View source for Rotational transform

The rotational transform (or field line pitch) ''&iota;/2&pi;'' 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>

:<math>\frac{\iota}{2 \pi} = \frac{d \psi}{d \Phi}</math>

where ''&psi;'' is the poloidal magnetic flux, and &Phi; the toroidal magnetic flux.