Here you can read what Wikipedia say about Tokamak. but keep reading I have added some notes between these lines
A tokamak (Russian: Токамáк) is a device which uses a powerful magnetic field to confine a hot plasma in the shape of a torus. The tokamak is one of several types of magnetic confinement devices being developed to produce controlled thermonuclear fusion power. As of 2016, it is the leading candidate for a practical fusion reactor.
Tokamaks were initially conceptualized in the 1950s by Soviet physicists Igor Tamm and Andrei Sakharov, inspired by a letter by Oleg Lavrentiev. Meanwhile, the first working tokamak was attributed to the work of Natan Yavlinskii on the T-1. It had been demonstrated that a stable plasma equilibrium requires magnetic field lines that wind around the torus in a helix. Devices like the z-pinch and stellarator had attempted this, but demonstrated serious instabilities. It was the development of the concept now known as the safety factor (labelled q in mathematical notation) that guided tokamak development; by arranging the reactor so this critical factor q was always greater than 1, the tokamaks strongly suppressed the instabilities which plagued earlier designs.
The first tokamak, the T-1, began operation in 1958. By the mid-1960s, the tokamak designs began to show greatly improved performance. Initial results were released in 1965, but were ignored; Lyman Spitzer dismissed them out of hand after noting potential problems in their system for measuring temperatures. A second set of results was published in 1968, this time claiming performance far in advance of any other machine, and was likewise considered unreliable. This led to the invitation of a delegation from the United Kingdom to make their own measurements. These confirmed the Soviet results, and their 1969 publication resulted in a stampede of tokamak construction.
One funny note is that the temperature that Russians had been measured was lower than what UK scientist and they had over design in measuring. :
By the mid-1970s, dozens of tokamaks were in use around the world. By the late 1970s, these machines had reached all of the conditions needed for practical fusion, although not at the same time nor in a single reactor. With the goal of breakeven now in sight, a new series of machines were designed that would run on a fusion fuel of deuterium and tritium. These machines, notably the Joint European Torus (JET), Tokamak Fusion Test Reactor (TFTR) and JT-60, had the explicit goal of reaching breakeven.
Instead, these machines demonstrated new problems that limited their performance. Solving these would require a much larger and more expensive machine, beyond the abilities of any one country. After an initial agreement between Ronald Reagan and Mikhail Gorbachev in November 1985, the International Thermonuclear Experimental Reactor (ITER) effort emerged and remains the primary international effort to develop practical fusion power. Many smaller designs, and offshoots like the spherical tokamak, continue to be used to investigate performance parameters and other issues.
Iter is a really huge Tokamak. As I heard it will eat 50 MW energy but it’s output worth what you spend on it. It will give us 500 MW energy and that is really exciting. but there is one problem. it doesn’t work continuously and we have some time limitation. if you need more to know about Iter click here.