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Rotational transform

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rotational transform

(Science: radiobiology) iota = 2PI/q) Due to the combination of applied toroidal field and induced poloidal field, the magnetic field lines wind helically around the torus (and on most flux surfaces they fill the surface ergodically). The rotational transform is a measure of this helicity, and is defined as the average angle the field line shifts in the poloidal direction per complete circuit in the toroidal direction. The quantity q = 2pi / iota is known as the safety factor because of its role in stability theory.

A magnetic field configuration is said to posses rotational transform if the lines of force, after one complete circuit around the configuration (for example, a torus) do not simply close exactly on themselves, but are instead rotated through some angle about the magnetic axies.