483 research outputs found
Thermal Detection of Turbulent and Laminar Dissipation in Vortex Front Motion
We report on direct measurements of the energy dissipated in the spin-up of
the superfluid component of 3He-B. A vortex-free sample is prepared in a
cylindrical container, where the normal component rotates at constant angular
velocity. At a temperature of 0.20Tc, seed vortices are injected into the
system using the shear-flow instability at the interface between 3He-B and
3He-A. These vortices interact and create a turbulent burst, which sets a
propagating vortex front into motion. In the following process, the free energy
stored in the initial vortex-free state is dissipated leading to the emission
of thermal excitations, which we observe with a bolometric measurement. We find
that the turbulent front contains less than the equilibrium number of vortices
and that the superfluid behind the front is partially decoupled from the
reference frame of the container. The final equilibrium state is approached in
the form of a slow laminar spin-up as demonstrated by the slowly decaying tail
of the thermal signal.Comment: 12 pages, 5 figures, to appear in Journal of Low Temperature Physic
Kelvin-Helmholtz instability of AB interface in superfluid 3He
The Kelvin-Helmholtz instability is well-known in classical hydrodynamics,
where it explains the sudden emergence of interfacial surface waves as a
function of the velocity of flow parallel to the interface. It can be carried
over to the inviscid two-fluid dynamics of superfluids, to study different
types of interfaces and phase boundaries in quantum fluids. We report
measurements on the stability of the phase boundary separating the two bulk
phases of superfluid 3He in rotating flow, while the boundary is localized with
the gradient of the magnetic field to a position perpendicular to the rotation
axis. The results demonstrate that the classic stability condition, when
modified for the superfluid environment, is obeyed down to 0.4 Tc, if a large
fraction of the magnetic polarization of the B-phase is attributed to a
parabolic reduction of the interfacial surface tension with increasing magnetic
field.Comment: 14 pages, 14 figure
Vortex core contribution to textural energy in 3He-B below 0.4Tc
Vortex lines affect the spatial order-parameter distribution in superfluid
3He-B owing to superflow circulating around vortex cores and due to the
interaction of the order parameter in the core and in the bulk as a result of
superfluid coherence over the whole volume. The step-like change of the latter
contribution at 0.6Tc (at a pressure of 29bar) signifies the transition from
axisymmetric cores at higher temperatures to broken-symmetry cores at lower
temperatures. We extended earlier measurements of the core contribution to
temperatures below 0.2Tc, in particular searching for a possible new core
transition to lower symmetries. As a measuring tool we track the energy levels
of magnon condensate states in a trap formed by the order-parameter texture.Comment: 13 pages, 10 figures, submitted to proceedings of the QFS2010
conferenc
Measurement of Turbulence in Superfluid 3He-B
The experimental investigation of superfluid turbulence in 3He-B is generally
not possible with the techniques which have been developed for 4He-II. We
describe a new method by which a transient burst of turbulent vortex expansion
can be generated in 3He-B. It is based on the injection of a few vortex loops
into rotating vortex-free flow. The time-dependent evolution of the quantized
vorticity is then monitored with NMR spectroscopy. Using these techniques the
transition between regular (i.e. vortex number conserving) and turbulent vortex
dynamics can be recorded at T ~ 0.6 Tc and a number of other characteristics of
turbulence can be followed down to a temperature of T ~ 0.4 Tc.Comment: 31 pages, 10 figure
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