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
3He Phase Diagram
Superfluid phases formed by Cooper pairs with S=1, L=1
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
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.
Quasiholes propagate through flow field
Quasiparticles Andreev Scattered into Quasiholes with very small momentum transfer
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)
Flow barrier independent of temperature below .22Tc
Flow barrier decreases above .22Tc
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.
In the presence of vortices, the change in width parameter is proportional to the fraction of excitations Andreev reflected.
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
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
Grid Mesh: 11m rectangular wires, 40m square holes.
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.