NSF World Materials Network: A Collaborative Experimental Investigation of Pure Quantum Turbulence in Superfluid He-4 at Very Low Temperatures

Turbulence in classical fluids is important and challenging both for theory and for many practical applications. The research aims to understand how classical turbulence is modified in a superfluid, in which flow is severely restricted by quantum conditions associated with the quantization of angular momentum.At higher temperatures, superfluids exhibit two-fluid behaviour, a normal fluid coexisting with the superfluid component in which the quantum effects are important. There is already strong evidence that at these temperatures turbulent structures on large length scales can be very similar to their classical counterparts, although dissipative processes acting on small scales are very different. It is suspected that a similar situation exists at very low temperatures, where the normal fluid is absent, and where simple mechanisms for the decay of the turbulence have disappeared. Our programme seeks to provide experimental evidence relating to these low temperatures, at which the fundamental behaviour of very pure forms of quantum turbulence ought to be observable.Quantum turbulence is of great intrinsic interest, and its study could lead to a better understanding of classical turbulence.The turbulence / comprised of a seemingly random tangle of quantized vortex lines / will be generated in the superfluid either by a steadily moving grid or by an oscillating grid. The mechanical behaviour of the oscillating grid will provide evidence about the nucleation of turbulence, and calorimetric observations or the detection of ion trapping will measure the turbulent decay, which reflects not only dissipation on small scales but also the overall turbulent structure. The quantitative application of the ion trapping detection technique is dependent on measurements of the trapping cross-section, currently in progress at the University of Manchester. Although the oscillating grid provides a proven technique for creating quantum turbulence in the mK temperature range, the turbulence is not well-characterised and nor is its spatial distribution known. In these senses, although technically far more demanding, the steadily moving grid is to be preferred because it will create quantum turbulence that is both well-characterised and spatially homogeneous. Some promising new, more sensitive, detection methods are appearing on the horizon, possibly including the spectroscopy of neutral excitations, and preliminary studies will be made to confirm that the excitations can indeed be trapped on quantized vortex lines.Success of the proposed experiments is dependent on the development of close interactive collaboration between Florida, Birmingham and Lancaster. The Lancaster experimental group and the Birmingham Co-Investigator are already funded by EPSRC for this work up to the end of June 2006. Their collaboration with the Florida group is being initiated by an EPSRC Visiting Fellowship for Professor Ihas to visit Lancaster during October 2005 / May 2006. The Florida group proposes to develop the work with the support of NSF Materials World Network funding; the Lancaster/Birmingham group will submit a research grant application for continuation and development of the Lancaster experimental programme early in 2006. In the present proposal they just request travel/subsistence to support their collaboration with the Florida group during the period of the proposed NSF MWN grant.