Tokamak

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. 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 for providing minimally polluting electrical power.

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 was first formulated by the Soviet physicist Oleg Lavrentiev in a mid-1950 paper. In 1951, Andrei Sakharov and Igor Tamm modified 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.

The first tokamak was built in the Soviet Union 1954. In 1968 the electronic plasma temperature of 1 keV was reached on the tokamak T-3, built at the Kurchatov Institute under the leadership of academician L. A. Artsimovich.

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 British scientists from the laboratory in Culham Centre for Fusion Energy (Nicol Peacock et al.) to the USSR with their equipment. Measurements on the T-3 confirmed the results, spurring a worldwide stampede of tokamak construction. 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 safety factor 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.

The Australian National University built and operated the first tokamak outside the Soviet Union in the 1960s. The Princeton Large Torus (or PLT), was built at the Princeton Plasma Physics Laboratory (PPPL). It was declared operational in December 1975. It was one of the first large scale tokamak machines and among the most powerful in terms of current and magnetic fields. It achieved a record for the peak ion temperature, eventually reaching 75 million K, well beyond the minimum needed for a practical fusion device.

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.

The Tokamak Fusion Test Reactor (TFTR), and the Joint European Torus (JET) performed extensive experiments studying and prefecting plasma discharges with high energy confinement and high fusion rates. TFTR discovered new modes of plasma discharges called supershots and enhanced reverse shear discharges. Jet prefected the High-confinement mode H-mode.

Both performed extensive experimebta campaigns with deuterium and tritium plasmas. As of 2025 they were the only tokamaks to do so. TFTR created 1.6 GJ of fusion energy during the three year campaign. The peak fusion power in one discharge was 10.3 MW. The peak in JET was 16 MW.

They achieved calculated values for the ratio of fusion power to applied heating power in the plasma center, Q_core of approximately 1.3 in JET and 0.8 in TFTR (discharge 80539). The achieved values of this ratio averaged over the entire plasmas, Q_DT were 0.63 and 0.28 (discharge 80539) respectively. As of 2025, a JET discharge remains the record holder for fusion output, with 69 MJ of energy output over a 5-second period.

Both TFTR and JET resulted in extensive studies of properties of the alpha particles resulting from the deuterium-tritium fusion reactions. The alpha particle heating of the plasma is necessary for sustaining burning conditions.

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.