DIRECT OBSERVATION OF THE TURBULENT emf AND TRANSPORT OF MAGNETIC FIELD IN A LIQUID SODIUM EXPERIMENT

Rahbarnia, K.; Brown, Benjamin; Clark, Mike; Kaplan, Elliot; Nornberg, M. D.; Rasmus, Alexander; Taylor, N.Z.; Forest, C. B.; Jenko, F.; Limone, Angelo; Pinton, Jean‐François; Plihon, Nicolas; Verhille, Gautier

doi:10.1088/0004-637x/759/2/80

Cited by 20 publications

(23 citation statements)

References 39 publications

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“…The installation of an equatorial baffle was found to reduce the amplitude of the largest-scale turbulent eddies and hence the α-effect [5]. In the same set-up, Rahbarnia et al [6] measured the local emf directly, finding contributions from both α and β, but with a dominant β-effect. They reported an increase in magnetic diffusivity of about 30%.…”

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confidence: 97%

“…We therefore extend our previous approach [9] to take into account the contribution of turbulent fluctuations to the mean magnetic field. Following earlier attempts [3,4,6], we choose to invert for α and β, but since we expect that fluctuations will strongly depend upon the intensity of the mean magnetic field, we allow them to vary with radius. Note that time-varying magnetic signals are particularly sensitive to the effective magnetic diffusivity, hence to β [4,13].…”

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Turbulence Reduces Magnetic Diffusivity in a Liquid Sodium Experiment

2014

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The contribution of small scale turbulent fluctuations to the induction of mean magnetic field is investigated in our liquid sodium spherical Couette experiment with an imposed magnetic field. An inversion technique is applied to a large number of measurements at Rm ≈ 100 to obtain radial profiles of the α and β effects and maps of the mean flow. It appears that the small scale turbulent fluctuations can be modeled as a strong contribution to the magnetic diffusivity that is negative in the interior region and positive close to the outer shell. Direct numerical simulations of our experiment support these results. The lowering of the effective magnetic diffusivity by small scale fluctuations implies that turbulence can actually help to achieve self-generation of large scale magnetic fields.The Earth, the Sun and many other astrophysical bodies produce their own magnetic field by dynamo action, where the induction of a magnetic field by fluid motion overcomes the Joule dissipation. In all astrophysical bodies, the conducting fluid undergoes turbulent motions, which can also significantly affect the induction of a largescale magnetic field by either enhancing it or weakening it. It is therefore of primary interest to quantify the role of these fluctuations in the dynamo problem.The induction equation for the mean magnetic field B reads:where U is the mean velocity field, η = (µ 0 σ) −1 is the magnetic diffusivity (involving the magnetic permeability µ 0 and the conductivity of the fluid σ), and E = ũ ×b is the mean electromotive force (emf) due to small scale fluctuating magneticb and velocityũ fields. The relative strength between the inductive and dissipative effects is given by the magnetic Reynolds number Rm = U L/η (U and L are characteristic velocity and the characteristic length-scale). When there is a scale separation between the turbulent fluctuations and the mean flow, we can follow the mean-field theory and expand the emf in terms of mean magnetic quantities: E = α B − β∇ × B . For homogeneous isotropic turbulence, α and β are scalar quantities. α is related to the flow helicity and results in an electrical current aligned with the mean magnetic field, whereas β can be interpreted as a turbulent diffusivity effectively increasing (β > 0) or decreasing (β < 0) electrical currents. The effective magnetic diffusivity η ef f = η + β can have tremendous effects on energy dissipation and on dynamo action by reducing or increasing the effective magnetic Reynolds number Rm ef f = U L/η ef f .However, direct determination of these small-scale contributions remains a challenging issue for experimental studies and numerical simulations.The first generation of dynamo experiments were designed to show that turbulent flows with strong geometrically-imposed helicity could self-generate their own magnetic fields. Since the success of Riga [1] and Karlsruhe [2] dynamos, several other liquid metal experiments have sought to overcome the effects of magnetohydrodynamic turbulence in less constrained, more geophysically rele...

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Turbulence Reduces Magnetic Diffusivity in a Liquid Sodium Experiment

2014

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“…In particular, Tu (1992, 1993) discovered evidence that the electromotive force follows a power-law spectrum in the solar wind (indicating turbulent electromotive force) and is not simply proportional in the mean magnetic field. The electromotive force is, in contrast to the limited space plasma studies, one of the primary study targets in the laboratory pinch plasmas (Fontana et al, 2000;Ji and Prager, 2002) and in the liquid sodium experiment (Rahbarnia et al, 2012). This paper is motivated by a wish to fill the gap in the application of the electromotive force between laboratory and space plasma studies.…”

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confidence: 99%

Evaluation of electromotive force in interplanetary space

Narita¹,

Vörös²

2018

Ann. Geophys.

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Abstract. Electromotive force plays a central role in the turbulent dynamo mechanism and carries important information on the nature of the turbulent fields. In this study, an analysis method is developed for the electromotive force and the transport coefficients such as those for the α effect (coefficient α) and the turbulent diffusivity (coefficient β). The method is applied to a magnetic cloud event observed by the Helios 2 spacecraft in the inner heliosphere. The electromotive force is enhanced together with the magnetic cloud event by 1 or 2 orders of magnitude, suggesting that the magnetic field can locally be amplified in the heliosphere, presumably for a short time.

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“…In addition, laminar dynamo action (Rm > Rm crit , Re < $ 1000) can be studied in the absence of fluid turbulence, which can suppress it. 8 To experimentally test theories about the dynamo mechanism in a plasma, the device operates in the flow-dominated regime as opposed to the magnetically dominated regime. Such an experiment requires not only that Rm be large but also that the flow energy dominates over the magnetic energy.…”

Section: Introductionmentioning

confidence: 99%

The Madison plasma dynamo experiment: A facility for studying laboratory plasma astrophysics

et al. 2014

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The Madison plasma dynamo experiment (MPDX) is a novel, versatile, basic plasma research device designed to investigate flow driven magnetohydrodynamic instabilities and other high-b 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 3 of nearly magnetic field free plasma that is well confined and highly ionized (>50%). At present, 8 lanthanum hexaboride (LaB 6 ) 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 power is planned for additional electron heating. The LaB 6 cathodes are positioned in the magnetized edge to drive toroidal rotation through J Â B torques that propagate into the unmagnetized core plasma. Dynamo studies on MPDX require a high magnetic Reynolds number Rm > 1000, and an adjustable fluid Reynolds number 10 < Re < 1000, in the regime where the kinetic energy of the flow exceeds the magnetic energy (M 2 A ¼ ðv=v A Þ 2 > 1). 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. V C 2014 AIP Publishing LLC.

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DIRECT OBSERVATION OF THE TURBULENT emf AND TRANSPORT OF MAGNETIC FIELD IN A LIQUID SODIUM EXPERIMENT

Cited by 20 publications

References 39 publications

Turbulence Reduces Magnetic Diffusivity in a Liquid Sodium Experiment

Turbulence Reduces Magnetic Diffusivity in a Liquid Sodium Experiment

Evaluation of electromotive force in interplanetary space

The Madison plasma dynamo experiment: A facility for studying laboratory plasma astrophysics

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