49 research outputs found
Numerical simulations of current generation and dynamo excitation in a mechanically-forced, turbulent flow
The role of turbulence in current generation and self-excitation of magnetic
fields has been studied in the geometry of a mechanically driven, spherical
dynamo experiment, using a three dimensional numerical computation. A simple
impeller model drives a flow which can generate a growing magnetic field,
depending upon the magnetic Reynolds number, Rm, and the fluid Reynolds number.
When the flow is laminar, the dynamo transition is governed by a simple
threshold in Rm, above which a growing magnetic eigenmode is observed. The
eigenmode is primarily a dipole field tranverse to axis of symmetry of the
flow. In saturation the Lorentz force slows the flow such that the magnetic
eigenmode becomes marginally stable. For turbulent flow, the dynamo eigenmode
is suppressed. The mechanism of suppression is due to a combination of a time
varying large-scale field and the presence of fluctuation driven currents which
effectively enhance the magnetic diffusivity. For higher Rm a dynamo reappears,
however the structure of the magnetic field is often different from the laminar
dynamo; it is dominated by a dipolar magnetic field which is aligned with the
axis of symmetry of the mean-flow, apparently generated by fluctuation-driven
currents. The fluctuation-driven currents have been studied by applying a weak
magnetic field to laminar and turbulent flows. The magnetic fields generated by
the fluctuations are significant: a dipole moment aligned with the symmetry
axis of the mean-flow is generated similar to those observed in the experiment,
and both toroidal and poloidal flux expulsion are observed.Comment: 14 pages, 14 figure
Observation of a Turbulence-Induced Large Scale Magnetic Field
An axisymmetric magnetic field is applied to a spherical, turbulent flow of
liquid sodium. An induced magnetic dipole moment is measured which cannot be
generated by the interaction of the axisymmetric mean flow with the applied
field, indicating the presence of a turbulent electromotive force. It is shown
that the induced dipole moment should vanish for any axisymmetric laminar flow.
Also observed is the production of toroidal magnetic field from applied
poloidal magnetic field (the omega-effect). Its potential role in the
production of the induced dipole is discussed.Comment: 5 pages, 4 figures Revisions to accomodate peer-reviewer concerns;
changes to main text including simplification of a proof, Fig. 2 updated, and
minor typos and clarifications; Added refrences. Resubmitted to Phys. Rev.
Let
The Madison plasma dynamo experiment: a facility for studying laboratory plasma astrophysics
The Madison plasma dynamo experiment (MPDX) is a novel, versatile, basic
plasma research device designed to investigate flow driven magnetohydrodynamic
(MHD) instabilities and other high- phenomena with astrophysically
relevant parameters. A 3 m diameter vacuum vessel is lined with 36 rings of
alternately oriented 4000 G samarium cobalt magnets which create an
axisymmetric multicusp that contains 14 m of nearly magnetic field
free plasma that is well confined and highly ionized . At present, 8
lanthanum hexaboride (LaB) cathodes and 10 molybdenum anodes are inserted
into the vessel and biased up to 500 V, drawing 40 A each cathode, ionizing a
low pressure Ar or He fill gas and heating it. Up to 100 kW of electron
cyclotron heating (ECH) power is planned for additional electron heating. The
LaB cathodes are positioned in the magnetized edge to drive toroidal
rotation through torques that propagate into the
unmagnetized core plasma. Dynamo studies on MPDX require a high magnetic
Reynolds number , and an adjustable fluid Reynolds number , in the regime where the kinetic energy of the flow exceeds the magnetic
energy (vv). Initial results from MPDX are presented
along with a 0-dimensional power and particle balance model to predict the
viscosity and resistivity to achieve dynamo action.Comment: 14 pages, 13 figure
Direct observation of the turbulent emf and transport of magnetic field in a liquid sodium experiment
International audienceFor the first time, we have directly measured the transport of a vector magnetic field by isotropic turbulence in a high Reynolds number liquid metal flow. In analogy with direct measurements of the turbulent Reynolds stress (turbulent viscosity) that governs momentum transport, we have measured the turbulent electromotive force (emf) by simultaneously measuring three components of velocity and magnetic fields, and computed the correlations that lead to mean-field current generation. Furthermore, we show that this turbulent emf tends to oppose and cancel out the local current, acting to increase the effective resistivity of the medium, i.e., it acts as an enhanced magnetic diffusivity. This has important implications for turbulent transport in astrophysical objects, particularly in dynamos and accretion disks
On the magnetic fields generated by experimental dynamos
We review the results obtained by three successful fluid dynamo experiments
and discuss what has been learnt from them about the effect of turbulence on
the dynamo threshold and saturation. We then discuss several questions that are
still open and propose experiments that could be performed to answer some of
them.Comment: 40 pages, 13 figure