Tokamak
A tokamak is a machine which uses a powerful magnetic field generated by external magnets to confine plasma in the shape of an axially symmetrical torus. The tokamak is the leading candidate of magnetic confinement fusion designs being developed to produce controlled thermonuclear fusion power.
Tokamaks use a combination of a central solenoid and toroidal and poloidal magnets to shape a ring of plasma. This is heated by a range of methods, including neutral-beam injection, electron and ion cyclotron resonance, lower hybrid resonance. Nuclear fusion may be achieved, measured by neutron detectors. Due to requiring a continuously changing magnetic field, modern tokamaks sustain "plasma discharges" on the timescales of seconds or minutes.
The world's largest tokamak by radius and plasma current is the JT-60SA in Japan. Other operational experiments include WEST in France and EAST in China. The International Thermonuclear Experimental Reactor tokamak is the primary global effort to research fusion power, under construction in France, and aimed for completion by 2034. Many smaller designs, and offshoots like the spherical tokamak, continue to be used to investigate performance parameters and other issues. Commercial fusion companies have also proposed tokamak construction in recent years, including Commonwealth Fusion Systems' SPARC design.
Tokamaks were first conceptualized by Soviet physicists Andrei Sakharov and Igor Tamm. Experiments were constructed from 1951 at Kurchatov Institute in Moscow led by Lev Artsimovich. Their 1958 T-1 device is sometimes considered the first tokamak. The same year, the USSR, US, and UK began sharing fusion research.
A "tokamak stampede" occurred after 1968, when British Culham Laboratory scientists verified high performance results of the Kurchatov Institute's T-3 tokamak. It had been demonstrated that a stable plasma equilibrium requires magnetic field lines that wind around the torus in a helix shape. The z-pinch and stellarator concepts of magnetic confinement had attempted this, but demonstrated serious instabilities. The safety factor guided tokamak development: tokamaks with q > 1 strongly suppressed these instabilities.
By the mid-1970s, dozens of tokamaks were in use around the world. The Princeton Plasma Physics Laboratory in the US became another center of research, beginning with the 1975 Princeton Large Torus. In the late 1970s, the Kurchatov Institute tested the first superconducting magnets and divertors in tokamaks, ubiquitous in modern large designs.
The Tokamak Fusion Test Reactor,
and the Joint European Torus
were the first to run experiments with an admixture of rare tritium to deuterium, previous reactors demonstrated fusion in a deuterium plasma.
Both reactors developed understanding of alpha particles' role in heating fusion plasma. JET also researched the high-confinement mode, and set magnetic confinement fusion power records standing as of 2025, for energy gain factor, total energy output, and fusion power.
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 project emerged; construction of the complex began in Cadarache, France in 2013, and tokamak assembly began in 2020.
Etymology
The word tokamak is a transliteration of the Russian word токамак, an acronym of either:or:
The term "tokamak" was coined in 1957 by Igor Golovin, a student of academician Igor Kurchatov. It originally sounded like "tokamag" — an acronym of the words "toroidal chamber magnetic", but Natan Yavlinsky, the author of the first toroidal system, proposed replacing "-mag" with "-mak" for euphony. Later, this name was borrowed by many languages.
The acronym "токамаг" ends in a г, which normally represents a voiced velar stop in Russian orthography, but in Russian this sound devoices to a voiceless velar stop at the end of a word, so both spellings are pronounced identically.
History
First steps
In 1934, Mark Oliphant, Paul Harteck and Ernest Rutherford were the first to achieve fusion on Earth, using a particle accelerator to shoot deuterium nuclei into metal foil containing deuterium or other atoms. This allowed them to measure the nuclear cross section of various fusion reactions, and determined that the deuterium–deuterium reaction occurred at a lower energy than other reactions, peaking at about 100,000 electronvolts.Accelerator-based fusion is not practical because the reactor cross section is tiny; most of the particles in the accelerator will scatter off the fuel, not fuse with it. These scatterings cause the particles to lose energy to the point where they can no longer undergo fusion. The energy put into these particles is thus lost, and it is easy to demonstrate this is much more energy than the resulting fusion reactions can release.
To maintain fusion and produce net energy output, the bulk of the fuel must be raised to high temperatures so its atoms are constantly colliding at high speed; this gives rise to the name thermonuclear due to the high temperatures needed to bring it about. In 1944, Enrico Fermi calculated the reaction would be self-sustaining at about 50,000,000 K; at that temperature, the rate that energy is given off by the reactions is high enough that they heat the surrounding fuel rapidly enough to maintain the temperature against losses to the environment, continuing the reaction.
During the Manhattan Project, the first practical way to reach these temperatures was created, using an atomic bomb. In 1944, Fermi gave a talk on the physics of fusion in the context of a then-hypothetical hydrogen bomb. However, some thought had already been given to controlled fusion, and James L. Tuck and Stanislaw Ulam had attempted such using shaped charges driving a metal foil infused with deuterium, although without success.
The first attempts to build a practical fusion machine took place in the United Kingdom, where George Paget Thomson had selected the pinch effect as a promising technique in 1945. After several failed attempts to gain funding, he gave up and asked two graduate students, Stanley W. Cousins and Alan Alfred Ware, to build something out of surplus radar equipment. This successfully operated in 1948, but showed no clear evidence of fusion and failed to gain the interest of the Atomic Energy Research Establishment.
Lavrentiev's letter
In 1950, Oleg Lavrentiev, then a Soviet Army sergeant stationed on Sakhalin, wrote a letter to the Central Committee of the Communist Party of the Soviet Union. The letter outlined the idea of using an atomic bomb to ignite fusion fuel, and then went on to describe a system that used electrostatic fields to contain hot plasma in a steady state for energy production.The letter was sent to Andrei Sakharov for comment. Sakharov noted that "the author formulates a very important and not necessarily hopeless problem", and found that his main concern in the arrangement was that the plasma would hit the electrode wires, and that "wide meshes and a thin current-carrying part which will have to reflect almost all incident nuclei back into the
reactor."
Some indication of the importance given to Lavrentiev's letter can be seen in the speed with which it was processed; the letter was received by the Central Committee on 29 July, Sakharov sent his review in on 18 August, by October, Sakharov and Igor Tamm had completed the first detailed study of a fusion reactor, and they had asked for funding to build it in January 1951.
Magnetic confinement
When heated to fusion temperatures, the electrons in atoms dissociate, resulting in a fluid of nuclei and electrons known as plasma. Unlike electrically neutral atoms, a plasma is electrically conductive, and can, therefore, be manipulated by electrical or magnetic fields.Sakharov's concern about the electrodes led him to consider using magnetic confinement instead of electrostatic. In the case of a magnetic field, the particles will circle around the lines of force. As the particles are moving at high speed, their resulting paths look like a helix. If one arranges a magnetic field so lines of force are parallel and close together, the particles orbiting adjacent lines may collide, and fuse.
Such a field can be created in a solenoid, a cylinder with magnets wrapped around the outside. The combined fields of the magnets create a set of parallel magnetic lines running down the length of the cylinder. This arrangement prevents the particles from moving sideways to the wall of the cylinder, but it does not prevent them from running out the end. The obvious solution to this problem is to bend the cylinder around into a donut shape, or torus, so that the lines form a series of continual rings. In this arrangement, the particles circle endlessly.
Sakharov discussed the concept with Igor Tamm, and by the end of October 1950 the two had written a proposal and sent it to Igor Kurchatov, the director of the atomic bomb project within the USSR, and his deputy, Igor Golovin. However, this initial proposal ignored a fundamental problem; when arranged along a straight solenoid, the external magnets are evenly spaced, but when bent around into a torus, they are closer together on the inside of the ring than the outside. This leads to uneven forces that cause the particles to drift away from their magnetic lines.
During visits to the Laboratory of Measuring Instruments of the USSR Academy of Sciences, the Soviet nuclear research centre, Sakharov suggested two possible solutions to this problem. One was to suspend a current-carrying ring in the centre of the torus. The current in the ring would produce a magnetic field that would mix with the one from the magnets on the outside. The resulting field would be twisted into a helix, so that any given particle would find itself repeatedly on the outside, then inside, of the torus. The drifts caused by the uneven fields are in opposite directions on the inside and outside, so over the course of multiple orbits around the long axis of the torus, the opposite drifts would cancel out. Alternately, he suggested using an external magnet to induce a current in the plasma itself, instead of a separate metal ring, which would have the same effect.
In January 1951, Kurchatov arranged a meeting at LIPAN to consider Sakharov's concepts. They found widespread interest and support, and in February a report on the topic was forwarded to Lavrentiy Beria, who oversaw the atomic efforts in the USSR. For a time, nothing was heard back.