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\title{Experiments on the Visualisation and Properties of Quantum Turbulence}

\author{Manish Amin}
\affiliation{Department of Physics, University of Florida, Gainesville, FL 32611, USA}
\author{Roman Chapurin}
\affiliation{Department of Physics, University of Florida, Gainesville, FL 32611, USA}
\author{Shaun N. Fisher}
\affiliation{Department of Physics, Lancaster University, Lancaster LA1 4YB, UK}
\author{Jian Gao}
\affiliation{Mechanical Engineering Department, Florida State University, Tallahassee, FL 32310, USA}
\affiliation{National High Magnetic Field Laboratory, 1800 E Paul Dirac Dr, Tallahassee, FL 32310, USA}
\author{Andrei I. Golov}
\affiliation{School of Physics and Astronomy, The University of Manchester, Manchester M13 9PL, UK}
\author{Wei~Guo}
\affiliation{Mechanical Engineering Department, Florida State University, Tallahassee, FL 32310, USA}
\affiliation{National High Magnetic Field Laboratory, 1800 E Paul Dirac Dr, Tallahassee, FL 32310, USA}
\author{Gary G. Ihas}
\affiliation{Department of Physics, University of Florida, Gainesville, FL 32611, USA}
\author{Alex Marakov}
\affiliation{Department of Physics, University of Florida, Gainesville, FL 32611, USA}
\affiliation{National High Magnetic Field Laboratory, 1800 E Paul Dirac Dr, Tallahassee, FL 32310, USA}
\author{\underbar{Peter V. E. McClintock}}
\affiliation{Department of Physics, Lancaster University, Lancaster LA1 4YB, UK}
\author{Daniel N. McKinsey}
\affiliation{Department of Physics, Yale University, New Haven, CT 06520, USA}
\author{Lydia~Munday}
\affiliation{Department of Physics, Lancaster University, Lancaster LA1 4YB, UK}
\affiliation{Department of Physics, University of Florida, Gainesville, FL 32611, USA}
\author{Fatemeh~Pakpour}
\affiliation{School of Physics and Astronomy, The University of Manchester, Manchester M13 9PL, UK}
\author{Kyle Thompson}
\affiliation{Department of Physics, University of Florida, Gainesville, FL 32611, USA}
\author{William F. Vinen}
\affiliation{School of Physics and Astronomy, University of Birmingham, Birmingham B15 2TT, UK}
\author{Paul M. Walmsley}
\affiliation{School of Physics and Astronomy, The University of Manchester, Manchester M13 9PL, UK}
\author{Jihee Yang}
\affiliation{Department of Physics, University of Florida, Gainesville, FL 32611, USA}
\author{Dmitry Zmeev}
\affiliation{School of Physics and Astronomy, The University of Manchester, Manchester M13 9PL, UK}
\affiliation{Department of Physics, Lancaster University, Lancaster LA1 4YB, UK}

%\date{February 19, 2014}
%\date{\today}

\maketitle

Recent experiments on quantum turbulence \footnote{Work supported by the UK Engineering and Physical Sciences Research Council (Grant No.\ EP/H04762X/1) and the US National Science Foundation (Grant Nos.\ DMR-1007974 and DMR-1007937) under the joint EPSRC/NSF Materials World Network programme.} in He\,II are reported and discussed. They introduce and exploit the use of metastable He$_2^{\ast}$ molecules as a means of visualising and tracing the turbulence, as well as using the scattering of very small vortex rings.

Experiments in the $T \rightarrow 0$ regime address what is, in principle, a very simple regime. In the almost complete absence of normal fluid component, quantum turbulence (QT) exists in the superfluid alone. It involves the chaotic motion of tangles of linear topological defects in the superfluid order parameter field -- quantized vortices. Each vortex line contributes a flow field defined by the Biot-Savart relation, and in $^4$He the velocity circulation around each vortex is equal to the ratio of Planck's constant to the $^4$He atomic mass: $\kappa=h/m=1.00\times 10^{-3}$\,cm$^2$/s. QT consists of a dynamically evolving tangle of such quantized vortices and is essentially a macroscopic quantum phenomenon. QT decays even at the lowest temperatures, and the mechanism for such decay in superfluid $^4$He is thought to involve a Kolmogorov-like energy cascade \cite{Zakharov:92} on the Kelvin waves leading eventually to the radiation of phonons \cite{Vinen:01b}.

Characterization of QT can in principle be effected by tagging the vortex lines, and monitoring the evolution of the tangle. Micron-sized particles of solid hydrogen have been used \cite{Bewley:06} for this purpose, but are unsuitable for use at very low $T$ because their introduction heats the liquid too much and because of their relatively large size. An alternative, which we discuss here, is the use of metastable He$_2^{\ast}$ molecules. They can be created by a laser pulse or in the course of field emission from a sharp metal tip immersed in the liquid. They are bound to the vortex cores. Their arrival can be detected either by the ionization they produce in a metal electrode \cite{Surko:68} or by their fluorescence \cite{McKinsey:05} when illuminated by a laser.

A long-term aim is an experiment on pure QT in the $T \rightarrow 0$ limit where the evolution of a decaying tangle of quantized vortex lines is monitored and measured quantitatively by He$_2^{\ast}$ fluorescence. All the elements for this are now in place: methods of creating QT frictionlessly, to minimise heating \cite{Munday:14}; confirmation that He$_2^{\ast}$ molecules can be created at high densities by laser excitation and that they fluoresce within the liquid \cite{Guo:13} above 1\,K; and a demonstration %(Fig.\ 1)
that the molecules are indeed trapped on vortex cores in the low $T$ limit \cite{Zmeev:13}.

%\begin{figure}[b]
%\includegraphics[width=8cm]{decoration.eps}
%\caption{Experiments demonstrating the decoration of quantized vortices with He$_2^{\ast}$ excimer molecules. After Zmeev {\it et al} %\cite{Zmeev:13}.}
%\label{}
%\end{figure}

Recent measurements \cite{Zmeev:14} have enabled a comparison to be made between the QT created by a moving grid, by ions, and by spin-down when the Manchester rotating cryostat is stopped, and will be discussed.

%\bibliography{lowtemp}

%\end{document}

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