Illustration of helical magnetic field lines. Particle travels along the track indicated by the yellow arrows. (Source: © vrx123 / stock.adobe.com)

The magnetic field necessary to the proper containment of the plasma, shaped like a helix wound around the torus. The particle orbiting the torus also orbits its axis. This eliminates the effect of the inhomogeneity of the toroidal magnetic field which is stronger inside the torus than on its outside. The helical magnetic field is created in stellarators by specially shaped magnetic coils. In tokamaks, it arises as a combination of the toroidal magnetic field generated by the coils and the electric current flowing through the plasma.