Drop Levitation Experiment

The dynamics of quantized vortex lines in a pure superfluid is a topic of interest in a wide range of research areas. For example, the nucleation of vortices through the phase transition can serve as a model for the formation of cosmological strings [1-2]. In a type II superconductor, the magnetic field lines are trapped in vortices, and the motion of these vortices can lead to energy dissipation and result in non-zero resistance [3-4]. A heavy neutron star consists of a solid crust and a liquid core of neutron-pair superfluid, and the de-pinning of vortex lines in the superfluid core has been invoked to explain the appearance of the glitches in neutron star rotation [5-6]. In quantum turbulence research, a key question is how the energy of a tangle of vortices decays in a pure superfluid with zero viscosity. It has been proposed that the energy can flow through a cascade of wave excitations on the vortex lines (Kelvin waves) [7-8], but this theory still awaits experimental verification. A systematic study of vortex-line dynamics promises broad significance spanning multiple physical science disciplines. In superfluid helium-4, vortex lines can be directly visualized by imaging tracer particles trapped on the lines. However, producing tracers in helium at low temperatures and imaging the trapped tracers remains challenging, and the container walls can often affect the vortex-line motion. This project employs a levitated helium-4 drop as the working system, in which the vortices can be produced via fast evaporative cooling and controllable drop rotation. These vortices will be decorated with metastable He₂ molecular tracers which can be imaged via laser-induced fluorescence (See Fig. 1). The objectives of this project include a thorough investigation on how the appearance of vortices can affect the stability of a rotating superfluid drop and an in-depth study of the evolution of a vortex tangle in a wall-free environment. This study will improve our understanding of superfluid drop morphology and will advance our knowledge about the turbulence dissipation mechanism in pure superfluid.

Figure 1: Schematic of the experimental apparatus inside the optical cryostat for visualizing quantized vortex lines in a magnetically levitated helium drop.

We have obtained an optical cryostat with a specially designed superconducting magnet for helium levitation. This magnet cryostat was built by our collaborator Prof. Maris’ group and was used to study the coalescence of helium drops [9]. A levitated drop of 1 cm in diameter can be quickly evaporatively cooled to below 0.6 K without the need of expensive dilution refrigerator [10]. The drop can be doped with He₂^* molecules which will bind to the vortex cores at this low temperature [11], allowing visual study of vortices in pure superfluid (see Fig. 1). However, it has been over 10 years since the last operation of this magnet cryostat, and it arrived with many leaks and no documentation on its structure. So far, we have successfully figured out the structure of the magnet cryostat, fixed some leaks, and conducted a number of cooling down tests. Our next step is to energize the superconducting coil and to test drop levitation.

A levitated helium drop may also serve as a low temperature matrix for doping a variety of atoms, molecules, van der Waals complexes, etc. High-resolution spectroscopy of the dopant particles can be made which allows the chemical properties and the relaxation dynamics of the dopant particles to be studied. The magnet system can be used to levitate other cryogenic liquids, i.e. liquid hydrogen. Studying fluid connection and management in zero gravity is valuable for the design of future propellant depots for on-orbit cryogenic propellants transfer and storage.

Reference:

[1]  D.R. Tilley and J. Tilley, Superfluidity and superconductivity. (A. Hilger; Published in association with the University of Sussex Press, Boston, 1986), 2nd ed.
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