Tokamak

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A tokamak (/ˈtoʊkəmæk/; Russian: токамáк) is a device which uses a powerful magnetic field generated by external magnets to confine plasma in the shape of an axially-symmetrical torus.^[1] The tokamak is one of several types of magnetic confinement devices being developed to produce controlled thermonuclear fusion power. The tokamak concept is currently one of the leading candidates for a practical fusion reactor.^[2]

The proposal to use controlled thermonuclear fusion for industrial purposes and a specific scheme using thermal insulation of high-temperature plasma by an electric field were first formulated by the Soviet physicist Oleg Lavrentiev in a mid-1950 paper.^[3] In 1951, Andrei Sakharov and Igor Tamm proposed to modify the scheme by proposing a theoretical basis for a thermonuclear reactor, where the plasma would have the shape of a torus and be held by a magnetic field. At the same time, the same idea was proposed by unknown American scientists, but "forgotten" until the 1970s.^[4]

The first tokamak was built in 1954,^[5] and for a long time it existed only in the USSR. In 1968 the electronic plasma temperature of 1 keV was reached on the tokamak T-3, built at the I. V. Kurchatov Institute of Atomic Energy under the leadership of academician L. A. Artsimovich.^[6][7][8] British scientists from the laboratory in Culham Centre for Fusion Energy (Nicol Peacock et al.) came to the USSR with their equipment,^[9] made measurements on the T-3 and confirmed the results,^[10][11] which they had originally viewed with great scepticism. This development spurred a worldwide tokamak boom. 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.

By the mid-1960s, the tokamak designs began to show greatly improved performance. The 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. When these were also met skeptically, the Soviets invited 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.

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 (a fusion energy gain factor equal to 1) 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) and Tokamak Fusion Test Reactor (TFTR), 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. As of 2024^[update], JET remains the record holder for fusion output, with 69 MJ of energy output over a 5-second period.^[12]