Studying the normal-fluid flow in Helium-II using metastable helium molecules

TL;DR

This paper demonstrates that metastable helium molecules can serve as tracers for visualizing normal-fluid flow in superfluid helium-4, using laser-induced fluorescence to observe flow patterns and measure molecular properties.

Contribution

It introduces a novel method employing metastable helium molecules as tracers for normal-fluid flow visualization in superfluid helium-4, including flow imaging and molecular lifetime measurement.

Findings

Visualization of normal-fluid jet impinging on a copper disc.

Observation of ring-shaped circulation structure.

Measurement of molecular fluorescence decay and density.

Abstract

We demonstrate that metastable helium molecules can be used as tracers to visualize the flow of the normal fluid in superfluid $^{4}$He using a laser-induced-fluorescence technique. The flow pattern of a normal-fluid jet impinging on the center of a copper disc is imaged. A ring-shaped circulation structure of the normal fluid is observed as the jet passes across the disc surface. The fluorescence signal for the molecules trapped in the circulation structure is measured as a function of time after we turn off the molecule source. The radiative lifetime and density of the molecules can be determined by fitting the measured data using a simple analytic model. We also discuss a proposed experiment on using a previously developed molecule tagging-imaging technique to visualize the normal-fluid velocity profile during the transition of quantum turbulence in a thermal counterflow channel.