Visualization Technique for Determining the Structure Functions of Normal-Fluid Turbulence in Superfluid Helium-4

Metastable helium molecules have been shown to be ideal tracers for the study of normal-fluid flow in superfluid He-4 above 1 K. These molecules can be visualized using a laser-induced-fluorescence technique. In order to extract quantitative information of the flow field, practical techniques need to be developed to study the density evolution of the molecules with a well defined initial distribution. In an on-going experiment, we use a focused femtosecond laser pulse to create a thin line of molecules in liquid helium. Such a line of molecules can be easily imaged. Studying the drift and distortion of the molecular line in a turbulent flow shall allow us to measure the instantaneous flow velocity field and hence determine the structure functions of the normal-fluid turbulence.