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Title: A study of quantum turbulence in super fluid 3He-B using vibrating structures
Author: Jackson, Martin James
Awarding Body: Lancaster University
Current Institution: Lancaster University
Date of Award: 2011
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Turbulence plays a large role in our everyday experiences, however, due to its enormous complexity, it is not fully understood. The hydrodynamic description of turbulence is greatly simplified by having zero viscosity, a property unique to superfluids. At sufficiently low temperatures, liquid helium is in a pure superfluid state providing us with an ideal medium to investigate turbulence. This thesis details experiments studying quantum turbulence using vibrating resonators in the B-phase of superfluid 3He below O.2Tc. At such low temperatures, there is a small number of ambient thermal quasi particle excitations which are highly ballistic, with intrinsic mean free paths approaching kilometre length scales. These few remaining quasi particles provide an ideal, non-invasive method to probe vortices and quantum turbulence in superfluid 3He-B via a process known as Andreev reflection. A macroscopic object moving through a superfluid will nucleate vorticity and create excitations once the Landau critical velocity is exceeded. In superfluid 3He, the Landau critical velocity is 27 mm/s at saturated vapour pressure. Both vorticity nucleation and the creation of excitations results in the increased damping of a vibrating object. We present damping measurements on various vibrating resonators in Superfluid 3He-B below O.2Tc and discuss the interplay between pair-breaking and vortex production, in contrast to superfluid 4He. We present evidence suggesting that in 3He-B, vortex production and pair-breaking are linked and that the onset of excess damping due to vortex production of a vibrating structure may be suppressed by submerging the resonator in vorticity produced by one of its neighbours. We investigate quantum turbulence generated by a vibrating grid resonator. The vortex line density and spatial dependence of quantum turbulence is probed by neighbouring vibrating wire resonators operating at low velocities. At low velocities, the grid emits ballistic vortex rings. As the grid velocity increases, the ring density increases, leading to the formation of a vortex tangle from the cascade of vortex ring collisions and reconnections. Using vortex line density fluctuations, we observe a 1-5/3 dependence in the power spectral density; a signature of the Kolmogorov spectrum in classical turbulence. At higher frequencies, the spectra show a 1-8 dependence, pointing to an extra dissipation mechanism. Using a beam of thermal ballistic quasiparticles and bolometric techniques, we investigated the transmission of excitations through a turbulent flow field in order to measure the absolute vortex line density. We have observed the Andreev reflection of the quasiparticle beam as vortex rings and the subsequent vortex tangle crosses the radiator's line-of-sight. From these measurements, we can estimate the propagation speed of the vortex tangle and infer the vortex line density.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available