The tokamak is the most investigated and furthest advanced configuration for the magnetic cage of a fusion plasma. In a tokamak the plasma is confined by two superposed magnetic fields: firstly by a toroidal field produced by external coils and secondly by the field of a ring current flowing in the plasma. The field lines in the combined field are then helical. In addition, the tokamak requires a third, vertical field that fixes the position of the current in the plasma vessel and shapes the plasma edge.
The plasma current is normally induced by a transformer coil in the plasma. A tokamak therefore does not operate continuously, but in pulsed mode like the transformer. Pulse times of a few hours are anticipated in a future power plant. For technical reasons, however, a power plant has to operate in continuous mode and so methods of producing a continuous current are being investigated. In contrast, the entire field in the stellarator is externally produced.