Rotational transform

Assuming the existence of toroidally nested magnetic flux surfaces, the rotational transform (field line pitch) is defined as

${\frac {\iota }{2\pi }}={\frac {d\psi }{d\phi }}$

where ψ is the poloidal magnetic flux, and φ the toroidal magnetic flux. Thus, ι/2π is the mean number of toroidal transits (n) divided by the mean number of poloidal transits (m) of a field line on a flux surface.

In tokamak research, the quantity q = 2π/ι is preferred (called the "safety factor"). In a circular tokamak, q can be approximated by q = r B_φ / R B_θ.