71 research outputs found
Observation of magnetocoriolis waves in a liquid metal Taylor-Couette experiment
The first observation of fast and slow magnetocoriolis (MC) waves in a
laboratory experiment is reported. Rotating nonaxisymmetric modes arising from
a magnetized turbulent Taylor-Couette flow of liquid metal are identified as
the fast and slow MC waves by the dependence of the rotation frequency on the
applied field strength. The observed slow MC wave is damped but the observation
provides a means for predicting the onset of the Magnetorotational Instability
High resolution charge-exchange spectroscopic measurements of aluminum impurity ions in a high temperature plasma
Charge-exchange recombination spectroscopy, which is generally used to measure low-Z impurities in fusion devices, has been used for measuring Al+11 and Al+13 impurities in the Madison Symmetric Torus reversed field pinch. To obtain the impurity ion temperature, the experimental emission spectrum is fitted with a model which includes fine structure in the atomic transition. Densities of these two ionization states, calculated from charge-exchange emission brightness, are used in combination with a collisional radiative model to estimate the abundance of all other charge states of aluminum in the plasma and the contribution of aluminum to the effective ionic charge of the plasma
Measurements of the magnetic field induced by a turbulent flow of liquid metal
Initial results from the Madison Dynamo Experiment provide details of the
inductive response of a turbulent flow of liquid sodium to an applied magnetic
field. The magnetic field structure is reconstructed from both internal and
external measurements. A mean toroidal magnetic field is induced by the flow
when an axial field is applied, thereby demonstrating the omega effect.
Poloidal magnetic flux is expelled from the fluid by the poloidal flow.
Small-scale magnetic field structures are generated by turbulence in the flow.
The resulting magnetic power spectrum exhibits a power-law scaling consistent
with the equipartition of the magnetic field with a turbulent velocity field.
The magnetic power spectrum has an apparent knee at the resistive dissipation
scale. Large-scale eddies in the flow cause significant changes to the
instantaneous flow profile resulting in intermittent bursts of non-axisymmetric
magnetic fields, demonstrating that the transition to a dynamo is not smooth
for a turbulent flow.Comment: 9 pages, 11 figures, invited talk by C. B. Forest at 2005 APS DPP
meeting, resubmitted to Physics of Plasma
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
Intermittent magnetic field excitation by a turbulent flow of liquid sodium
The magnetic field measured in the Madison Dynamo Experiment shows
intermittent periods of growth when an axial magnetic field is applied. The
geometry of the intermittent field is consistent with the fastest growing
magnetic eigenmode predicted by kinematic dynamo theory using a laminar model
of the mean flow. Though the eigenmodes of the mean flow are decaying, it is
postulated that turbulent fluctuations of the velocity field change the flow
geometry such that the eigenmode growth rate is temporarily positive.
Therefore, it is expected that a characteristic of the onset of a turbulent
dynamo is magnetic intermittency.Comment: 5 pages, 7 figure
Zoneamento edáfico para o sorgo (Sorghum bicolor L. Moench) no estado do Rio Grande do Sul na escala 1:250.000.
bitstream/item/168463/1/DOCUMENTO-433.pd
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