Tokamak is a toroidal, (doughnut shaped) magnetic plasma confinement device, the leading candidate for magnetic fusion energy. Since no known material can withstand the hundred-million degree temperatures required for any nuclear fusion reaction only magnetic fields can confine the plasma. Confining the plasma in a magnetic field also assures a good insulation which makes it easier to heat. The term tokamak comes from the Russian words: "toroidalnaya", "kamera", and "magnitnaya", which mean "toroidal, chamber, magnetic". It was invented in the 1950s by Igor Yevgenyevich Tamm and Andrei Sakharov.
The tokamak is characterized by the use of the plasma current to generate the helical component of the magnetic field necessary for stable equilibria. This can be compared to another toroidal magnetic confinement device, the stellarator, in which all of the confining magnetic fields are produced by external coils and there is a negligible current flowing in the plasma.