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A tokamak is a toroidal (doughnut-shaped) magnetic plasma confinement device, the leading candidate for producing magnetic fusion energy. The term tokamak comes from the Russian words: "toroidalnaya", "kamera", and "magnitnaya", which mean "toroidal, chamber, magnetic". The last letter g was replaced by k to avoid analogy with the word magic. 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 through the plasma.
Why doughnut shaped? The distinctive shape of the fusion reactor is necessary because of a particular property of a doughnut that a sphere (for example) does not have. Essentialy the problem is the hairy ball theorem. If a sphere has hair growing out of it then it is impossible to comb it so that no hair sticks up. However a hairy doughnut can be so combed.
This is important because a fusion reactor is a hairy doughnut with the hair being the magnetic field lines. A strand of hair that is standing on end would be an instability in the reactor.
Many tokamaks have been built and all the big machines can do something that none of the others can do.