In the physics of magnetically confined plasmas, a measure of magnetic field 'windiness'.

When in ideal MHD equilibrium, the magnetic flux surfaces in a plasma will be nested toroids (in a tokamak). The magnetic field will snake around these flux surfaces.

On certain flux surfaces (known as rational flux surfaces), the magnetic field eventually swallows its own tail, the safety factor q is the number of toroidal turns m divided by the number of poloidal turns n

If the magnetic field never joins up, (an ergodic flux surface) then the safety factor may be defined by the pitch of the magnetic field

where φ is the toroidal angle and θ the poloidal angle.

The safety factor profile (q profile) describes how q varies from the magnetic axis at the plasma centre to the plasma boundary. It can be controlled by careful plasma heating. In a standard plasma the q profile increases monotonically from the magnetic axis to the edge of the plasma. Optimised Shear discharges are plasmas in which the q profile is non-monotonic.

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