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The rotational transform (or field line pitch) ''ι/2π'' is defined as the number of poloidal transits per single toroidal transit of a field line on a toroidal flux surface. | |||
The definition can be relaxed somewhat to include field lines moving in a spatial volume between two nested toroidal surfaces (e.g., a stochastic field region). | |||
Assuming the existence of toroidally nested magnetic [[Flux surface|flux surfaces]], the rotational transform on such a surface may also be defined as | |||
<ref>[http://link.aps.org/doi/10.1103/RevModPhys.76.1071 A.H. Boozer, ''Physics of magnetically confined plasmas'', Rev. Mod. Phys. '''76''' (2004) 1071]</ref> | |||
where ''ψ'' is the poloidal magnetic flux, and | :<math>\frac{\iota}{2 \pi} = \frac{d \psi}{d \Phi}</math> | ||
where ''ψ'' is the poloidal magnetic flux, and Φ the toroidal magnetic flux. | |||
== Safety factor == | |||
In [[Tokamak|tokamak]] research, the quantity ''q = 2π/ι'' is preferred (called the "safety factor"). | In [[Tokamak|tokamak]] research, the quantity ''q = 2π/ι'' is preferred (called the "safety factor"). | ||
In a circular [[ | In a circular tokamak, | ||
''q | the equations of a field line on the flux surface are, approximately: | ||
<ref>K. Miyamoto, ''Plasma Physics and Controlled Nuclear Fusion'', Springer-Verlag (2005) {{ISBN|3540242171}}</ref> | |||
:<math>\frac{r d\theta}{B_\theta} = \frac{Rd\varphi}{B_\varphi}</math> | |||
where <math>\phi</math> and ''θ'' are the [[Toroidal coordinates|toroidal and poloidal angles]], respectively. | |||
Thus <math>q = m/n = \left \langle d\varphi /d\theta \right \rangle </math> can be approximated by | |||
:<math>q \simeq \frac{r B_\varphi}{R B_\theta}</math> | |||
Where the poloidal magnetic field <math>{B_\theta}</math> is mostly produced by a toroidal plasma current. The principal significance of the safety factor ''q'' is that if <math>q \leq 2</math> at the last closed flux surface (the edge), the plasma is [[:Wikipedia:Magnetohydrodynamics|magnetohydrodynamically]] unstable.<ref>Wesson J 1997 Tokamaks 2nd edn (Oxford: Oxford University Press) p280 {{ISBN|0198509227}}</ref> | |||
In [[Tokamak|tokamaks]] with a [[divertor]], ''q'' approaches infinity at the [[separatrix]], so it is more useful to consider ''q'' just inside the separatrix. It is customary to use ''q'' at the 95% flux surface (the flux surface that encloses 95% of the poloidal flux), ''q<sub>95</sub>''. | |||
== See also == | |||
* [[Magnetic island]] | |||
* [[Magnetic shear]] | |||
== References == | |||
<references /> | |||