First steps In 1934,
Mark Oliphant,
Paul Harteck and
Ernest Rutherford were the first to achieve fusion on Earth, using a
particle accelerator to shoot
deuterium nuclei into metal foil containing deuterium or other atoms. This allowed them to measure the
nuclear cross section of various fusion reactions, and determined that the deuterium–deuterium reaction occurred at a lower energy than other reactions, peaking at about 100,000
electronvolts (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 due to the high temperatures needed to bring it about. In 1944,
Enrico Fermi calculated the reaction would be self-sustaining at about 50,000,000 K; at that temperature, the rate that energy is given off by the reactions is high enough that they heat the surrounding fuel rapidly enough to maintain the temperature against losses to the environment, continuing the reaction. During the
Manhattan Project, the first practical way to reach these temperatures was created, using an
atomic bomb. In 1944, Fermi gave a talk on the physics of fusion in the context of a then-hypothetical
hydrogen bomb. However, some thought had already been given to
controlled fusion, and
James L. Tuck and
Stanislaw Ulam had attempted such using
shaped charges driving a metal foil infused with deuterium, although without success. The first attempts to build a practical fusion machine took place in the
United Kingdom, where
George Paget Thomson had selected the
pinch effect as a promising technique in 1945. After several failed attempts to gain funding, he gave up and asked two graduate students, Stanley (Stan) W. Cousins and Alan Alfred Ware (1924–2010), to build something out of surplus
radar equipment. This successfully operated in 1948, but showed no clear evidence of fusion and failed to gain the interest of the
Atomic Energy Research Establishment.
Lavrentiev's letter In 1950,
Oleg Lavrentiev, then a
Soviet Army sergeant stationed on
Sakhalin, wrote a letter to the
Central Committee of the Communist Party of the Soviet Union. The letter outlined the idea of using an
atomic bomb to ignite fusion fuel, and then went on to describe a system that used
electrostatic fields to contain hot plasma in a steady state for energy production. The letter was sent to
Andrei Sakharov for comment. Sakharov noted that "the author formulates a very important and not necessarily hopeless problem", and found that his main concern in the arrangement was that the plasma would hit the electrode wires, and that "wide meshes and a thin current-carrying part which will have to reflect almost all incident nuclei back into the reactor." Some indication of the importance given to Lavrentiev's letter can be seen in the speed with which it was processed; the letter was received by the Central Committee on 29 July, Sakharov sent his review in on 18 August, by October, Sakharov and
Igor Tamm had completed the first detailed study of a fusion reactor, and they had asked for funding to build it in January 1951.
Magnetic confinement When heated to fusion temperatures, the
electrons in atoms dissociate, resulting in a fluid of nuclei and electrons known as
plasma. Unlike electrically neutral atoms, a plasma is electrically conductive, and can, therefore, be manipulated by electrical or magnetic fields. Sakharov's concern about the electrodes led him to consider using magnetic confinement instead of electrostatic. In the case of a magnetic field, the particles will circle around the
lines of force. As the particles are moving at high speed, their resulting paths look like a helix. If one arranges a magnetic field so lines of force are parallel and close together, the particles orbiting adjacent lines may collide, and fuse. Such a field can be created in a
solenoid, a cylinder with magnets wrapped around the outside. The combined fields of the magnets create a set of parallel magnetic lines running down the length of the cylinder. This arrangement prevents the particles from moving sideways to the wall of the cylinder, but it does not prevent them from running out the end. The obvious solution to this problem is to bend the cylinder around into a donut shape, or torus, so that the lines form a series of continual rings. In this arrangement, the particles circle endlessly. Sakharov discussed the concept with
Igor Tamm, and by the end of October 1950 the two had written a proposal and sent it to
Igor Kurchatov, the director of the atomic bomb project within the USSR, and his deputy,
Igor Golovin. However, this initial proposal ignored a fundamental problem; when arranged along a straight solenoid, the external magnets are evenly spaced, but when bent around into a torus, they are closer together on the inside of the ring than the outside. This leads to uneven forces that cause the particles to drift away from their magnetic lines. During visits to the
Laboratory of Measuring Instruments of the USSR Academy of Sciences (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
orbits around the long axis of the
torus, the opposite drifts would cancel out. Alternately, he suggested using an external magnet to induce a current in the
plasma itself, instead of a separate metal ring, which would have the same effect. In January 1951, Kurchatov arranged a meeting at LIPAN to consider Sakharov's concepts. They found widespread interest and support, and in February a report on the topic was forwarded to
Lavrentiy Beria, who oversaw the atomic efforts in the USSR. For a time, nothing was heard back.
Richter and the birth of fusion research (right). Richter's claims sparked off fusion research around the world. On 25 March 1951, Argentine President
Juan Perón 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 1951, he was awarded $50,000 to begin research on his
stellarator 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. In mid-April 1951, 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 1951, the proposal had been signed by
Joseph Stalin.
New ideas , with visible light radiation dominated by the
hydrogen alpha line emitting 656 nm light. By October 1951, Sakharov and Tamm had completed a far more detailed consideration of their original proposal, calling for something 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 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 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
neutrons 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. 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, it penetrated only the outer areas of the plasma. 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 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 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. 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. Petersburg 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 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 had been part of the team working on isotopic separation of
uranium. 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 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. 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 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, 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 of a plasma was reduced as the temperature increased, 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. 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 (ITER) the largest tokamak in the world, which began construction in 2013 and is scheduled to begin its initial phase of operations in 2034/2035. It is intended as a demonstration that a practical
fusion reactor is possible, and will produce 500 megawatts of power. Human figure in blue at bottom for scale. At the same time these experiments were demonstrating problems, much of the impetus for the US's massive funding disappeared; in 1986
Ronald Reagan declared the
1970s energy crisis 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 and
Leonid Brezhnev, 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 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 Stronger field magnets enable high energy gain in a much smaller tokamak; concepts such as FIRE, IGNITOR, and the
Compact Ignition Tokamak (CIT) were early proposals. 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 the 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 planning on building a net-energy
spherical tokamak using HTS magnets. 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 ==