quantum turbulence in superfluid 3 he-b at ultra low temperatures
DESCRIPTION
Quantum Turbulence in Superfluid 3 He-B at Ultra Low Temperatures. D.I.Bradley D.O.Clubb S.N.Fisher A.M.Guenault. A.J.Hale R.P.Haley M.R.Lowe C.Mathhews. I.E.Miller M.G.Ward. G.R.Pickett R.Rahm K.Zaki. Introduction Vibrating Wires in superfluid 3He-B Observation of Turbulence - PowerPoint PPT PresentationTRANSCRIPT
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Quantum Turbulence in Superfluid 3He-B at Ultra Low Temperatures.
D.I.BradleyD.O.ClubbS.N.FisherA.M.Guenault
A.J.HaleR.P.Haley M.R.Lowe C.Mathhews
•Introduction
•Vibrating Wires in superfluid 3He-B
•Observation of Turbulence
•The Spatial Extent of Turbulence
•Direct measurements of Andreev scattering from Turbulence
•Grid Turbulence
G.R.PickettR.RahmK.Zaki
I.E.MillerM.G.Ward
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3He Phase Diagram
Superfluid phases formed by Cooper pairs with S=1, L=1
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Vortices in the B-phase
Formed by a 2 phase shift around the core
superfluid flows around core with velocity,
vS=/2r
vortices are singly quantised with circulation :
=h/2m3
Superfluid is distorted in the core,
core size depends on pressure: 0~ 65nm to 15nm
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Decrease in damping at higher temperatures implies that the damping from thermal quasiparticles is reduced.
i.e. thermal quasiparticles are prevented from scattering with the detector wire.
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Quasiholes propagate through flow field
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Quasiparticles Andreev Scattered into Quasiholes with very small momentum transfer
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Fraction of flux Reflected=0.5*[1-exp(-pFv(r)/kBT)]
v(r)=/2r, =h/2m3
Shadow half Width= pF/2kBTln2
~8m @ 100K
(vortex core size 0 ~ 65nm @low P)
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Flow barrier independent of temperature below .22Tc
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Flow barrier decreases above .22Tc
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The heat input to the radiator (applied heat and heat leak) is balanced by a beam of ballistic quasiparticle excitations emitted from the radiator orifice.
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In the presence of vortices, the change in width parameter is proportional to the fraction of excitations Andreev reflected.
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Take a thin slab of homogeneous vortex tangle of unit area, line density L and thickness x
Probability of qp passing within distance r of a vortex core is L x r
Mean qp energy =kBT
Qps are Andreev scattered if pFv(r)> kBT
v(r)=2r, so qps scattered if they approach within a distance, r ~ pF /2kBT
Simple Estimate of vortex Line Density
Fraction of qps Andreev scattered after traveling x through tangle, Lx pF /2kBT
Total fraction transmitted through tangle of thickness x is exp(-x/),
~2kBT / LpF
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Decay time of vortex tangle
From Simulations by C.F.Barenghi and D.C.Samuels, PRL 89 155302 (2002)
Tangle disperses by evaporating small rings of size R~L-1/2
Rings form after a time ~1/(L) [~0.3s for our line densities]
The tangle then expands at the self induced velocity of the rings, vR
Time scale for tangle to disperse ~ S0/ vR
~5s for our line densities
VWR measurements show the tangle disperses in ~ 3-4s
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Grid Mesh: 11m rectangular wires, 40m square holes.
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SummaryTurbulence in 3He-B Generated by VWRs:• Generated above pair-breaking critical velocity vC=vL/3 ~ 9mm/s @ P=0
• Spatial extent ~2mm
• Line densities up to ~5x107m-2, line spacing ~ 150m
• Disperses on a time scale of a few seconds, explained by ‘ring evaporation’
Turbulence in 3He-B Generated by a Vibrating Grid:• Generated above a velocity ~ 1mm/s• Estimated Spatial extent ~2mm • Estimated Line densities up to ~5x108m-2, line spacing ~ 50m• Disperses on a time scale of: seconds above ~4mm/s <0.1s at lower velocities (sharp cross-over at 3.5 mm/s)
- Possible explanation: The Grid is only generating fast propagating vortex rings at low velocities which become turbulent at high velocities.