Quantum Turbulence in 4He, Oscillating Grids, and Where Do We Go Next?

Abstract

Experimental approaches to the study of quantum turbulence (QT) in superfluid 4He in the low temperature limit, where the normal fluid density is effectively zero, are considered. A succinct general introduction covers liquid 4He, superfluidity, critical velocities for the onset of dissipation, quantized vortex lines and QT. The QT can be created mechanically by the oscillation of wires or grids above characteristic critical velocities. The interesting dynamics of the oscillating grid are discussed. It exhibits an enhanced effective mass due to backflow, as expected from classical hydrodynamics. It is found that the critical velocity attributable to the onset of QT production rises with increasing temperature. Oscillating objects like grids or wires create QT that is not well-characterized in terms of length scale, and the QT is not spatially homogeneous. The QT can be detected by the trapping of negative ions on vortex cores. Although the corresponding capture cross-section has not yet been measured, it is evidently very small, so that the technique cannot be expected to be a very sensitive one. In the future it is hoped to create well-characterized, homogeneous QT by means of a drawn grid. Improved sensitivity in the detection of QT is being sought through calorimetric techniques that monitor the temperature rise of the liquid caused by the decay of the vortex lines.