A tokamak fusion reactor uses a toroidal coil array to create a magnetic field capable of containing plasma at the temperatures required for fusion. As well as a primary toroidal field, transformers are used to induce a current in the plasma itself, creating a poloidal magnetic field (at right angles to the toroidal magnetic field) the two fields combine to create a total helical magnetic field, which spirals round the toriod plasma.Heating is acheived by passing a current through the plasma, neutral beam heating, or radio frequency heating.
The original reason for the use of the toroidal reactor shape was because it was the most efficient way for the plasma to be contained, as it did not require "end" fields. Tokamak fusion reactors include JET and the planned future energy-producing ITER.