The word tokamak means "toroidal chamber" in Russian. It is a magnetic fusion device that is in a shape of a torus (e.g. a doughnut), and which depends on external windings for generating a strong toroidal magnetic field (i.e. in the direction along the doughnut).
Poloidal magnetic fields (in the direction of the doughnut's cross-section) are created primarily by a toroidal current inside the plasma itself. This combination of toroidal and poloidal magnetic fields generates an overall nested helical structure which is necessary to keep the plasma stable.(See the image on top) The tokamak is presently the leading candidate design for a future "working" magnetic fusion device.
newton@Nuc.Berkeley.EDU