title: Producing and imaging a thin line of He*₂ molecular tracers in helium-4.
abstract: Cryogenic helium-4 has long been recognized as a useful material in fluids research. The unique properties of helium-4 in the gaseous phase and the normal liquid phase allow for the generation of turbulent flows with exceptionally high Reynolds and Rayleigh numbers. In the superfluid phase, helium-4 exhibits two-fluid hydrodynamics and possesses fascinating properties due to its quantum nature. However, studying the flows in helium-4 has been very challenging largely due to the lack of effective visualization and velocimetry techniques. In this article, we discuss the development of novel instrumentation for flow-visualization in helium based on the generation and imaging of thin lines of metastable He*₂ tracer molecules. These molecular tracers are created via femtosecond-laser field-ionization of helium atoms and can be imaged using a laser-induced fluorescence technique. By observing the displacement and distortion of the tracer lines in helium, quantitative information about the flow field can be extracted. We present experimental results in the study of thermal counterflow in superfluid helium that validate the concept of this technique. We also discuss anticipated future developments of this powerful visualization technique.
authors.affiliation: Mechanical Engineering Department, Florida State University, Tallahassee, Florida 32310, USA.
authors.forename: J
authors.initials: J
authors.lastname: Gao
authors.affiliation: National High Magnetic Field Laboratory, 1800 East Paul Dirac Drive, Tallahassee, Florida 32310, USA.
authors.forename: A
authors.initials: A
authors.lastname: Marakov
authors.affiliation: Mechanical Engineering Department, Florida State University, Tallahassee, Florida 32310, USA.
authors.forename: W
authors.initials: W
authors.lastname: Guo
authors.affiliation: National High Magnetic Field Laboratory, 1800 East Paul Dirac Drive, Tallahassee, Florida 32310, USA.
authors.forename: B T
authors.initials: BT
authors.lastname: Pawlowski
authors.affiliation: Mechanical Engineering Department, Florida State University, Tallahassee, Florida 32310, USA.
authors.forename: S W
authors.initials: SW
authors.lastname: Van Sciver
authors.affiliation: Department of Physics, University of Florida, Gainesville, Florida 32611, USA.
authors.forename: G G
authors.initials: GG
authors.lastname: Ihas
authors.affiliation: Department of Physics, Yale University, New Haven, Connecticut 06515, USA.
authors.forename: D N
authors.initials: DN
authors.lastname: McKinsey
authors.affiliation: School of Physics and Astronomy, University of Birmingham, Birmingham B15 2TT, United Kingdom.
authors.forename: W F
authors.initials: WF
authors.lastname: Vinen