Understanding the dynamics of quantized vortex rings in superfluid helium (He II)
Much of the universe is still unknown due to its vastness; light years, for example, eventually became the standard for measuring distance in space, because kilometers were too small as a unit. Now, as more research is done about pulsars—a type of neutron star that periodically radiates pulses of radioactive waves—measuring the passage of time in the universe is becoming more practical. To do this, however, requires an understanding of the superfluids that make up pulsars. This is where Dr. Wei Guo and his colleagues play a significant role. Their research focuses on the dynamics and quantized vortices in superfluid helium at the FSU National High Magnetic Field Laboratory.
To get a better understanding of their research and its significance, it is imperative to understand the properties of superfluids themselves, which form when gases liquify and exist without viscosity or friction. This can happen only when certain gases are heated a few degrees above absolute zero. In Dr. Guo’s research, cooling helium two degrees above absolute zero results in the formation of the “superfluid helium phase,” also known as He II. Superfluids such as He II can be considered as a mixture of two miscible fluid components: a normal viscous component and a superfluid component that has zero viscosity. Such as a two-fluid system conducts heat in a non-classical way, i.e., the normal component carries the heat away from the heat source while the superfluid component moves in opposite direction to compensate the fluid mass. This counterflow heat transfer mode is extremely effective, which makes He II a superior heat conductor. Engineers at the Maglab utilize He II to cool their superconducting magnets, and physicists at Fermilab also use He II to cool their particle accelerators.
Upon further experimentation and hands-on research with the He II superfluid component, Dr. Guo and his team highlight the formation of strings of vortex rings when He II fluid is disturbed. While these vortices are significantly small (about one angstrom in diameter), their formation and collision with one another impede the flow of the normal fluid in He II, which can result in quenching of the superconducting magnet. “This is why understanding the [behavior of] the quantized vortex rings is important to study,” Dr. Guo mentions. By inserting tracer particles in the form of hydrogen isotopes (D2), his team can visualize the motion of the quantized vortex rings and study how the vortex rings decay. Quantitative information about how the two fluid components interact can be extracted from this research.
Along with the resources provided at the FSU MagLab to collect data, Dr. Guo says that the Research Computing Center’s HPC cluster has played an important role. His team needs to conduct model simulations to compare with the experimental data in order to develop a complete understanding of the vortex dynamics. The HPC clusters helped to simulate vortex rings in action, playing a critical role in his groundbreaking research. It has significantly reduced the time needed to collect data “from months to a few weeks' time.”