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safety factor

(Science: radiobiology) The number of times a field line goes around a torus the long way for each time around the short way.

In a tokamak, this number is typically near unity in the centre of the plasma and between two and 6 or 8 at the edge. So-called because it helps to determine the degree of stability the plasma has against certain instabilities. The safety factor is the inverse of the rotational transform, and can be expressed mathematically as q = (r Bt)/(R bp), where r and R are the minor and major radii of the torus, and Bt and Bp are the toroidal and poloidal magnetic fields.