Measurement of the dynamo effect in a plasma

Ji, Hantao; Prager, S. C.; Almagri, A. F.; Sarff, J. S.; Yagi, Y.; Hirano, Y.; Hattori, Kenichi; Toyama, Hiroshi

doi:10.1063/1.871989

Cited by 52 publications

(48 citation statements)

References 29 publications

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“…In the low density plasmas, the MHD dynamo dominates while at the highest density, the diamagnetic dynamo dominates. Although the underlying mechanisms for the transition are still unclear, this observation is consistent [26] with the early measurements in a dense plasma where no significant MHD dynamo was detected [18].…”

Section: Diamagnetic Dynamosupporting

confidence: 78%

The Alpha Dynamo Effects in Laboratory Plasmas

Ji¹,

Prager²

2001

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A concise review of observations of the α dynamo effect in laboratory plasmas is given. Unlike many astrophysical systems, the laboratory pinch plasmas are driven magnetically. When the system is overdriven, the resultant instabilities cause magnetic and flow fields to fluctuate, and their correlation induces electromotive forces along the mean magnetic field. This α-effect drives mean parallel electric current, which, in turn, modifies the initial background mean magnetic structure towards the stable regime. This drive-andrelax cycle, or the so-called self-organization process, happens in magnetized plasmas in a time scale much shorter than resistive diffusion time, thus it is a fast and unquenched dynamo process. The observed α-effect redistributes magnetic helicity (a measure of twistedness and knottedness of magnetic field lines) but conserves its total value. It can be shown that fast and unquenched dynamos are natural consequences of a driven system where fluctuations are statistically either not stationary in time or not homogeneous in space, or both. Implications to astrophysical phenomena will be discussed.

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Section: Diamagnetic Dynamosupporting

confidence: 78%

The Alpha Dynamo Effects in Laboratory Plasmas

Ji¹,

Prager²

2001

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“…The agreement is striking, and confirms earlier measurements with a complex Langmuir probe. 7 However, the same measurement made deeper into the plasma, around the q = 0 surface at r/a ≈ 0.80, do not exhibit the same agreement with simple Ohm's law modeling. The cause of this disagreement is currently unknown; it may be an instrumental artifact resulting from perturbation of the plasma as the probe is inserted more deeply, or it may be that terms other than v ˜ H B ˜ are required to balance Ohm's law.…”

Section: A Previous Workmentioning

confidence: 70%

Measurement of core velocity fluctuations and the dynamo in a reversed-field pinch

et al. 1999

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“…Not all liquid-metal experiments have had such results: Frick et al have reported [13] that the mean flow accounts for all magnetic fields in their torus-shaped gallium experiment, and Peffley, Cawthorne and Lathrop [14] have observed no such effects. It should also be noted that an α-effect has been observed in the core of magnetically-confined plasmas [15,16].In this Letter we report measurements of the magnetic field induced by applying an axisymmetric magnetic field to a turbulent, axisymmetric flow of liquid sodium. An induced dipole moment is measured which cannot be generated by the mean flow, indicating the presence of a turbulent EMF.…”

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confidence: 96%

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confidence: 99%

Observation of a Turbulence-Induced Large Scale Magnetic Field

et al. 2006

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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 ω-effect). Its potential role in the production of the induced dipole is discussed. Many stars and planets generate their own nearlyaxisymmetric magnetic fields. Understanding the mechanism by which these fields are generated is a problem of fundamental importance to astrophysics. These dynamos are sometimes modeled using two components: a process which generates toroidal magnetic field from poloidal field and a feedback mechanism which reinforces the poloidal field [1]. The first process is easily modeled in an axisymmetric system: toroidal differential rotation of a highly-conducting fluid sweeps the pre-existing poloidal field in the toroidal direction creating toroidal field. This phenomenon, known as the ω-effect, is efficient at producing magnetic field and has been observed experimentally [2,3,4]. The second ingredient to the model is more subtle, as toroidal currents must be generated to reinforce the original axisymmetric poloidal field. Cowling's theorem [5] excludes the possibility of an axisymmetric flow generating such currents so some symmetry-breaking mechanism is required.The usual mechanism invoked [6] is a turbulent electromotive force (EMF), E = ṽ ×b , whereby small scale fluctuations in the velocity and magnetic fields break the symmetry and interact coherently to generate the large scale magnetic field. This EMF is sometimes expanded [7] in terms of transport coefficients about the mean magnetic field: E = αB + β∇ × B + γ × B; α is characterized by helicity in the turbulence, β by enhanced diffusion and γ by a gradient in the intensity of the turbulence. α is of particular interest as it results in current flowing parallel to a magnetic field, and when coupled with the ω-effect can generate the toroidal currents needed to reinforce the poloidal field. Experimental evidence for mean-field EMFs (such as the α-effect) in turbulent flows has been scarce. Three experiments, relying on a laminar α-effect, have generated an EMF [8] and dynamo action [9,10], but heavilyconstrained flow geometries were used to produce the needed helicity; the role of turbulence was ambiguous. Experiments with unconstrained flows have provided evidence for turbulent EMFs, though not the turbulent α- effect. Reighard and Brown [11] have attributed a measured reduction in the conductivity of a turbulent flow of sodium to the β-effect. Pétrélis et al. have observed [12] distortion of a magnetic field similar to an α-effect (currents generated in the direction of an applied magnetic field) and postulate that turbulence may be responsible for...

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Measurement of the dynamo effect in a plasma

Cited by 52 publications

References 29 publications

The Alpha Dynamo Effects in Laboratory Plasmas

The Alpha Dynamo Effects in Laboratory Plasmas

Measurement of core velocity fluctuations and the dynamo in a reversed-field pinch

Observation of a Turbulence-Induced Large Scale Magnetic Field

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