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| [[File:Toroidal coordinates.png|400px|thumb|right|A simple toroidal coordinate system]]
| | That saves me. Tanhks for being so sensible! |
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| Coordinate systems used in toroidal systems:
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| == Cartesian coordinates ==
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| (''X'', ''Y'', ''Z'')
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| <ref>[[:Wikipedia:Cartesian coordinate system]]</ref>
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| == Cylindrical coordinates ==
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| (''R'', ''φ'', ''Z''), where
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| <ref>[[:Wikipedia:Cylindrical coordinate system]]</ref>
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| * ''R<sup>2</sup> = X<sup>2</sup> + Y<sup>2</sup>'', and
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| * tan ''φ'' = ''Y/X''.
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| ''φ'' is called the ''toroidal angle'' (and not the ''cylindrical'' angle, at least not in the context of magnetic confinement).
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| Cylindrical symmetry (symmetry under rotation over φ) is referred to as ''axisymmetry''.
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| == Simple toroidal coordinates ==
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| (''r'', ''φ'', ''θ''), where
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| * ''R'' = ''R<sub>0</sub>'' + ''r'' cos ''θ'', and
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| * ''Z'' = ''r'' sin ''θ''
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| ''R<sub>0</sub>'' corresponds to the torus axis and is called the ''major radius'', while ''r'' is called the ''minor radius'', and ''θ'' the ''poloidal angle''.
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| In order to adapt this simple system better to the [[Flux surface|magnetic surfaces]] of an axisymmetric [[MHD equilibrium]], it may be enhanced by
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| <ref>[http://link.aip.org/link/?PHPAEN/5/973/1 R.L. Miller et al, ''Noncircular, finite aspect ratio, local equilibrium model'', Phys. Plasmas '''5''' (1998) 973]</ref>
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| * letting ''R<sub>0</sub>'' depend on ''r'' (to account for the [[Shafranov shift]] of flux surfaces) <ref>R.D. Hazeltine, J.D. Meiss, ''Plasma confinement'', Courier Dover Publications (2003) ISBN 0486432424</ref>
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| * adding ellipticity (''ε''), triangularity (''κ''), etc. (to account for non-circular flux surface cross sections)
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| == Toroidal coordinates ==
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| (''ζ'', ''η'', ''φ''), where
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| <ref>Morse and Feshbach, ''Methods of theoretical physics'', McGraw-Hill, New York, 1953 ISBN 007043316X</ref>
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| <ref>[[:Wikipedia:Toroidal coordinates]]</ref>
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| :<math>R = R_p \frac{\sinh \zeta}{\cosh \zeta - \cos \eta}</math>
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| :<math>Z = R_p \frac{\sin \eta}{\cosh \zeta - \cos \eta}</math>
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| where ''R<sub>p</sub>'' is the pole of the coordinate system.
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| Surfaces of constant ''ζ'' are tori with major radii ''R = R<sub>p</sub>''/tanh ''ζ'' and minor radii ''r = R<sub>p</sub>''/sinh ''ζ''.
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| At ''R = R<sub>p</sub>'', ''ζ'' = ∞, while at infinity and at ''R = 0, ζ = 0''.
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| The coordinate ''η'' is a poloidal angle and runs from 0 to 2π.
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| This system is orthogonal.
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| The Laplace equation separates in this system of coordinates, thus allowing an expansion of the vacuum magnetic field in toroidal harmonics.
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| <ref>F. Alladio, F. Crisanti, ''Analysis of MHD equilibria by toroidal multipolar expansions'', Nucl. Fusion '''26''' (1986) 1143</ref>
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| <ref>[http://dx.doi.org/10.1016/0010-4655(94)90112-0 B.Ph. van Milligen and A. Lopez Fraguas, ''Expansion of vacuum magnetic fields in toroidal harmonics'', Computer Physics Communications '''81''', Issues 1-2 (1994) 74-90]</ref>
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| == Magnetic coordinates ==
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| Magnetic coordinates are a particular type of [[flux coordinates]] in which the magnetic field lines are straight lines. Magnetic coordinates adapt to the magnetic field, and therefore to the [[MHD equilibrium]] (also see [[Flux surface]]).
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| Magnetic coordinates simplify the description of the magnetic field.
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| In 3 dimensions (not assuming axisymmetry), the most commonly used coordinate systems are:
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| <ref name='Dhaeseleer'>W.D. D'haeseleer, ''Flux coordinates and magnetic field structure: a guide to a fundamental tool of plasma theory'', Springer series in computational physics, Springer-Verlag (1991) ISBN 3540524193</ref>
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| * [[Hamada coordinates]]. <ref>S. Hamada, Nucl. Fusion '''2''' (1962) 23</ref><ref>[http://dx.doi.org/10.1063/1.1706651 J.M. Greene and J.L Johnson, ''Stability Criterion for Arbitrary Hydromagnetic Equilibria'', Phys. Fluids '''5''' (1962) 510]</ref> In these coordinates, both the field lines and current lines corresponding to the [[MHD equilibrium]] are straight.
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| * [[Boozer coordinates]]. <ref>[http://dx.doi.org/10.1063/1.863297 A.H. Boozer, ''Plasma equilibrium with rational magnetic surfaces'', Phys. Fluids '''24''' (1981) 1999]</ref><ref>[http://dx.doi.org/10.1063/1.863765 A.H. Boozer, ''Establishment of magnetic coordinates for a given magnetic field'', Phys. Fluids '''25''' (1982) 520]</ref> In these coordinates, the field lines corresponding to the [[MHD equilibrium]] are straight and so are the ''diamagnetic lines '', i.e. the integral lines of <math>\nabla\psi\times\mathbf{B}</math>.
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| These two coordinate systems are related.
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| <ref>K. Miyamoto, ''Controlled fusion and plasma physics'', Vol. 21 of Series in
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| Plasma Physics, CRC Press (2007) ISBN 1584887095</ref>
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| == References ==
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| <references />
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