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A tokamak (; ) is a device which uses a powerful
magnetic field A magnetic field (sometimes called B-field) is a physical field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular ...
generated by external magnets to confine plasma in the shape of an axially symmetrical
torus In geometry, a torus (: tori or toruses) is a surface of revolution generated by revolving a circle in three-dimensional space one full revolution about an axis that is coplanarity, coplanar with the circle. The main types of toruses inclu ...
. The tokamak is one of several types of magnetic confinement devices being developed to produce controlled
thermonuclear Nuclear fusion is a reaction in which two or more atomic nuclei combine to form a larger nuclei, nuclei/neutron by-products. The difference in mass between the reactants and products is manifested as either the release or absorption of ener ...
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 The Union of Soviet Socialist Republics. (USSR), commonly known as the Soviet Union, was a List of former transcontinental countries#Since 1700, transcontinental country that spanned much of Eurasia from 1922 until Dissolution of the Soviet ...
1954. In 1968 the electronic plasma temperature of 1 keV was reached on the tokamak T-3, built at the
Kurchatov Institute The Kurchatov Institute (, National Research Centre "Kurchatov Institute") is Russia's leading research and development institution in the field of nuclear power, nuclear energy. It is named after Igor Kurchatov and is located at 1 Kurchatov Sq ...
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 A stellarator confines Plasma (physics), plasma using external magnets. Scientists aim to use stellarators to generate fusion power. It is one of many types of magnetic confinement fusion devices. The name "stellarator" refers to stars because ...
had attempted this, but demonstrated serious instabilities. It was the development of the concept now known as the
safety factor In engineering, a factor of safety (FoS) or safety factor (SF) expresses how much stronger a system is than it needs to be for its specified maximum load. Safety factors are often calculated using detailed analysis because comprehensive testing i ...
(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 Princeton Plasma Physics Laboratory (PPPL) is a United States Department of Energy national laboratory for plasma physics and nuclear fusion science. Its primary mission is research into and development of fusion as an energy source. It is know ...
(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 Deuterium (hydrogen-2, symbol H or D, also known as heavy hydrogen) is one of two stable isotopes of hydrogen; the other is protium, or hydrogen-1, H. The deuterium nucleus (deuteron) contains one proton and one neutron, whereas the far more c ...
and
tritium Tritium () or hydrogen-3 (symbol T or H) is a rare and radioactive isotope of hydrogen with a half-life of ~12.33 years. The tritium nucleus (t, sometimes called a ''triton'') contains one proton and two neutrons, whereas the nucleus of the ...
. 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. 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 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 were 0.63 and 0.28 (discharge 80539) respectively. , 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 Ronald Wilson Reagan (February 6, 1911 – June 5, 2004) was an American politician and actor who served as the 40th president of the United States from 1981 to 1989. He was a member of the Republican Party (United States), Republican Party a ...
and
Mikhail Gorbachev Mikhail Sergeyevich Gorbachev (2 March 1931 – 30 August 2022) was a Soviet and Russian politician who served as the last leader of the Soviet Union from 1985 to dissolution of the Soviet Union, the country's dissolution in 1991. He served a ...
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.


Etymology

The word ''tokamak'' is a
transliteration Transliteration is a type of conversion of a text from one script to another that involves swapping letters (thus '' trans-'' + '' liter-'') in predictable ways, such as Greek → and → the digraph , Cyrillic → , Armenian → or L ...
of the Russian word , 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" ("токамаг") — an acronym of the words "''to''roidal ''cha''mber magnetic" ("''то''роидальная ''ка''мера ''маг''нитная"), 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.


History


First steps

In 1934, Mark Oliphant, Paul Harteck and
Ernest Rutherford Ernest Rutherford, 1st Baron Rutherford of Nelson (30 August 1871 – 19 October 1937) was a New Zealand physicist who was a pioneering researcher in both Atomic physics, atomic and nuclear physics. He has been described as "the father of nu ...
were the first to achieve fusion on Earth, using a
particle accelerator A particle accelerator is a machine that uses electromagnetic fields to propel electric charge, charged particles to very high speeds and energies to contain them in well-defined particle beam, beams. Small accelerators are used for fundamental ...
to shoot
deuterium Deuterium (hydrogen-2, symbol H or D, also known as heavy hydrogen) is one of two stable isotopes of hydrogen; the other is protium, or hydrogen-1, H. The deuterium nucleus (deuteron) contains one proton and one neutron, whereas the far more c ...
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 
electronvolt In physics, an electronvolt (symbol eV), also written electron-volt and electron volt, is the measure of an amount of kinetic energy gained by a single electron accelerating through an Voltage, electric potential difference of one volt in vacuum ...
s (100 keV). 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 Nuclear fusion is a reaction in which two or more atomic nuclei combine to form a larger nuclei, nuclei/neutron by-products. The difference in mass between the reactants and products is manifested as either the release or absorption of ener ...
'' 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 Manhattan Project was a research and development program undertaken during World War II to produce the first nuclear weapons. It was led by the United States in collaboration with the United Kingdom and Canada. From 1942 to 1946, the ...
, 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 A thermonuclear weapon, fusion weapon or hydrogen bomb (H-bomb) is a second-generation nuclear weapon design. Its greater sophistication affords it vastly greater destructive power than first-generation nuclear bombs, a more compact size, a lo ...
. However, some thought had already been given to a ''controlled'' fusion device, 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 The United Kingdom of Great Britain and Northern Ireland, commonly known as the United Kingdom (UK) or Britain, is a country in Northwestern Europe, off the coast of European mainland, the continental mainland. It comprises England, Scotlan ...
, 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 (Stan) W. Cousins and Alan Alfred Ware (1924–2010), to build a device out of surplus
radar Radar is a system that uses radio waves to determine the distance ('' ranging''), direction ( azimuth and elevation angles), and radial velocity of objects relative to the site. It is a radiodetermination method used to detect and track ...
equipment. This was successfully operated in 1948, but showed no clear evidence of fusion and failed to gain the interest of the
Atomic Energy Research Establishment The Atomic Energy Research Establishment (AERE), also known as Harwell Laboratory, was the main Headquarters, centre for nuclear power, atomic energy research and development in the United Kingdom from 1946 to the 1990s. It was created, owned ...
.


Lavrentiev's letter

In 1950, Oleg Lavrentiev, then a
Red Army The Workers' and Peasants' Red Army, often shortened to the Red Army, was the army and air force of the Russian Soviet Republic and, from 1922, the Soviet Union. The army was established in January 1918 by a decree of the Council of People ...
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 a fusion fuel, and then went on to describe a system that used
electrostatic Electrostatics is a branch of physics that studies slow-moving or stationary electric charges. Since classical times, it has been known that some materials, such as amber, attract lightweight particles after rubbing. The Greek word (), mean ...
fields to contain a 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 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. In all likelihood, this requirement is incompatible with the mechanical strength of the device." 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
electron The electron (, or in nuclear reactions) is a subatomic particle with a negative one elementary charge, elementary electric charge. It is a fundamental particle that comprises the ordinary matter that makes up the universe, along with up qua ...
s 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 upright=1.20, An illustration of a solenoid upright=1.20, Magnetic field created by a seven-loop solenoid (cross-sectional view) described using field lines A solenoid () is a type of electromagnet formed by a helix, helical coil of wire whos ...
, 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 (LIPAN), 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
orbit In celestial mechanics, an orbit (also known as orbital revolution) is the curved trajectory of an object such as the trajectory of a planet around a star, or of a natural satellite around a planet, or of an artificial satellite around an ...
s around the long axis of the
torus In geometry, a torus (: tori or toruses) is a surface of revolution generated by revolving a circle in three-dimensional space one full revolution about an axis that is coplanarity, coplanar with the circle. The main types of toruses inclu ...
, 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.


Richter and the birth of fusion research

On 25 March 1951, Argentine President
Juan Perón Juan Domingo Perón (, , ; 8 October 1895 – 1 July 1974) was an Argentine military officer and Statesman (politician), statesman who served as the History of Argentina (1946-1955), 29th president of Argentina from 1946 to Revolución Libertad ...
announced that a former German scientist, Ronald Richter, had succeeded in producing fusion at a laboratory scale as part of what is now known as the Huemul Project. Scientists around the world were excited by the announcement, but soon concluded it was not true; simple calculations showed that his experimental setup could not produce enough energy to heat the fusion fuel to the needed temperatures. Although dismissed by nuclear researchers, the widespread news coverage meant politicians were suddenly aware of, and receptive to, fusion research. In the UK, Thomson was suddenly granted considerable funding. Over the next months, two projects based on the pinch system were up and running. In the US, Lyman Spitzer read the Huemul story, realized it was false, and set about designing a machine that would work. In May he was awarded $50,000 to begin research on his
stellarator A stellarator confines Plasma (physics), plasma using external magnets. Scientists aim to use stellarators to generate fusion power. It is one of many types of magnetic confinement fusion devices. The name "stellarator" refers to stars because ...
concept. Jim Tuck had returned to the UK briefly and saw Thomson's pinch machines. When he returned to Los Alamos he also received $50,000 directly from the Los Alamos budget. Similar events occurred in the
USSR The Union of Soviet Socialist Republics. (USSR), commonly known as the Soviet Union, was a List of former transcontinental countries#Since 1700, transcontinental country that spanned much of Eurasia from 1922 until Dissolution of the Soviet ...
. In mid-April, Dmitri Efremov of the Scientific Research Institute of Electrophysical Apparatus stormed into Kurchatov's study with a magazine containing a story about Richter's work, demanding to know why they were beaten by the Argentines. Kurchatov immediately contacted Beria with a proposal to set up a separate fusion research laboratory with Lev Artsimovich as director. Only days later, on 5 May, the proposal had been signed by
Joseph Stalin Joseph Vissarionovich Stalin (born Dzhugashvili; 5 March 1953) was a Soviet politician and revolutionary who led the Soviet Union from 1924 until Death and state funeral of Joseph Stalin, his death in 1953. He held power as General Secret ...
.


New ideas

By October, Sakharov and Tamm had completed a much more detailed consideration of their original proposal, calling for a device with a major radius (of the torus as a whole) of and a minor radius (the interior of the cylinder) of . The proposal suggested the system could produce of
tritium Tritium () or hydrogen-3 (symbol T or H) is a rare and radioactive isotope of hydrogen with a half-life of ~12.33 years. The tritium nucleus (t, sometimes called a ''triton'') contains one proton and two neutrons, whereas the nucleus of the ...
a day, or breed of U233 a day. As the idea was further developed, it was realized that a current in the plasma could create a field that was strong enough to confine the plasma as well, removing the need for the external coils. At this point, the Soviet researchers had re-invented the pinch system being developed in the UK, although they had come to this design from a very different starting point. Once the idea of using the pinch effect for confinement had been proposed, a much simpler solution became evident. Instead of a large toroid, one could simply induce the current into a linear tube, which could cause the plasma within to collapse down into a filament. This had a huge advantage; the current in the plasma would heat it through normal resistive heating, but this would not heat the plasma to fusion temperatures. However, as the plasma collapsed, the
adiabatic process An adiabatic process (''adiabatic'' ) is a type of thermodynamic process that occurs without transferring heat between the thermodynamic system and its Environment (systems), environment. Unlike an isothermal process, an adiabatic process transf ...
would result in the temperature rising dramatically, more than enough for fusion. With this development, only Golovin and Natan Yavlinsky continued considering the more static toroidal arrangement.


Instability

On 4 July 1952, Nikolai Filippov's group measured
neutron The neutron is a subatomic particle, symbol or , that has no electric charge, and a mass slightly greater than that of a proton. The Discovery of the neutron, neutron was discovered by James Chadwick in 1932, leading to the discovery of nucle ...
s being released from a linear pinch machine. Lev Artsimovich demanded that they check everything before concluding fusion had occurred, and during these checks, they found that the neutrons were not from fusion at all. This same linear arrangement had also occurred to researchers in the UK and US, and their machines showed the same behaviour. But the great secrecy surrounding the type of research meant that none of the groups were aware that others were also working on it, let alone having the identical problem. After much study, it was found that some of the released neutrons were produced by instabilities in the plasma. There were two common types of instability, the ''sausage'' that was seen primarily in linear machines, and the ''kink'' which was most common in the toroidal machines. Groups in all three countries began studying the formation of these instabilities and potential ways to address them. Important contributions to the field were made by Martin David Kruskal and Martin Schwarzschild in the US, and Shafranov in the USSR. One idea that came from these studies became known as the "stabilized pinch". This concept added additional coils to the outside of the chamber, which created a magnetic field that would be present in the plasma before the pinch discharge. In most concepts, the externally induced field was relatively weak, and because a plasma is
diamagnetic Diamagnetism is the property of materials that are repelled by a magnetic field; an applied magnetic field creates an induced magnetic field in them in the opposite direction, causing a repulsive force. In contrast, paramagnetic and ferromagn ...
, it penetrated only the outer areas of the plasma. When the pinch discharge occurred and the plasma quickly contracted, this field became "frozen in" to the resulting filament, creating a strong field in its outer layers. In the US, this was known as "giving the plasma a backbone". Sakharov revisited his original toroidal concepts and came to a slightly different conclusion about how to stabilize the plasma. The layout would be the same as the stabilized pinch concept, but the role of the two fields would be reversed. Instead of weak externally induced magnetic fields providing stabilization and a strong pinch current responsible for confinement, in the new layout, the external field would be much more powerful in order to provide the majority of confinement, while the current would be much smaller and responsible for the stabilizing effect.


Steps toward declassification

In 1955, with the linear approaches still subject to instability, the first toroidal device was built in the USSR. TMP was a classic pinch machine, similar to models in the UK and US of the same era. The vacuum chamber was made of ceramic, and the spectra of the discharges showed silica, meaning the plasma was not perfectly confined by magnetic field and hitting the walls of the chamber. Two smaller machines followed, using copper shells. The conductive shells were intended to help stabilize the plasma, but were not completely successful in any of the machines that tried it. With progress apparently stalled, in 1955, Kurchatov called an All Union conference of Soviet researchers with the ultimate aim of opening up fusion research within the USSR. In April 1956, Kurchatov travelled to the UK as part of a widely publicized visit by
Nikita Khrushchev Nikita Sergeyevich Khrushchev (– 11 September 1971) was the General Secretary of the Communist Party of the Soviet Union, First Secretary of the Communist Party of the Soviet Union from 1953 to 1964 and the Premier of the Soviet Union, Chai ...
and Nikolai Bulganin. He offered to give a talk at Atomic Energy Research Establishment, at the former RAF Harwell, where he shocked the hosts by presenting a detailed historical overview of the Soviet fusion efforts. He took time to note, in particular, the neutrons seen in early machines and warned that neutrons did not mean fusion. Unknown to Kurchatov, the British
ZETA Zeta (, ; uppercase Ζ, lowercase ζ; , , classical or ''zē̂ta''; ''zíta'') is the sixth letter of the Greek alphabet. In the system of Greek numerals, it has a value of 7. It was derived from the Phoenician alphabet, Phoenician letter zay ...
stabilized pinch machine was being built at the far end of the former runway. ZETA was, by far, the largest and most powerful fusion machine to date. Supported by experiments on earlier designs that had been modified to include stabilization, ZETA intended to produce low levels of fusion reactions. This was apparently a great success, and in January 1958, they announced the fusion had been achieved in ZETA based on the release of neutrons and measurements of the plasma temperature. Vitaly Shafranov and Stanislav Braginskii examined the news reports and attempted to figure out how it worked. One possibility they considered was the use of weak "frozen in" fields, but rejected this, believing the fields would not last long enough. They then concluded ZETA was essentially identical to the devices they had been studying, with strong external fields.


First tokamaks

By this time, Soviet researchers had decided to build a larger toroidal machine along the lines suggested by Sakharov. In particular, their design considered one important point found in Kruskal's and Shafranov's works; if the helical path of the particles made them circulate around the plasma's circumference more rapidly than they circulated the long axis of the torus, the kink instability would be strongly suppressed. (To be clear, Electrical current in coils wrapping around the torus produces a toroidal magnetic field inside the torus; a pulsed magnetic field through the hole in the torus induces the axial current in the torus which has a poloidal magnetic field surrounding it; there may also be rings of current above and below the torus that create additional poloidal magnetic field. The combined magnetic fields form a helical magnetic structure inside the torus.) Today this basic concept is known as the ''
safety factor In engineering, a factor of safety (FoS) or safety factor (SF) expresses how much stronger a system is than it needs to be for its specified maximum load. Safety factors are often calculated using detailed analysis because comprehensive testing i ...
''. The ratio of the number of times the particle orbits the major axis compared to the minor axis is denoted ''q'', and the ''Kruskal-Shafranov Limit'' stated that the kink will be suppressed as long as ''q'' > 1. This path is controlled by the relative strengths of the externally induced magnetic field compared to the field created by the internal current. To have ''q'' > 1, the external magnets must be much more powerful, or alternatively, the internal current has to be reduced. Following this criterion, design began on a new reactor, T-1, which today is known as the first real tokamak. T-1 used both stronger external magnetic fields and a reduced current compared to stabilized pinch machines like ZETA. The success of the T-1 resulted in its recognition as the first working tokamak. For his work on "powerful impulse discharges in a gas, to obtain unusually high temperatures needed for thermonuclear processes", Yavlinskii was awarded the Lenin Prize and the Stalin Prize in 1958. Yavlinskii was already preparing the design of an even larger model, later built as T-3. With the apparently successful ZETA announcement, Yavlinskii's concept was viewed very favourably. Details of ZETA became public in a series of articles in ''Nature'' later in January. To Shafranov's surprise, the system did use the "frozen in" field concept. He remained sceptical, but a team at the Ioffe Institute in St. Petersberg began plans to build a similar machine known as Alpha. Only a few months later, in May, the ZETA team issued a release stating they had not achieved fusion, and that they had been misled by erroneous measures of the plasma temperature. T-1 began operation at the end of 1958. It demonstrated very high energy losses through radiation. This was traced to impurities in the plasma due to the vacuum system causing outgassing from the container materials. In order to explore solutions to this problem, another small device was constructed, T-2. This used an internal liner of corrugated metal that was baked at to cook off trapped gasses.


Atoms for Peace and the doldrums

As part of the second Atoms for Peace meeting in
Geneva Geneva ( , ; ) ; ; . is the List of cities in Switzerland, second-most populous city in Switzerland and the most populous in French-speaking Romandy. Situated in the southwest of the country, where the Rhône exits Lake Geneva, it is the ca ...
in September 1958, the Soviet delegation released many papers covering their fusion research. Among them was a set of initial results on their toroidal machines, which at that point had shown nothing of note. The "star" of the show was a large model of Spitzer's stellarator, which immediately caught the attention of the Soviets. In contrast to their designs, the stellarator produced the required twisted paths in the plasma without driving a current through it, using a series of external coils (producing internal magnetic fields) that could operate in the steady state rather than the pulses of the induction system that produced the axial current. Kurchatov began asking Yavlinskii to change their T-3 design to a stellarator, but they convinced him that the current provided a useful second role in heating, something the stellarator lacked. At the time of the show, the stellarator had suffered a long string of minor problems that were just being solved. Solving these revealed that the diffusion rate of the plasma was much faster than theory predicted. Similar problems were seen in all the contemporary designs, for one reason or another. The stellarator, various pinch concepts and the magnetic mirror machines in both the US and USSR all demonstrated problems that limited their confinement times. From the first studies of controlled fusion, there was a problem lurking in the background. During the Manhattan Project,
David Bohm David Joseph Bohm (; 20 December 1917 – 27 October 1992) was an American scientist who has been described as one of the most significant Theoretical physics, theoretical physicists of the 20th centuryDavid Peat Who's Afraid of Schrödinger' ...
had been part of the team working on isotopic separation of
uranium Uranium is a chemical element; it has chemical symbol, symbol U and atomic number 92. It is a silvery-grey metal in the actinide series of the periodic table. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons. Ura ...
. In the post-war era he continued working with plasmas in magnetic fields. Using basic theory, one would expect the plasma to diffuse across the lines of force at a rate inversely proportional to the square of the strength of the field, meaning that small increases in force would greatly improve confinement. But based on their experiments, Bohm developed an empirical formula, now known as Bohm diffusion, that suggested the rate was linear with the magnetic force, not its square. If Bohm's formula was correct, there was no hope one could build a fusion reactor based on magnetic confinement. To confine the plasma at the temperatures needed for fusion, the magnetic field would have to be orders of magnitude greater than any known magnet. Spitzer ascribed the difference between the Bohm and classical diffusion rates to turbulence in the plasma, and believed the steady fields of the stellarator would not suffer from this problem. Various experiments at that time suggested the Bohm rate did not apply, and that the classical formula was correct. But by the early 1960s, with all of the various designs leaking plasma at a prodigious rate, Spitzer himself concluded that the Bohm scaling was an inherent quality of plasmas, and that magnetic confinement would not work. The entire field descended into what became known as "the doldrums", a period of intense pessimism.


Progress in the 1960s

In contrast to the other designs, the experimental tokamaks appeared to be progressing well, so well that a minor theoretical problem was now a real concern. In the presence of gravity, there is a small pressure gradient in the plasma, formerly small enough to ignore but now becoming something that had to be addressed. This led to the addition of yet another set of coils in 1962, which produced a vertical magnetic field that offset these effects. These were a success, and by the mid-1960s the machines began to show signs that they were beating the Bohm limit. At the 1965 Second
International Atomic Energy Agency The International Atomic Energy Agency (IAEA) is an intergovernmental organization that seeks to promote the peaceful use of nuclear technology, nuclear energy and to inhibit its use for any military purpose, including nuclear weapons. It was ...
Conference on fusion at the UK's newly opened Culham Centre for Fusion Energy, Artsimovich reported that their systems were surpassing the Bohm limit by 10 times. Spitzer, reviewing the presentations, suggested that the Bohm limit may still apply; the results were within the range of experimental error of results seen on the stellarators, and the temperature measurements, based on the magnetic fields, were simply not trustworthy. The next major international fusion meeting was held in August 1968 in
Novosibirsk Novosibirsk is the largest city and administrative centre of Novosibirsk Oblast and the Siberian Federal District in Russia. As of the 2021 Russian census, 2021 census, it had a population of 1,633,595, making it the most populous city in Siber ...
. By this time two additional tokamak designs had been completed, TM-2 in 1965, and T-4 in 1968. Results from T-3 had continued to improve, and similar results were coming from early tests of the new reactors. At the meeting, the Soviet delegation announced that T-3 was producing electron temperatures of 1000 eV (equivalent to 10 million degrees Celsius) and that confinement time was at least 50 times the Bohm limit. These results were at least 10 times that of any other machine. If correct, they represented an enormous leap for the fusion community. Spitzer remained skeptical, noting that the temperature measurements were still based on the indirect calculations from the magnetic properties of the plasma. Many concluded they were due to an effect known as runaway electrons, and that the Soviets were measuring only those extremely energetic electrons and not the bulk temperature. The Soviets countered with several arguments suggesting the temperature they were measuring was Maxwellian, and the debate raged.


Culham Five

In the aftermath of ZETA, the UK teams began the development of new plasma diagnostic tools to provide more accurate measurements. Among these was the use of a
laser A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The word ''laser'' originated as an acronym for light amplification by stimulated emission of radi ...
to directly measure the temperature of the bulk electrons using Thomson scattering. This technique was well known and respected in the fusion community; Artsimovich had publicly called it "brilliant". Artsimovich invited Bas Pease, the head of Culham, to use their devices on the Soviet reactors. At the height of the
Cold War The Cold War was a period of global Geopolitics, geopolitical rivalry between the United States (US) and the Soviet Union (USSR) and their respective allies, the capitalist Western Bloc and communist Eastern Bloc, which lasted from 1947 unt ...
, in what is still considered a major political manoeuvre on Artsimovich's part, British physicists were allowed to visit the Kurchatov Institute, the heart of the Soviet nuclear bomb effort. The British team, nicknamed "The Culham Five", arrived late in 1968. After a lengthy installation and calibration process, the team measured the temperatures over a period of many experimental runs. Initial results were available by August 1969; the Soviets were correct, their results were accurate. The team phoned the results home to Culham, who then passed them along in a confidential phone call to Washington. The final results were published in ''Nature'' in November 1969. The results of this announcement have been described as a "veritable stampede" of tokamak construction around the world. One serious problem remained. Because the electrical current in the plasma was much lower and produced much less compression than a pinch machine, this meant the temperature of the plasma was limited to the resistive heating rate of the current. First proposed in 1950, Spitzer resistivity stated that the
electrical resistance The electrical resistance of an object is a measure of its opposition to the flow of electric current. Its reciprocal quantity is , measuring the ease with which an electric current passes. Electrical resistance shares some conceptual paral ...
of a plasma was reduced as the temperature increased, meaning the heating rate of the plasma would slow as the devices improved and temperatures were pressed higher. Calculations demonstrated that the resulting maximum temperatures while staying within ''q'' > 1 would be limited to the low millions of degrees. Artsimovich had been quick to point this out in Novosibirsk, stating that future progress would require new heating methods to be developed.


US turmoil

One of the people attending the Novosibirsk meeting in 1968 was Amasa Stone Bishop, one of the leaders of the US fusion program. One of the few other devices to show clear evidence of beating the Bohm limit at that time was the multipole concept. Both Lawrence Livermore and the
Princeton Plasma Physics Laboratory Princeton Plasma Physics Laboratory (PPPL) is a United States Department of Energy national laboratory for plasma physics and nuclear fusion science. Its primary mission is research into and development of fusion as an energy source. It is know ...
(PPPL), home of Spitzer's stellarator, were building variations on the multipole design. While moderately successful on their own, T-3 greatly outperformed either machine. Bishop was concerned that the multipoles were redundant and thought the US should consider a tokamak of its own. When he raised the issue at a December 1968 meeting, directors of the labs refused to consider it. Melvin B. Gottlieb of Princeton was exasperated, asking "Do you think that this committee can out-think the scientists?" With the major labs demanding they control their own research, one lab found itself left out. Oak Ridge had originally entered the fusion field with studies for reactor fueling systems, but branched out into a mirror program of their own. By the mid-1960s, their DCX designs were running out of ideas, offering nothing that the similar program at the more prestigious and politically powerful Livermore did not. This made them highly receptive to new concepts. After a considerable internal debate, Herman Postma formed a small group in early 1969 to consider the tokamak. They came up with a new design, later christened Ormak, that had several novel features. Primary among them was the way the external field was created in a single large copper block, fed power from a large
transformer In electrical engineering, a transformer is a passive component that transfers electrical energy from one electrical circuit to another circuit, or multiple Electrical network, circuits. A varying current in any coil of the transformer produces ...
below the torus. This was as opposed to traditional designs that used electric current windings on the outside. They felt the single block would produce a much more uniform field. It would also have the advantage of allowing the torus to have a smaller major radius, lacking the need to route cables through the donut hole, leading to a lower ''
aspect ratio The aspect ratio of a geometry, geometric shape is the ratio of its sizes in different dimensions. For example, the aspect ratio of a rectangle is the ratio of its longer side to its shorter side—the ratio of width to height, when the rectangl ...
'', which the Soviets had already suggested would produce better results.


Tokamak race in the US

In early 1969, Artsimovich visited MIT, where he was hounded by those interested in fusion. He finally agreed to give several lectures in April and then allowed lengthy question-and-answer sessions. As these went on, MIT itself grew interested in the tokamak, having previously stayed out of the fusion field for a variety of reasons. Bruno Coppi was at MIT at the time, and following the same concepts as Postma's team, came up with his own low-aspect-ratio concept, Alcator. Instead of Ormak's toroidal transformer, Alcator used traditional ring-shaped magnetic field coils but required them to be much smaller than existing designs. MIT's Francis Bitter Magnet Laboratory was the world leader in magnet design and they were confident they could build them. During 1969, two additional groups entered the field. At General Atomics, Tihiro Ohkawa had been developing multipole reactors, and submitted a concept based on these ideas. This was a tokamak that would have a non-circular plasma cross-section; the same math that suggested a lower aspect-ratio would improve performance also suggested that a C or D-shaped plasma would do the same. He called the new design Doublet. Meanwhile, a group at
University of Texas at Austin The University of Texas at Austin (UT Austin, UT, or Texas) is a public university, public research university in Austin, Texas, United States. Founded in 1883, it is the flagship institution of the University of Texas System. With 53,082 stud ...
was proposing a relatively simple tokamak to explore heating the plasma through deliberately induced turbulence, the Texas Turbulent Tokamak. When the members of the Atomic Energy Commissions' Fusion Steering Committee met again in June 1969, they had "tokamak proposals coming out of our ears". The only major lab working on a toroidal design that was not proposing a tokamak was Princeton, who refused to consider it in spite of their Model C stellarator being just about perfect for such a conversion. They continued to offer a long list of reasons why the Model C should not be converted. When these were questioned, a furious debate broke out about whether the Soviet results were reliable. Watching the debate take place, Gottlieb had a change of heart. There was no point moving forward with the tokamak if the Soviet electron temperature measurements were not accurate, so he formulated a plan to either prove or disprove their results. While swimming in the pool during the lunch break, he told Harold Furth his plan, to which Furth replied: "well, maybe you're right." After lunch, the various teams presented their designs, at which point Gottlieb presented his idea for a "stellarator-tokamak" based on the Model C. The Standing Committee noted that this system could be complete in six months, while Ormak would take a year. It was only a short time later that the confidential results from the Culham Five were released. When they met again in October, the Standing Committee released funding for all of these proposals. The Model C's new configuration, soon named
Symmetrical Tokamak Symmetry () in everyday life refers to a sense of harmonious and beautiful proportion and balance. In mathematics, the term has a more precise definition and is usually used to refer to an object that is invariant under some transformations ...
, intended to simply verify the Soviet results, while the others would explore ways to go well beyond T-3.


Heating: US takes the lead

Experiments on the Symmetric Tokamak began in May 1970, and by early the next year they had confirmed the Soviet results and then surpassed them. The stellarator was abandoned, and PPPL turned its considerable expertise to the problem of heating the plasma. Two concepts seemed to hold promise. PPPL proposed using magnetic compression, a pinch-like technique to compress a warm plasma to raise its temperature, but providing that compression through magnets rather than current. Oak Ridge suggested neutral beam injection, small particle accelerators that would shoot fuel atoms through the surrounding magnetic field where they would collide with the plasma and heat it. PPPL's Adiabatic Toroidal Compressor (ATC) began operation in May 1972, followed shortly thereafter by a neutral-beam equipped Ormak. Both demonstrated significant problems, but PPPL leapt past Oak Ridge by fitting beam injectors to ATC and provided clear evidence of successful heating in 1973. This success "scooped" Oak Ridge, who fell from favour within the Washington Steering Committee. By this time a much larger design based on beam heating was under construction, the Princeton Large Torus, or PLT. PLT was designed specifically to "give a clear indication whether the tokamak concept plus auxiliary heating can form a basis for a future fusion reactor". PLT was an enormous success, continually raising its internal temperature until it hit 60 million Celsius (8,000 eV, eight times T-3's record) in 1978. This is a key point in the development of the tokamak; fusion reactions become self-sustaining at temperatures between 50 and 100 million Celsius, PLT demonstrated that this was technically achievable. These experiments, especially PLT, put the US far in the lead in tokamak research. This is due largely to budget; a tokamak cost about $500,000 and the US annual fusion budget was around $25 million at that time. They could afford to explore all of the promising methods of heating, ultimately discovering neutral beams to be among the most effective. During this period, Robert Hirsch took over the Directorate of fusion development in the U.S. Atomic Energy Commission. Hirsch felt that the program could not be sustained at its current funding levels without demonstrating tangible results. He began to reformulate the entire program. What had once been a lab-led effort of mostly scientific exploration was now a Washington-led effort to build a working power-producing reactor. This was given a boost by the
1973 oil crisis In October 1973, the Organization of Arab Petroleum Exporting Countries (OAPEC) announced that it was implementing a total oil embargo against countries that had supported Israel at any point during the 1973 Yom Kippur War, which began after Eg ...
, which led to greatly increased research into
alternative energy Renewable energy (also called green energy) is energy made from renewable resource, renewable natural resources that are replenished on a human lifetime, human timescale. The most widely used renewable energy types are solar energy, wind pow ...
systems.


1980s: great hope, great disappointment

By the late-1970s, tokamaks had reached all the conditions needed for a practical fusion reactor; in 1978 PLT had demonstrated ignition temperatures, the next year the Soviet T-7 successfully used superconducting magnets for the first time, Doublet proved to be a success and led to almost all future designs adopting this "shaped plasma" approach. It appeared all that was needed to build a power-producing reactor was to put all of these design concepts into a single machine, one that would be capable of running with the radioactive
tritium Tritium () or hydrogen-3 (symbol T or H) is a rare and radioactive isotope of hydrogen with a half-life of ~12.33 years. The tritium nucleus (t, sometimes called a ''triton'') contains one proton and two neutrons, whereas the nucleus of the ...
in its fuel mix. During the 1970s, four major second-generation proposals were funded worldwide. The Soviets continued their development lineage with the T-15, while a pan-European effort was developing the Joint European Torus (JET) and Japan began the JT-60 effort (originally known as the "Breakeven Plasma Test Facility"). In the US, Hirsch began formulating plans for a similar design, skipping over proposals for another stepping-stone design directly to a tritium-burning one. This emerged as the Tokamak Fusion Test Reactor (TFTR), run directly from Washington and not linked to any specific lab. Originally favouring Oak Ridge as the host, Hirsch moved it to PPPL after others convinced him they would work the hardest on it because they had the most to lose. The excitement was so widespread that several commercial ventures to produce commercial tokamaks began around this time. Best known among these, in 1978, Bob Guccione, publisher of Penthouse Magazine, met Robert Bussard and became the world's biggest and most committed private investor in fusion technology, ultimately putting $20 million of his own money into Bussard's Compact Tokamak. Funding by the Riggs Bank led to this effort being known as the Riggatron. TFTR won the construction race and began operation in 1982, followed shortly by JET in 1983 and JT-60 in 1985. JET quickly took the lead in critical experiments, moving from test gases to deuterium and increasingly powerful "shots". But it soon became clear that none of the new systems were working as expected. A host of new instabilities appeared, along with a number of more practical problems that continued to interfere with their performance. On top of this, dangerous "excursions" of the plasma hitting with the walls of the reactor were evident in both TFTR and JET. Even when working perfectly, plasma confinement at fusion temperatures, the so-called " fusion triple product", continued to be far below what would be needed for a practical reactor design. Through the mid-1980s the reasons for many of these problems became clear, and various solutions were offered. However, these would significantly increase the size and complexity of the machines. A follow-on design incorporating these changes would be both enormous and vastly more expensive than either JET or TFTR. A new period of pessimism descended on the fusion field.


ITER

At the same time these experiments were demonstrating problems, much of the impetus for the US's massive funding disappeared; in 1986
Ronald Reagan Ronald Wilson Reagan (February 6, 1911 – June 5, 2004) was an American politician and actor who served as the 40th president of the United States from 1981 to 1989. He was a member of the Republican Party (United States), Republican Party a ...
declared the
1970s energy crisis The 1970s energy crisis occurred when the Western world, particularly the United States, Canada, Western Europe, Australia, and New Zealand, faced substantial petroleum shortages as well as elevated prices. The two worst crises of this period wer ...
was over, and funding for advanced energy sources had been slashed in the early 1980s. Some thought of an international reactor design had been ongoing since June 1973 under the name INTOR, for INternational TOkamak Reactor. This was originally started through an agreement between
Richard Nixon Richard Milhous Nixon (January 9, 1913April 22, 1994) was the 37th president of the United States, serving from 1969 until Resignation of Richard Nixon, his resignation in 1974. A member of the Republican Party (United States), Republican ...
and
Leonid Brezhnev Leonid Ilyich Brezhnev (19 December 190610 November 1982) was a Soviet politician who served as the General Secretary of the Communist Party of the Soviet Union from 1964 until Death and state funeral of Leonid Brezhnev, his death in 1982 as w ...
, but had been moving slowly since its first real meeting on 23 November 1978. During the Geneva Summit in November 1985, Reagan raised the issue with
Mikhail Gorbachev Mikhail Sergeyevich Gorbachev (2 March 1931 – 30 August 2022) was a Soviet and Russian politician who served as the last leader of the Soviet Union from 1985 to dissolution of the Soviet Union, the country's dissolution in 1991. He served a ...
and proposed reforming the organization. "... The two leaders emphasized the potential importance of the work aimed at utilizing controlled thermonuclear fusion for peaceful purposes and, in this connection, advocated the widest practicable development of international cooperation in obtaining this source of energy, which is essentially inexhaustible, for the benefit for all mankind." The next year, an agreement was signed between the US, Soviet Union, European Union and Japan, creating the International Thermonuclear Experimental Reactor organization. Design work began in 1988, and since that time the ITER reactor has been the primary tokamak design effort worldwide.


High Field Tokamaks

It has been known for a long time that stronger field magnets would enable high energy gain in a much smaller tokamak, with concepts such a
FIRE, IGNITOR
and the Compact Ignition Tokamak (CIT) being proposed decades ago. The commercial availability of high temperature superconductors (HTS) in the 2010s opened a promising pathway to building the higher field magnets required to achieve ITER-like levels of energy gain in a compact device. To leverage this new technology, the MIT Plasma Science and Fusion Center (PSFC) and MIT spinout Commonwealth Fusion Systems (CFS) successfully built and tested th
Toroidal Field Model Coil (TFMC)
in 2021 to demonstrate the necessary 20 Tesla magnetic field needed to build SPARC, a device designed to achieve a similar fusion gain as ITER but with only ~1/40th ITER's plasma volume. British startup Tokamak Energy is also planning on building a net-energy tokamak using HTS magnets, but with the spherical tokamak variant. The joint EU/Japan JT-60SA reactor achieved first plasma on October 23, 2023, after a two-year delay caused by an electrical short.


Design


Basic problem

Positively charged ions and negatively charged
electron The electron (, or in nuclear reactions) is a subatomic particle with a negative one elementary charge, elementary electric charge. It is a fundamental particle that comprises the ordinary matter that makes up the universe, along with up qua ...
s in a fusion plasma are at very high temperatures, and have correspondingly large velocities. In order to maintain the fusion process, particles from the hot plasma must be confined in the central region, or the plasma will rapidly cool. Magnetic confinement fusion devices exploit the fact that charged particles in a magnetic field experience a Lorentz force and follow helical paths along the field lines. The simplest magnetic confinement system is a
solenoid upright=1.20, An illustration of a solenoid upright=1.20, Magnetic field created by a seven-loop solenoid (cross-sectional view) described using field lines A solenoid () is a type of electromagnet formed by a helix, helical coil of wire whos ...
. A plasma in a solenoid will spiral about the lines of field running down its center, preventing motion towards the sides. However, this does not prevent motion towards the ends. The obvious solution is to bend the solenoid around into a circle, forming a torus. However, it was demonstrated that such an arrangement is not uniform; for purely geometric reasons, the field on the outside edge of the torus is lower than on the inside edge. This asymmetry causes the electrons and ions to drift across the field, and eventually hit the walls of the torus. The solution is to shape the lines so they do not simply run around the torus, but twist around like the stripes on a barber pole or candycane. In such a field any single particle will find itself at the outside edge where it will drift one way, then as it follows its magnetic line around the torus it will find itself on the inside edge, where it will drift the other way. This cancellation is not perfect, but calculations showed it was enough to allow the fuel to remain in the reactor for a useful time.


Tokamak solution

The two first solutions to making a design with the required twist were the
stellarator A stellarator confines Plasma (physics), plasma using external magnets. Scientists aim to use stellarators to generate fusion power. It is one of many types of magnetic confinement fusion devices. The name "stellarator" refers to stars because ...
which did so through a mechanical arrangement, twisting the entire torus, and the z-pinch design which ran an electrical current through the plasma to create a second magnetic field to the same end. Both demonstrated improved confinement times compared to a simple torus, but both also demonstrated a variety of effects that caused the plasma to be lost from the reactors at rates that were not sustainable. The tokamak is essentially identical to the z-pinch concept in its physical layout. Its key innovation was the realization that the instabilities that were causing the pinch to lose its plasma could be controlled. The issue was how "twisty" the fields were; fields that caused the particles to transit inside and out more than once per orbit around the long axis torus were much more stable than devices that had less twist. This ratio of twists to orbits became known as the ''
safety factor In engineering, a factor of safety (FoS) or safety factor (SF) expresses how much stronger a system is than it needs to be for its specified maximum load. Safety factors are often calculated using detailed analysis because comprehensive testing i ...
'', denoted ''q''. Previous devices operated at ''q'' about , while the tokamak operates at . This increases stability by orders of magnitude. When the problem is considered even more closely, the need for a vertical (parallel to the axis of rotation) component of the magnetic field arises. The Lorentz force of the toroidal plasma current in the vertical field provides the inward force that holds the plasma torus in equilibrium.


Other issues

Early tokamaks had an ion temperature limited by the empirical Artsimovich formula: T_i = 5.9 \times 10^ (n_e I B_t R^2 A^)^ While the tokamak addresses the issue of plasma stability in a gross sense, plasmas are also subject to a number of dynamic instabilities. One of these, the kink instability, is strongly suppressed by the tokamak layout, a side-effect of the high safety factors of tokamaks. The lack of kinks allowed the tokamak to operate at much higher temperatures than previous machines, and this allowed a host of new phenomena to appear. One of these, the banana orbits, is caused by the wide range of particle energies in a tokamak – much of the fuel is hot, but a certain percentage is much cooler. Due to the high twist of the fields in the tokamak, particles following their lines of force rapidly move towards the inner edge and then outer. As they move inward they are subject to increasing magnetic fields due to the smaller radius concentrating the field. The low-energy particles in the fuel will reflect off this increasing field and begin to travel backwards through the fuel, colliding with the higher energy nuclei and scattering them out of the plasma. This process causes fuel to be lost from the reactor, although this process is slow enough that a practical reactor is still well within reach. Another instability is tearing instability. In 2024 researchers used
reinforcement learning Reinforcement learning (RL) is an interdisciplinary area of machine learning and optimal control concerned with how an intelligent agent should take actions in a dynamic environment in order to maximize a reward signal. Reinforcement learnin ...
against a multimodal dynamic model to measure and forecast such instabilities based on signals from multiple diagnostics and actuators at 25 millisecond intervals. This forecast was used to reduce tearing instabilities in DIII-D6, in the US. The reward function balanced the conflicting objectives of maximum plasma pressure and instability risks. In particular, the plasma actively tracked the stable path while maintaining H-mode performance.


Breakeven, ''Q'', and ignition

One of the first goals for any controlled fusion device is to reach '' breakeven'', the point where the energy being released by the fusion reactions is equal to the amount of energy being used to maintain the reaction. The ratio of output to input energy is denoted ''Q'', and breakeven corresponds to a ''Q'' of 1. A ''Q'' of more than one is needed for the reactor to generate net energy, but for practical reasons, it is desirable for it to be much higher. Once breakeven is reached, further improvements in confinement generally lead to a rapidly increasing ''Q''. That is because some of the energy being given off by the fusion reactions of the most common fusion fuel, a 50-50 mix of
deuterium Deuterium (hydrogen-2, symbol H or D, also known as heavy hydrogen) is one of two stable isotopes of hydrogen; the other is protium, or hydrogen-1, H. The deuterium nucleus (deuteron) contains one proton and one neutron, whereas the far more c ...
and
tritium Tritium () or hydrogen-3 (symbol T or H) is a rare and radioactive isotope of hydrogen with a half-life of ~12.33 years. The tritium nucleus (t, sometimes called a ''triton'') contains one proton and two neutrons, whereas the nucleus of the ...
, is in the form of
alpha particle Alpha particles, also called alpha rays or alpha radiation, consist of two protons and two neutrons bound together into a particle identical to a helium-4 nucleus. They are generally produced in the process of alpha decay but may also be produce ...
s. These can collide with the fuel nuclei in the plasma and heat it, reducing the amount of external heat needed. At some point, known as ''ignition'', this internal self-heating is enough to keep the reaction going without any external heating, corresponding to an infinite ''Q''. In the case of the tokamak, this self-heating process is maximized if the alpha particles remain in the fuel long enough to guarantee they will collide with the fuel. As the alphas are electrically charged, they are subject to the same fields that are confining the fuel plasma. The amount of time they spend in the fuel can be maximized by ensuring their orbit in the field remains within the plasma. It can be demonstrated that this occurs when the electrical current in the plasma is about 3 MA.


Advanced tokamaks

In the early 1970s, studies at Princeton into the use of high-power superconducting magnets in future tokamak designs examined the layout of the magnets. They noticed that the arrangement of the main toroidal coils meant that there was significantly more tension between the magnets on the inside of the curvature where they were closer together. Considering this, they noted that the tensional forces within the magnets would be evened out if they were shaped like a D, rather than an O. This became known as the "Princeton D-coil". This was not the first time this sort of arrangement had been considered, although for entirely different reasons. The safety factor varies across the axis of the machine; for purely geometrical reasons, it is always smaller at the inside edge of the plasma closest to the machine's center because the long axis is shorter there. That means that a machine with an average ''q'' = 2 might still be less than 1 in certain areas. In the 1970s, it was suggested that one way to counteract this and produce a design with a higher average ''q'' would be to shape the magnetic fields so that the plasma only filled the outer half of the torus, shaped like a D or C when viewed end-on, instead of the normal circular cross section. One of the first machines to incorporate a D-shaped plasma was the JET, which began its design work in 1973. This decision was made both for theoretical reasons as well as practical; because the force is larger on the inside edge of the torus, there is a large net force pressing inward on the entire reactor. The D-shape also had the advantage of reducing the net force, as well as making the supported inside edge flatter so it was easier to support. Code exploring the general layout noticed that a non-circular shape would slowly drift vertically, which led to the addition of an active feedback system to hold it in the center. Once JET had selected this layout, the General Atomics Doublet III team redesigned that machine into the D-IIID with a D-shaped cross-section, and it was selected for the Japanese JT-60 design as well. This layout has been largely universal since then. One problem seen in all fusion reactors is that the presence of heavier elements causes energy to be lost at an increased rate, cooling the plasma. During the very earliest development of fusion power, a solution to this problem was found, the '' divertor'', essentially a large
mass spectrometer Mass spectrometry (MS) is an analytical technique that is used to measure the mass-to-charge ratio of ions. The results are presented as a '' mass spectrum'', a plot of intensity as a function of the mass-to-charge ratio. Mass spectrometry is us ...
that would cause the heavier elements to be flung out of the reactor. This was initially part of the
stellarator A stellarator confines Plasma (physics), plasma using external magnets. Scientists aim to use stellarators to generate fusion power. It is one of many types of magnetic confinement fusion devices. The name "stellarator" refers to stars because ...
designs, where it is easy to integrate into the magnetic windings. However, designing a divertor for a tokamak proved to be a very difficult design problem. Another problem seen in all fusion designs is the heat load that the plasma places on the wall of the confinement vessel. There are materials that can handle this load, but they are generally undesirable and expensive heavy metals. When such materials are sputtered in collisions with hot ions, their atoms mix with the fuel and rapidly cool it. A solution used on most tokamak designs is the ''limiter'', a small ring of light metal that projected into the chamber so that the plasma would hit it before hitting the walls. This eroded the limiter and caused its atoms to mix with the fuel, but these lighter materials cause less disruption than the wall materials. When reactors moved to the D-shaped plasmas it was quickly noted that the escaping particle flux of the plasma could be shaped as well. Over time, this led to the idea of using the fields to create an internal divertor that flings the heavier elements out of the fuel, typically towards the bottom of the reactor. There, a pool of liquid
lithium Lithium (from , , ) is a chemical element; it has chemical symbol, symbol Li and atomic number 3. It is a soft, silvery-white alkali metal. Under standard temperature and pressure, standard conditions, it is the least dense metal and the ...
metal is used as a sort of limiter; the particles hit it and are rapidly cooled, remaining in the lithium. This internal pool is much easier to cool, due to its location, and although some lithium atoms are released into the plasma, its very low mass makes it a much smaller problem than even the lightest metals used previously. As machines began to explore this newly shaped plasma, they noticed that certain arrangements of the fields and plasma parameters would sometimes enter what is now known as the high-confinement mode, or H-mode, which operated stably at higher temperatures and pressures. Operating in the H-mode, which can also be seen in stellarators, is now a major design goal of the tokamak design. Finally, it was noted that when the plasma had a non-uniform density it would give rise to internal electrical currents. This is known as the '' bootstrap current''. This allows a properly designed reactor to generate some of the internal current needed to twist the magnetic field lines without having to supply it from an external source. This has a number of advantages, and modern designs all attempt to generate as much of their total current through the bootstrap process as possible. By the early 1990s, the combination of these features and others collectively gave rise to the "advanced tokamak" concept. This forms the basis of modern research, including ITER.


Plasma disruptions

Tokamaks are subject to events known as "disruptions" that cause confinement to be lost in milliseconds. There are two primary mechanisms. In one, the "vertical displacement event" (VDE), the entire plasma moves vertically until it touches the upper or lower section of the vacuum chamber. In the other, the "major disruption", long wavelength, non-axisymmetric magnetohydrodynamical instabilities cause the plasma to be forced into non-symmetrical shapes, often squeezed into the top and bottom of the chamber. When the plasma touches the vessel walls it undergoes rapid cooling, or "thermal quenching". In the major disruption case, this is normally accompanied by a brief increase in plasma current as the plasma concentrates. Quenching ultimately causes the plasma confinement to break up. In the case of the major disruption the current drops again, the "current quench". The initial increase in current is not seen in the VDE, and the thermal and current quench occurs at the same time. In both cases, the thermal and electrical load of the plasma is rapidly deposited on the reactor vessel, which has to be able to handle these loads. ITER is designed to handle 2600 of these events over its lifetime.Runaway Electrons in Tokamaks and Their Mitigation in ITER
, S. Putvinski, ITER Organization
For modern high-energy devices, where plasma currents are on the order of 15 mega
ampere The ampere ( , ; symbol: A), often shortened to amp,SI supports only the use of symbols and deprecates the use of abbreviations for units. is the unit of electric current in the International System of Units (SI). One ampere is equal to 1 c ...
s in ITER, it is possible the brief increase in current during a major disruption will cross a critical threshold. This occurs when the current produces a force on the electrons that is higher than the frictional forces of the collisions between particles in the plasma. In this event, electrons can be rapidly accelerated to relativistic velocities, creating so-called "runaway electrons" in the relativistic runaway electron avalanche. These retain their energy even as the current quench is occurring on the bulk of the plasma. When confinement finally breaks down, these runaway electrons follow the path of least resistance and impact the side of the reactor. These can reach 12 megaamps of current deposited in a small area, well beyond the capabilities of any mechanical solution. In one famous case, the Tokamak de Fontenay aux Roses had a major disruption where the runaway electrons burned a hole through the vacuum chamber. The occurrence of major disruptions in running tokamaks has always been rather high, of the order of a few percent of the total numbers of the shots. In currently operated tokamaks, the damage is often large but rarely dramatic. In the ITER tokamak, it is expected that the occurrence of a limited number of major disruptions will definitively damage the chamber with no possibility to restore the device. The development of systems to counter the effects of runaway electrons is considered a must-have piece of technology for the operational level ITER. A large amplitude of the central current density can also result in internal disruptions, or sawteeth, which do not generally result in termination of the discharge. Densities over the Greenwald limit, a bound depending on the plasma current and the minor radius, typically leads to disruptions. It has been exceeded up to factors of 10, but it remains an important concept describing the phenomenology of the transition of the plasma flow, which still needs to be understood.


Plasma heating

In an operating fusion reactor, part of the energy generated will serve to maintain the plasma temperature as fresh
deuterium Deuterium (hydrogen-2, symbol H or D, also known as heavy hydrogen) is one of two stable isotopes of hydrogen; the other is protium, or hydrogen-1, H. The deuterium nucleus (deuteron) contains one proton and one neutron, whereas the far more c ...
and
tritium Tritium () or hydrogen-3 (symbol T or H) is a rare and radioactive isotope of hydrogen with a half-life of ~12.33 years. The tritium nucleus (t, sometimes called a ''triton'') contains one proton and two neutrons, whereas the nucleus of the ...
are introduced. However, in the startup of a reactor, either initially or after a temporary shutdown, the plasma will have to be heated to its
operating temperature An operating temperature is the allowable temperature range of the local ambient environment at which an electrical or mechanical device operates. The device will operate effectively within a specified temperature range which varies based on the de ...
of greater than 10 keV (over 100 million degrees Celsius). In current tokamak (and other) magnetic fusion experiments, insufficient fusion energy is produced to maintain the plasma temperature, and constant external heating must be supplied. Chinese researchers set up the Experimental Advanced Superconducting Tokamak (EAST) in 2006, which can supposedly sustain a plasma temperature of 100 million degree Celsius for initiating fusion between hydrogen atoms, according to a November 2018 test.


Ohmic heating ~ inductive mode

Since the plasma is an electrical conductor, it is possible to heat the plasma by inducing a current through it; the induced current that provides most of the poloidal field is also a major source of initial heating. The heating caused by the induced current is called ohmic (or resistive) heating; it is the same kind of heating that occurs in an electric light bulb or in an electric heater. The heat generated depends on the resistance of the plasma and the amount of electric current running through it. But as the temperature of heated plasma rises, the resistance decreases and ohmic heating becomes less effective. It appears that the maximum plasma temperature attainable by ohmic heating in a tokamak is 20–30 million degrees Celsius. To obtain still higher temperatures, additional heating methods must be used. The current is induced by continually increasing the current through an electromagnetic winding linked with the plasma torus: the plasma can be viewed as the secondary winding of a transformer. This is inherently a pulsed process because there is a limit to the current through the primary (there are also other limitations on long pulses). Tokamaks must therefore either operate for short periods or rely on other means of heating and current drive.


Magnetic compression

A gas can be heated by sudden compression. In the same way, the temperature of a plasma is increased if it is compressed rapidly by increasing the confining magnetic field. In a tokamak, this compression is achieved simply by moving the plasma into a region of higher magnetic field (i.e., radially inward). Since plasma compression brings the ions closer together, the process has the additional benefit of facilitating attainment of the required density for a fusion reactor. Magnetic compression was an area of research in the early "tokamak stampede", and was the purpose of one major design, the ATC. The concept has not been widely used since then, although a somewhat similar concept is part of the General Fusion design.


Neutral-beam injection

Neutral-beam injection involves the introduction of high energy (rapidly moving) atoms or molecules into an ohmically heated, magnetically confined plasma within the tokamak. The high energy atoms originate as ions in an arc chamber before being extracted through a high voltage grid set. The term "ion source" is used to generally mean the assembly consisting of a set of electron emitting filaments, an arc chamber volume, and a set of extraction grids. A second device, similar in concept, is used to separately accelerate electrons to the same energy. The much lighter mass of the electrons makes this device much smaller than its ion counterpart. The two beams then intersect, where the ions and electrons recombine into neutral atoms, allowing them to travel through the magnetic fields. Once the neutral beam enters the tokamak, interactions with the main plasma ions occur. This has two effects. One is that the injected atoms re-ionize and become charged, thereby becoming trapped inside the reactor and adding to the fuel mass. The other is that the process of being ionized occurs through impacts with the rest of the fuel, and these impacts deposit energy in that fuel, heating it. This form of heating has no inherent energy (temperature) limitation, in contrast to the ohmic method, but its rate is limited to the current in the injectors. Ion source extraction voltages are typically on the order of 50–100 kV, and high voltage, negative ion sources (-1 MV) are being developed for ITER. The ITER Neutral Beam Test Facility in Padova will be the first ITER facility to start operation. While neutral beam injection is used primarily for plasma heating, it can also be used as a diagnostic tool and in feedback control by making a pulsed beam consisting of a string of brief 2–10 ms beam blips. Deuterium is a primary fuel for neutral beam heating systems and hydrogen and helium are sometimes used for selected experiments.


Radio-frequency heating

High-frequency electromagnetic waves are generated by oscillators (often by gyrotrons or klystrons) outside the torus. If the waves have the correct frequency (or wavelength) and polarization, their energy can be transferred to the charged particles in the plasma, which in turn collide with other plasma particles, thus increasing the temperature of the bulk plasma. Various techniques exist including electron cyclotron resonance heating (ECRH) and ion cyclotron resonance heating. This energy is usually transferred by microwaves.


Particle inventory

Plasma discharges within the tokamak's vacuum chamber consist of energized ions and atoms. The energy from these particles eventually reaches the inner wall of the chamber through radiation, collisions, or lack of confinement. The heat from the particles is removed via conduction through the chamber's inner wall to a water-cooling system, where the heated water proceeds to an external cooling system through convection. Turbomolecular or diffusion pumps allow for particles to be evacuated from the bulk volume and cryogenic pumps, consisting of a liquid helium-cooled surface, serve to effectively control the density throughout the discharge by providing an energy sink for condensation to occur. When done correctly, the fusion reactions produce large amounts of high energy
neutron The neutron is a subatomic particle, symbol or , that has no electric charge, and a mass slightly greater than that of a proton. The Discovery of the neutron, neutron was discovered by James Chadwick in 1932, leading to the discovery of nucle ...
s. Being electrically neutral and relatively tiny, the neutrons are not affected by the magnetic fields nor are they stopped much by the surrounding vacuum chamber. The neutron flux is reduced significantly at a purpose-built neutron shield boundary that surrounds the tokamak in all directions. Shield materials vary but are generally materials made of atoms which are close to the size of neutrons because these work best to absorb the neutron and its energy. Good candidate materials include those with much hydrogen, such as water and plastics. Boron atoms are also good absorbers of neutrons. Thus, concrete and polyethylene doped with boron make inexpensive neutron shielding materials. Once freed, the neutron has a relatively short half-life of about 10 minutes before it decays into a proton and electron with the emission of energy. When the time comes to actually try to make electricity from a tokamak-based reactor, some of the neutrons produced in the fusion process would be absorbed by a liquid metal blanket and their kinetic energy would be used in heat transfer processes to ultimately turn a generator.


Experimental tokamaks


Currently in operation

(in chronological order of start of operations) *1960s: TM1-MH (since 1977 as Castor; since 2007 as Golem) in
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,
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. In operation in
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since the early 1960s but renamed to Castor in 1977 and moved to IPP CAS, Prague. In 2007 moved to FNSPE, Czech Technical University in Prague and renamed to Golem. * 1975: T-10, in
Kurchatov Institute The Kurchatov Institute (, National Research Centre "Kurchatov Institute") is Russia's leading research and development institution in the field of nuclear power, nuclear energy. It is named after Igor Kurchatov and is located at 1 Kurchatov Sq ...
,
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, Russia (formerly
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); 2 MW * 1986: DIII-D, in
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, United States; operated by General Atomics since the late 1980s * 1987: STOR-M, University of Saskatchewan, Canada; its predecessor, STOR1-M built in 1983, was used for the first demonstration of alternating current in a tokamak. * 1988: Tore Supra,Tore Supra
but renamed to WEST in 2016, at the CEA,
Cadarache Cadarache () in Southern France is the largest technological research and development centre for energy in Europe. It includes French Alternative Energies and Atomic Energy Commission, CEA research activities and ITER. CEA Cadarache is one of th ...
, France * 1989: Aditya, at Institute for Plasma Research (IPR) in
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, India * 1989:
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, in
Prague Prague ( ; ) is the capital and List of cities and towns in the Czech Republic, largest city of the Czech Republic and the historical capital of Bohemia. Prague, located on the Vltava River, has a population of about 1.4 million, while its P ...
,
Czech Republic The Czech Republic, also known as Czechia, and historically known as Bohemia, is a landlocked country in Central Europe. The country is bordered by Austria to the south, Germany to the west, Poland to the northeast, and Slovakia to the south ...
; in operation since 2008, previously operated from 1989 to 1999 in Culham, United Kingdom * 1990: FTU, in Frascati, Italy * 1991: ISTTOK, at the Instituto de Plasmas e Fusão Nuclear,
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, Portugal * 1991: ASDEX Upgrade, in Garching, Germany * 1992: H-1NF (H-1 National Plasma Fusion Research Facility)Fusion Research: Australian Connections, Past and Future
B. D. Blackwell, M.J. Hole, J. Howard and J. O'Connor
based on the H-1 Heliac device built by Australia National University's plasma physics group and in operation since 1992 * 1992: Tokamak à configuration variable (TCV), at the Swiss Plasma Center, EPFL, Switzerland * 1993: HBT-EP Tokamak, at
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in
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* 1994: TCABR, at the
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,
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, Brazil; this tokamak was transferred from CRPP (now Swiss Plasma Center) in Switzerland * 1996: Pegasus Toroidal Experiment at the
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; in operation since the late 1990s * 1999: NSTX in
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* 1999: Globus-M in Ioffe Institute,
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, Russia * 2000: ETE at the National Institute for Space Research,
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, Brazil * 2002: HL-2A, in
Chengdu Chengdu; Sichuanese dialects, Sichuanese pronunciation: , Standard Chinese pronunciation: ; Chinese postal romanization, previously Romanization of Chinese, romanized as Chengtu. is the capital city of the Chinese province of Sichuan. With a ...
, China * 2006:
EAST East is one of the four cardinal directions or points of the compass. It is the opposite direction from west and is the direction from which the Sun rises on the Earth. Etymology As in other languages, the word is formed from the fact that ea ...
(HT-7U), in
Hefei Hefei is the Capital city, capital of Anhui, China. A prefecture-level city, it is the political, economic, and cultural center of Anhui. Its population was 9,369,881 as of the 2020 census. Its built-up (or ''metro'') area is made up of four u ...
, at The Hefei Institutes of Physical Science, China ( ITER member) * 2007: QUEST, in
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, JAPAN https://www.triam.kyushu-u.ac.jp/QUEST_HP/suben/history.html * 2008: KSTAR, in Daejon, South Korea ( ITER member) * 2012: Medusa CR, in Cartago, at the Costa Rica Institute of Technology,
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* 2012: SST-1, in Gandhinagar, at the Institute for Plasma Research, India ( ITER member) * 2012: IR-T1, Islamic Azad University, Science and Research Branch, Tehran,
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* 2015: ST25-HTS at Tokamak Energy Ltd in Culham, United Kingdom * 2017: KTM – this is an experimental thermonuclear facility for research and testing of materials under energy load conditions close to ITER and future energy fusion reactors, Kazakhstan * 2018: ST40 at Tokamak Energy Ltd in
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, United Kingdom * 2020: HL-2M China National Nuclear Corporation and the Southwestern Institute of Physics, China * 2020: MAST Upgrade, in Culham, United Kingdom * 2023: JT-60SA, in Naka, Japan ( ITER member); upgraded from the JT-60. * 2024: HH70, China


Previously operated

* 1960s: T-3 and T-4, in
Kurchatov Institute The Kurchatov Institute (, National Research Centre "Kurchatov Institute") is Russia's leading research and development institution in the field of nuclear power, nuclear energy. It is named after Igor Kurchatov and is located at 1 Kurchatov Sq ...
, Moscow, Russia (formerly Soviet Union); T-4 in operation in 1968. * 1963: LT-1, Australia National University's plasma physics group built a device to explore toroidal configurations, independently discovering the tokamak layout * 1970: Stellarator C reopens as the Symmetric Tokamak in May at PPPL * 1971–1980: Texas Turbulent Tokamak,
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, US * 1972: The Adiabatic Toroidal Compressor begins operation at PPPL * 1973–1976: Tokamak de Fontenay aux Roses (TFR), near Paris, France * 1973–1979: Alcator A, MIT, US * 1975: Princeton Large Torus begins operation at PPPL * 1978–1987: Alcator C, MIT, US * 1978–2013: TEXTOR, in Jülich, Germany * 1979–1998: MT-1 Tokamak, Budapest, Hungary (Built at the Kurchatov Institute, Russia, transported to Hungary in 1979, rebuilt as MT-1M in 1991) * 1980–1990: Tokoloshe Tokamak, Atomic Energy Board, South Africa * 1980–2004: TEXT/TEXT-U,
University of Texas at Austin The University of Texas at Austin (UT Austin, UT, or Texas) is a public university, public research university in Austin, Texas, United States. Founded in 1883, it is the flagship institution of the University of Texas System. With 53,082 stud ...
, US * 1982–1997: TFTR,
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, US * 1983–2023: Joint European Torus (JET), in Culham, United Kingdom * 1983–2000: Novillo Tokamak, at the Instituto Nacional de Investigaciones Nucleares, in
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, Mexico * 1984–1992: HL-1 Tokamak, in
Chengdu Chengdu; Sichuanese dialects, Sichuanese pronunciation: , Standard Chinese pronunciation: ; Chinese postal romanization, previously Romanization of Chinese, romanized as Chengtu. is the capital city of the Chinese province of Sichuan. With a ...
, China * 1985–2010: JT-60, in Naka,
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, Japan; (Being upgraded 2015–2018 to Super, Advanced model) * 1987–1999: Tokamak de Varennes; Varennes, Canada; operated by
Hydro-Québec Hydro-Québec () is a Canadian Crown corporations of Canada#Quebec, Crown corporation public utility headquartered in Montreal, Quebec. It manages the electricity generation, generation, electric power transmission, transmission and electricity ...
and used by researchers from '' Institut de recherche en électricité du Québec'' (IREQ) and the ''Institut national de la recherche scientifique'' (INRS) * 1988–2005: T-15, in
Kurchatov Institute The Kurchatov Institute (, National Research Centre "Kurchatov Institute") is Russia's leading research and development institution in the field of nuclear power, nuclear energy. It is named after Igor Kurchatov and is located at 1 Kurchatov Sq ...
, Moscow, Russia (formerly Soviet Union); 10 MW * 1991–1998:
START Start can refer to multiple topics: * Takeoff, the phase of flight where an aircraft transitions from moving along the ground to flying through the air * Starting lineup in sports * Track and field#Starts use in race, Starts use in sport race * S ...
, in Culham, United Kingdom * 1990s–2001:
COMPASS A compass is a device that shows the cardinal directions used for navigation and geographic orientation. It commonly consists of a magnetized needle or other element, such as a compass card or compass rose, which can pivot to align itself with No ...
, in Culham, United Kingdom * 1994–2001: HL-1M Tokamak, in
Chengdu Chengdu; Sichuanese dialects, Sichuanese pronunciation: , Standard Chinese pronunciation: ; Chinese postal romanization, previously Romanization of Chinese, romanized as Chengtu. is the capital city of the Chinese province of Sichuan. With a ...
, China * 1999–2006: UCLA Electric Tokamak, in
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, US * 1999–2014: MAST, in Culham, United Kingdom * 1992–2016:
Alcator C-Mod Alcator C-Mod was a tokamak (a type of magnetically confined fusion device) that operated between 1991 and 2016 at the Massachusetts Institute of Technology (MIT) Plasma Science and Fusion Center (PSFC). Notable for its high toroidal magnetic ...
, MIT,
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, US * 1995–2013: HT-7, at the Institute of Plasma Physics,
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, China


Planned

* ITER, international project in
Cadarache Cadarache () in Southern France is the largest technological research and development centre for energy in Europe. It includes French Alternative Energies and Atomic Energy Commission, CEA research activities and ITER. CEA Cadarache is one of th ...
, France; 500 MW; construction began in 2010, first plasma expected in 2025. Expected fully operational by 2035. * DEMO; 2000 MW, continuous operation, connected to power grid. Planned successor to ITER; construction to begin in 2040 according to EUROfusion 2018 timetable. * CFETR, also known as "China Fusion Engineering Test Reactor"; 200 MW; Next generation Chinese fusion reactor, is a new tokamak device. * K-DEMO in South Korea; 2200–3000 MW, a net electric generation on the order of 500 MW is planned; construction is targeted by 2037. * Spherical Tokamak for Energy Production (STEP), a UK project planning to produce a burning plasma by 2035. * SPARC a development of Commonwealth Fusion Systems (CFS) in collaboration with the
Massachusetts Institute of Technology The Massachusetts Institute of Technology (MIT) is a Private university, private research university in Cambridge, Massachusetts, United States. Established in 1861, MIT has played a significant role in the development of many areas of moder ...
(MIT) Plasma Science and Fusion Center (PSFC) in Devens, Massachusetts.Verma, Pranshu
Nuclear fusion power inches closer to reality.
The Washington Post, August 26, 2022.
Expected to achieve energy gain in 2026 with a fraction of ITERs size by utilizing high magnetic fields.


See also

* Edge-localized mode, a tokamak plasma instability * Reversed-field pinch, an alternative design * Ball-pen probe * Dimensionless parameters in tokamaks in the article on Plasma scaling * Lawson criterion, and triple product, needed for break-even and ignition * Fusion power § Records, inc beta, Q * ARC fusion reactor, an MIT tokamak design


Notes


References


Citations


Bibliography

* * * * * * * * * * * * * *


External links


CCFE
– site from the UK fusion research centre CCFE.
Int'l Tokamak research
– various that relate to ITER

– site on tokamaks from the French CEA.
Fusion Programs
at General Atomics, including the DIII-D National Fusion Facility, an experimental tokamak.
General Atomics DIII-D Program

Fusion and Plasma Physics Seminar
at MIT OCW
Unofficial ITER fan club
– fans of the biggest tokamak planned to be built in near future.
All-the-Worlds-Tokamaks
Extensive list of current and historic tokamaks from around the world.
SSTC-1
Overview video of a small scale tokamak concept. * Section View Video of a small scale tokamak concept. * Fly Through Video of a small scale tokamak concept.

Information on conditions necessary for nuclear reaction in a tokamak reactor *
Observer Newspaper Article on Tokomak
Nuclear fusion and the promise of a brighter tomorrow {{Authority control Science and technology in the Soviet Union Soviet inventions Deuterium Tritium