Tokamak

A toroidal magnetic-confinement system combining an externally imposed toroidal field with a poloidal field supplied largely by toroidal plasma current. The resulting helical field-line winding supports nested magnetic flux surfaces and improves orbit confinement compared with a purely toroidal field. Confinement remains limited by orbit effects, collisions and instabilities.

A measure of toroidal field-line turns per poloidal turn. In the circular large-aspect-ratio approximation, geometry gives $q_s\simeq r{B}_{\varphi }/(R_0{B}_{\theta })$. With Ampère's law, $B_{\theta }(a)\simeq {\mu }_0{I}_p/(2\pi a)$, 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.