What is a Reversed Field Pinch?

Escande, Dominique

doi:10.1142/9789814644839_0009

Cited by 11 publications

(8 citation statements)

References 79 publications

(132 reference statements)

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“…The process by which the RFP velocity field u RFP is created is a conversion of magnetic energy to kinetic energy through a kink instability. 17 As expected for a RFP equilibrium, the field reversal parameter F = B z / B z decreases as a function of the pinch ratio, where B z denotes the wall averaged and B z the volume-average axial magnetic field. In the present investigation, in both the laminar and turbulent statistically steady states, the equilibrium profile is characterized by a reduced axial magnetic field at the walls B z as compared to the imposed field.…”

Section: Please Cite This Article Assupporting

confidence: 54%

“…The current profile observed in RFPs was proposed to be sustained by a dynamo regime due to continuous chaotic or turbulent motion [13][14][15] . Investigations in various regimes were reported over the last two decades [16][17][18][19][20] . More recently, a dynamo electromotive force was proposed to explain stationary non-sawtoothing regimes in tokamaks, often referred to as "flux pumping" [21][22][23] .…”

Section: Introductionmentioning

confidence: 99%

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The dynamo properties of the reversed field pinch velocity field

2022

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Reversed field pinch (RFP) is a toroidal device aiming at magnetic confinement of a plasma in order to reach conditions of thermonuclear reactions. In RFPs, the magnetic and velocity fields self-organize to a saturated state determined by their nonlinear interplay and the values of the transport-coefficients. The question addressed in this article is whether this saturated velocity field is capable of amplifying a seed magnetic field, the so-called dynamo-effect for the astrophysical community. It is shown, using numerical simulations in periodic cylinders, that the RFP velocity field can amplify a passively advected seed-field, but this is only observed for values of the magnetic Prandtl number above unity. These observations are reported for both laminar and turbulent RFP flows. We further assess the difference in behavior between a passively advected vector field and the true magnetic field and show that their difference is associated with the detailed alignment properties of the fields.

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Section: Please Cite This Article Assupporting

confidence: 54%

Section: Introductionmentioning

confidence: 99%

The dynamo properties of the reversed field pinch velocity field

2022

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“…This conjecture, together with associated theory and further extensive experimental evidence, was reviewed by Taylor (1986), and has become part of the ‘received knowledge’ concerning turbulent relaxation processes in plasma physics. Escande (2015) provides a recent review of relaxation in the RFP context, and of variants of the Taylor conjecture. An excellent review of the role of magnetic helicity in laboratory and astrophysical plasma dynamics has been provided by Berger (1999).…”

Section: Introductionmentioning

confidence: 99%

Magnetic relaxation and the Taylor conjecture

Moffatt

2015

J. Plasma Phys.

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A one-dimensional model of magnetic relaxation in a pressureless low-resistivity plasma is considered. The initial two-component magnetic field b(x, t) is strongly helical, with non-uniform helicity density. The magnetic pressure gradient drives a velocity field that is dissipated by viscosity. Relaxation occurs in two phases. The first is a rapid initial phase in which the magnetic energy drops sharply and the magnetic pressure becomes approximately uniform; the helicity density is redistributed during this phase but remains non-uniform, and although the total helicity remains relatively constant, a Taylor state is not established. The second phase is one of slow diffusion, in which the velocity is weak, though still driven by persistent weak non-uniformity of magnetic pressure; during this phase, magnetic energy and helicity decay slowly and at constant ratio through the combined effects of pressure equalisation and finite resistivity. The density field, initially uniform, develops rapidly (in association with the magnetic field) during the initial phase, and continues to evolve, developing sharp maxima, throughout the diffusive stage. Finally it is proved that, if the resistivity is zero, the spatial mean (b · ∇ × b)/b 2 is an invariant of the governing one-dimensional induction equation.

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“…and is plotted in direction and as such is akin to a Reversed Field Pinch (e.g. see Escande (2015) for a laboratory interpretation): this configuration may be of use in the study of astrophysical jets, see Li et al (2006) for example. The value k = 1/2 gives zero Bz at r = 0, and as such is the value that distinguishes the two different classes of field configuration, namely unidirectional (k ≥ 1/2) or including field reversal (k < 1/2).…”

Section: Gh Flux Tube Plus Background Field (Gh+b)mentioning

confidence: 99%

Theory of one-dimensional Vlasov-Maxwell equilibria: with applications to collisionless current sheets and flux tubes

Allanson

2017

Preprint

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Vlasov-Maxwell equilibria are characterised by the self-consistent descriptions of the steady-states of collisionless plasmas in particle phase-space, and balanced macroscopic forces. We study the theory of Vlasov-Maxwell equilibria in one spatial dimension, as well as its application to current sheet and flux tube models.The 'inverse problem' is that of determining a Vlasov-Maxwell equilibrium distribution function self-consistent with a given magnetic field. We develop the theory of inversion using expansions in Hermite polynomial functions of the canonical momenta. Sufficient conditions for the convergence of a Hermite expansion are found, given a pressure tensor. For large classes of DFs, we prove that nonnegativity of the distribution function is contingent on the magnetisation of the plasma, and make conjectures for all classes.The inverse problem is considered for nonlinear 'force-free Harris sheets'. By applying the Hermite method, we construct new models that can describe sub-unity values of the plasma beta (β pl ) for the first time. Whilst analytical convergence is proven for all β pl , numerical convergence is attained for β pl = 0.85, and then β pl = 0.05 after a 're-gauging' process.We consider the properties that a pressure tensor must satisfy to be consistent with 'asymmetric Harris sheets', and construct new examples. It is possible to analytically solve the inverse problem in some cases, but others must be tackled numerically. We present new exact Vlasov-Maxwell equilibria for asymmetric current sheets, which can be written as a sum of shifted Maxwellian distributions. This is ideal for implementations in particle-in-cell simulations.We study the correspondence between the microscopic and macroscopic descriptions of equilibrium in cylindrical geometry, and then attempt to find Vlasov-Maxwell equilibria for the nonlinear force-free 'Gold-Hoyle' model. However, it is necessary to include a background field, which can be arbitrarily weak if desired. The equilibrium can be electrically non-neutral, depending on the bulk flows.

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What is a Reversed Field Pinch?

Cited by 11 publications

References 79 publications

The dynamo properties of the reversed field pinch velocity field

The dynamo properties of the reversed field pinch velocity field

Magnetic relaxation and the Taylor conjecture

Theory of one-dimensional Vlasov-Maxwell equilibria: with applications to collisionless current sheets and flux tubes

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