Turbulence in a Pure Superfluid

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Turbulence, the chaotic swirling motion of a fluid, is often described as the most important unsolved problem of classical physics. Yet turbulence is something that everyone has some familiarity with, from getting bumped around while flying on an aircraft to the water gushing out of the taps while running a bath. It occurs in fluid flows on all scales, from the microscopic to the galactic and has been studied intensively for well over a century but a full understanding remains elusive. Part of the problem is due to the constantly changing nature of the vortices, or eddies, that make up the turbulence in a classical fluid (such as water).Turbulent flow in a pure superfluid, such as 4He cooled to within half a degree of absolute zero is an ideal model system for investigating turbulent phenomena. Quantum mechanics dictates that the only way such a fluid can display rotational motion is through the creation of very fine (0.1 nm diameter) identical filaments where all the vorticity is concentrated and around which the fluid circulation has to take a particular fixed value. These objects are quantized vortices and the turbulent flow is completely described by a messy tangle of these lines.My proposal seeks to answer the fundamental question as to how this special type turbulence decays. There is no frictional mechanism due to viscosity, such as occurs in a classical fluid, so it is believed that dissipation occurs due to sound being emitted from high frequency waves on individual vortex lines but this needs to be checked experimentally. How energy can be transferred from the large scales (centimetres) where the fluid is stirred down to these small length (nanometres) scales is even less well understood and there are several competing theoretical ideas. Thus, even though turbulence takes a conceptually simple form in this fluid, there is plenty of new physics to explore, making it ripe for new experimental research.To significantly advance our understanding, I will perform several different types of experiment that will probe the turbulence over a huge range of length scales, spanning six orders of magnitude. This will involve monitoring how the density of vortex lines in the tangle decays with time by observing how a beam of micron-sized vortex rings is scattered by the turbulent tangle. Homogeneous turbulence will be generated by suddenly stopping a rotating container which will induce superfluid flow through a grid. In addition, sensitive calorimetry will be used to measure the heat released due to dissipation at microscopic scales, providing insight into the decay mechanism. I will also develop techniques to discover what happens on the scale of individual lines, such as by looking how initially straight vortex lines behave when their ends are shaken. This will allow a type of wave turbulence, where energy is transferred to shorter wavelength waves due to non-linear interactions, to be probed. The final type of experiment will check whether an exotic type of probe particle, metastable helium molecules, can be trapped on the cores of the vortex lines. If so, then in the future these particles could be used to visualize the turbulent vortex tangles. The common feature of all these experiments is the need to rotate the apparatus to create rectilinear vortex lines (during steady rotation) and generate turbulence (by suddenly stopping). Thus, a new rotating millikelvin cryostat will be constructed through the extensive refurbishment of an old rotating cryostat. The new state-of-the-art instrument will be vital for the proposed experiments on quantum turbulence, but it will also be capable, in the future, of probing many of the mysteries that still remain in our understanding of liquid and solid helium.