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Title: Investigation of quantum turbulence in superfluid ⁴He using injected ions and He₂* molecules in the zero temperature limit
Author: Pakpour, Fatemeh
ISNI:       0000 0004 5364 5670
Awarding Body: University of Manchester
Current Institution: University of Manchester
Date of Award: 2014
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The decay of quantum turbulence in superfluid (_^4)He in the zero temperature limit wasinvestigated. Quantum turbulence was created by impulsive spin-down from a specific angular velocity to rest. A measurement of the vortex line density was performed using the scattering of charged vortex rings. The vortex line density decayed as L ∝ t^(-3/2) which is the characteristic of quasiclassical turbulence forced at large length scales. The interaction of metastable spin-triplet 〖He〗_2^* molecules with quantized vortex lines in superfluid (_^4)He at temperatures below 200 mK was studied too. The molecules were generated during an injection of electrons from a sharp metal tip at high voltage. They were detected as a current into a metal collector after ionization upon colliding with the collector surface. The detected current was suppressed by even a small rotation indicating the trapping of the molecules on quantized vortices. The presence of (_^3)He impurities at the level of 0.3 ppb strongly suppressed the detected signal. The temperature dependence of the detected signal showed a sharp peak, most probably associated with the condensation of (_^3)He atoms on vortex cores. The time of flight of the molecules as a function of temperature was measured. It was observed that there are three regimes for 〖He〗_2^* molecules transportation in superfluid (_^4)He : ballistic regime for T < 100 mK, diffusive for T > 200 mK, and an intermediate regime between them. The vortex lines were created by either rotation or ion injection. The trapping diameter of the molecules on quantized vortices was found to be 96 ± 6 nm at pressure of 0.1 bar and 27 ± 5 nm at 5.0 bar. It was also demonstrated that a tangle of vortices moving in superfluid (_^4)He are capable of conveying the 〖He〗_2^* molecules through the drift region.
Supervisor: Golov, Andrei Sponsor: School of Physics and Astronomy, University of Manchester
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Quantum turbulence ; ?He?_2^* molecules ; Superfluid helium