude higher than conventional torsion oscillators without being short-circuited by direct touches. The prototype is working well, but we have yet to test it in a vacuum or under cryogenic conditions. Although designed specifically for our project on dissipation and vortex creation in superfluid helium, it is clear that a very sensitive pressure gauge could be developed from the same technology. 
Type Of Technology Detection Devices 
Year Produced 2018 
Impact It is too soon for impacts. 
 
Company Name LANCASTER HELIUM LTD 
Description Lancaster Helium Ltd is a spin-out company of Lancaster University. It supplies the isotopically pure He-4 gas needed for a diversity of applications including - •Coolant gas in a nuclear reactor •The down-scattering medium for ultra-cold neutron (UCN) production •Measurement of the Landau critical velocity •Experiments on quantum turbulence •Cyclotron resonance of ions below the superfluid surface •Production of excited helium molecules (long-lived excimers) •Ultra-low-temperature experiments on solid He-4 The gas has many other possible uses. Isotopic separation is effected by heat flush in liquid helium below its superfluid transition temperature at 2.17 K. The He-4 isotopic purity achieved within the company's present machine is believed to be perfect. Thus, any He-3 in the product must arise from subsequent contamination, e.g. by the compressor or pipework or containers. The gas may contain traces of air, water and oil vapour but isotopically it is of extremely high purity. 
Year Established 2016 
Impact Sale of 12 kg of isotopically purified He-4 to Technical University of Munich (2018) for use in a nuclear reactor forming the core of their new ultra-cold neutron (UCN) source.
Website http://www.lancasterhelium.uk/