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Register a new userExperiments on the twisted vortex state in superfluid 3He-B
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We have performed measurements and numerical simulations on a bundle of vortex lines which is expanding along a rotating column of initially vortex-free 3He-B. Expanding vortices form a propagating front: Within the front the superfluid is involved in rotation and behind the front the twisted vortex state forms, which eventually relaxes to the equilibrium vortex state. We have measured the magnitude of the twist and its relaxation rate as function of temperature above 0.3Tc. We also demonstrate that the integrity of the propagating vortex front results from axial superfluid flow, induced by the twist.
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A profound change occurs in the stability of quantized vortices in externally applied flow of superfluid 3He-B at temperatures ~ 0.6 Tc, owing to the rapidly decreasing damping in vortex motion with decreasing temperature. At low damping an evolving vortex may become unstable and generate a new independent vortex loop. This single-vortex instability is the generic precursor to turbulence. We investigate the instability with non-invasive NMR measurements on a rotating cylindrical sample in the intermediate temperature regime (0.3 - 0.6) Tc. From comparisons with numerical calculations we interpret that the instability occurs at the container wall, when the vortex end moves along the wall in applied flow.
Vortex dynamics in 3He-B is divided by the temperature dependent damping into a high-temperature regime, where the number of vortices is conserved, and a low-temperature regime, where rapid vortex multiplication takes place in a turbulent burst. We investigate experimentally the hydrodynamic transition between these two regimes by injecting seed vortex loops into vortex-free rotating flow. The onset temperature of turbulence is dominated by the roughly exponential temperature dependence of vortex friction, but its exact value is found to depend on the injection method.
The flow of quantized vortex lines in superfluid 3He-B is laminar at high temperatures, but below 0.6 Tc turbulence becomes possible, owing to the rapidly decreasing mutual friction damping. In the turbulent regime a vortex evolving in applied flow may become unstable, create new vortices, and start turbulence. We monitor this single-vortex instability with NMR techniques in a rotating cylinder. Close to the onset temperature of turbulence, an oscillating component in NMR absorption has been observed, while the instability generates new vortices at a low rate ~ 1 vortex/s, before turbulence sets in. By comparison to numerical calculations, we associate the oscillations with spiral vortex motion, when evolving vortices expand to rectilinear lines.
Vortex flow remains laminar up to large Reynolds numbers (Re~1000) in a cylinder filled with 3He-B. This is inferred from NMR measurements and numerical vortex filament calculations where we study the spin up and spin down responses of the superfluid component, after a sudden change in rotation velocity. In normal fluids and in superfluid 4He these responses are turbulent. In 3He-B the vortex core radius is much larger which reduces both surface pinning and vortex reconnections, the phenomena, which enhance vortex bending and the creation of turbulent tangles. Thus the origin for the greater stability of vortex flow in 3He-B is a quantum phenomenon. Only large flow perturbations are found to make the responses turbulent, such as the walls of a cubic container or the presence of invasive measuring probes inside the container.
In a rotating two-phase sample of 3He-B and magnetic-field stabilized 3He-A the large difference in mutual friction dissipation at 0.20 Tc gives rise to unusual vortex flow responses. We use noninvasive NMR techniques to monitor spin down and spin up of the B-phase superfluid component to a sudden change in the rotation velocity. Compared to measurements at low field with no A-phase, where these responses are laminar in cylindrically symmetric flow, spin down with vortices extending across the AB interface is found to be faster, indicating enhanced dissipation from turbulence. Spin up in turn is slower, owing to rapid annihilation of remanent vortices before the rotation increase. As confirmed by both our NMR signal analysis and vortex filament calculations, these observations are explained by the additional force acting on the B-phase vortex ends at the AB interface.
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