Tokamak safety factor
= Tokamak safety factor
{title2=$q_s\simeq rB_\phi/(R_0B_\theta)$}
A measure of toroidal field-line turns per poloidal turn. In the circular large-aspect-ratio approximation, geometry gives $q_s\simeq rB_\phi/(R_0B_\theta)$. With <Ampère's law>, $B_\theta(a)\simeq\mu_0I_p/(2\pi a)$, so the edge value relates the field-line pitch to the toroidal plasma current. This geometrical estimate alone does not establish stability for a complete current profile.