Tokamak

A tokamak (/ˈtoʊkəmæk/; Russian: токамáк) is

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

The proposal to use controlled thermonuclear

fusion for industrial purposes and a specific The reaction chamber of the DIII-D, an experimental
scheme using thermal insulation of high- tokamak fusion reactor operated by General Atomics in
temperature plasma by an electric field was first San Diego, which has been used in research since it
formulated by the Soviet physicist Oleg was completed in the late 1980s. The characteristic
torus-shaped chamber is clad with graphite to help
Lavrentiev in a mid-1950 paper.[3] In 1951,
withstand the extreme heat.
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.[4]

The first tokamak was built in 1954,[5] and for over a decade this technology existed only in the USSR. In
1968 the electronic plasma temperature of 1 keV was reached on the tokamak T-3, built at the I. V.
Kurchatov Institute of Atomic Energy under the leadership of academician L. A. Artsimovich.[6][7][8]

By the mid-1960s, the tokamak designs began to show greatly improved performance. The initial results
were released in 1965, but were ignored; Lyman Spitzer dismissed them out of hand after noting potential
problems in their system for measuring temperatures. A second set of results was published in 1968, this
time claiming performance far in advance of any other machine. When these were also met skeptically,
the Soviets invited British scientists from the laboratory in Culham Centre for Fusion Energy (Nicol
Peacock et al.) to the USSR with their equipment.[9] Measurements on the T-3 confirmed the
results,[10][11] spurring a worldwide stampede of tokamak construction. It had been demonstrated that a
stable plasma equilibrium requires magnetic field lines that wind around the torus in a helix. Devices like
the z-pinch and stellarator had attempted this, but demonstrated serious instabilities. It was the
development of the concept now known as the safety factor (labelled q in mathematical notation) that
guided tokamak development; by arranging the reactor so this critical factor q was always greater than 1,
the tokamaks strongly suppressed the instabilities which plagued earlier designs.

By the mid-1970s, dozens of tokamaks were in use around the world. By the late 1970s, these machines
had reached all of the conditions needed for practical fusion, although not at the same time nor in a single
reactor. With the goal of breakeven (a fusion energy gain factor equal to 1) now in sight, a new series of
machines were designed that would run on a fusion fuel of deuterium and tritium. These machines,
notably the Joint European Torus (JET) and Tokamak Fusion Test Reactor (TFTR), had the explicit goal
of reaching breakeven.

Instead, these machines demonstrated new problems that limited their performance. Solving these would
require a much larger and more expensive machine, beyond the abilities of any one country. After an
initial agreement between Ronald Reagan and Mikhail Gorbachev in November 1985, the International
Thermonuclear Experimental Reactor (ITER) effort emerged and remains the primary international effort
to develop practical fusion power. Many smaller designs, and offshoots like the spherical tokamak,
continue to be used to investigate performance parameters and other issues. As of 2024, JET remains the
record holder for fusion output, with 69 MJ of energy output over a 5-second period.[12]

Etymology
The word tokamak is a transliteration of the Russian word токамак, an acronym of either:

тороидальная камера с
toroidal'naya kamera s
toroidal chamber with
магнитными катушками
magnitnymi katushkami
magnetic coils
or:

тороидальная камера с
toroidal'naya kamera s
toroidal chamber with
аксиальным магнитным полем
aksial'nym magnitnym polem
axial magnetic field

[13]

The term "tokamak" was coined in 1957[14] by Igor Golovin, a student of academician Igor Kurchatov. It
originally sounded like "tokamag" ("токамаг") — an acronym of the words «toroidal chamber magnetic»
(«тороидальная камера магнитная»), but Natan Yavlinsky, the author of the first toroidal system,
proposed replacing "-mag" with "-mak" for euphony.[15] Later, this name was borrowed by many
languages.

History

First steps
In 1934, Mark Oliphant, Paul Harteck and Ernest Rutherford were the first
to achieve fusion on Earth, using a particle accelerator to shoot deuterium
nuclei into metal foil containing deuterium or other atoms.[16] 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).[17][a]

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 A USSR stamp, 1987:
energy than the resulting fusion reactions can release.[19] Tokamak thermonuclear
system
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.[19]

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 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.[20]

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[21]), to build a device out of surplus radar 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.[22]

Lavrentiev's letter
In 1950, Oleg Lavrentiev, then a Red 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 a fusion fuel, and then went on to describe a system that used electrostatic fields to contain
a hot plasma in a steady state for energy production.[23][24][b]

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."[23]
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.[25]

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.[26]

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.[26] 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.[27]

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.[27]

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.[27] 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.[28][29]

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.[28]

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.[28]

Richter and the birth of fusion research

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.[30] Ronald Richter (left) with Juan Domingo Perón (right).
Richter's claims sparked off fusion research around the
Although dismissed by nuclear researchers, the world.
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.[31] In the US, Lyman Spitzer read the Huemul
story, realized it was false, and set about designing a machine that would work.[32] In May he was
awarded $50,000 to begin research on his stellarator concept.[33] 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.[34]

Similar events occurred in the USSR. 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.[28]

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 12 metres
(39 ft) and a minor radius (the interior of the cylinder) of 2 metres
(6 ft 7 in). The proposal suggested the system could produce 100
grams (3.5 oz) of tritium a day, or breed 10 kilograms (22 lb) of
U233 a day.[28]

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.[35] At
this point, the Soviet researchers had re-invented the pinch system
being developed in the UK,[20] although they had come to this
design from a very different starting point. Red plasma in EAST, with visible
light radiation dominated by the
hydrogen alpha line emitting 656 nm
Once the idea of using the pinch effect for confinement had been
light.
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.[35]

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.[35] 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.[36]

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.[36] Groups in all three
countries began studying the formation of these instabilities and potential ways to address them.[37]
Important contributions to the field were made by Martin David Kruskal and Martin Schwarzschild in the
US, and Shafranov in the USSR.[38]

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.[36] 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".[39]

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.[35]

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.[35] Two smaller
machines followed, using copper shells.[40] The conductive shells were intended to help stabilize the
plasma, but were not completely successful in any of the machines that tried it.[41]

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.[42] 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.[43] He took time to note, in particular, the neutrons
seen in early machines and warned that neutrons did not mean
fusion.[44]

Unknown to Kurchatov, the British ZETA stabilized pinch

machine was being built at the far end of the former runway.
Khrushchev (roughly centred, bald),
ZETA was, by far, the largest and most powerful fusion machine
Kurchatov (to the right, bearded),
to date. Supported by experiments on earlier designs that had been and Bulganin (to the right, white-
modified to include stabilization, ZETA intended to produce low haired) visited Harwell on 26 April
levels of fusion reactions. This was apparently a great success, and 1956. Cockcroft stands across from
in January 1958, they announced the fusion had been achieved in them (in glasses), while a presenter
ZETA based on the release of neutrons and measurements of the points to mockups of various
plasma temperature.[45] materials being tested in the newly
opened DIDO reactor.
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.[43]

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.[37]

(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. 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.[37]

Following this criterion, design began on a new reactor, T-1, which today is known as the first real
tokamak.[40] 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.[46][47][48][49] 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.[43][50]

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.[43] 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.[51]

T-1 began operation at the end of 1958.[52][c] 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 550 °C (1,022 °F) to cook off
trapped gasses.[52]

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.[53]

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.[53]

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.[52]

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.[54]

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,[55] 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.[54]

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.[52] The entire field descended into what became known as "the doldrums",[56] a period
of intense pessimism.[35]

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.[57]

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.[57]

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.[58]

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.[59]

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;[60] 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.[61]
The British team, nicknamed "The Culham Five",[62] 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.[63] The final results were published in Nature in November 1969.[64] The results of this
announcement have been described as a "veritable stampede" of tokamak construction around the
world.[65]

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,[66] 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.[67]

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 (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.[68]

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?"[69] 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.[70]

After a considerable internal debate, Herman Postma formed a small group in early 1969 to consider the
tokamak.[70] 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 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, which the Soviets had already
suggested would produce better results.[71]

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[67] 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.[67]

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.[72] Meanwhile, a group at University of Texas at Austin was proposing a relatively
simple tokamak to explore heating the plasma through deliberately induced turbulence, the Texas
Turbulent Tokamak.[73]

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".[73] 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.[73]

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."[63] After lunch, the various
teams presented their designs, at which point Gottlieb presented his idea for a "stellarator-tokamak" based
on the Model C.[63]

The Standing Committee noted that this system could be complete in six months, while Ormak would
take a year.[63] 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, intended to simply
verify the Soviet results, while the others would explore ways to go well beyond T-3.[74]

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.[75] 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.[76]

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.[77]
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".[78] 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 Overhead view of the Princeton Large Torus in 1975.
PLT set numerous records and demonstrated that the
technically achievable.[78]
temperatures needed for fusion were possible.

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.[58] They could afford to explore all of the
promising methods of heating, ultimately discovering neutral beams to be among the most effective.[79]

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.[79] This was given a boost by the 1973 oil crisis, which led to greatly increased
research into alternative energy systems.[80]

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,[81] 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 in its fuel
mix.[82]
Joint European Torus (JET), in operation from 1983 to
2023
During the 1970s, four major second-generation
proposals were funded worldwide. The Soviets
continued their development lineage with the T-15,[81] 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.[82] 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.[83]

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.[84]

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 declared the 1970s energy crisis was over,[85] 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.[86]

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."[87]

The next year, an agreement was signed between the US, Soviet Union, European Union and Japan,
creating the International Thermonuclear Experimental Reactor organization.[88][89]
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 as
FIRE, IGNITOR (https://fire.pppl.gov/snowmass
02.html), 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
Cutaway diagram of the International Thermonuclear
energy gain in a compact device. To leverage this Experimental Reactor (ITER) the largest tokamak in
new technology, the MIT Plasma Science and the world, which began construction in 2013 and is
Fusion Center (PSFC) and MIT spinout projected to begin full operation in 2035. It is intended
Commonwealth Fusion Systems (CFS) as a demonstration that a practical fusion reactor is
successfully built and tested the Toroidal Field possible, and will produce 500 megawatts of power.
Blue human figure at bottom shows scale.
Model Coil (TFMC) (https://news.mit.edu/2021/
MIT-CFS-major-advance-toward-fusion-energy-
0908) 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.[90][91]

Design

Basic problem
Positively charged ions and negatively charged electrons 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.[92]

The simplest magnetic confinement system is a solenoid. 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.[29]

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
Magnetic fields in a tokamak
outside edge where it will drift one way, say up,
and 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.[92]

Tokamak solution
The two first solutions to making a design with the required twist were the
stellarator 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.[93] Its key innovation was the realization that the instabilities that Tokamak magnetic field and
were causing the pinch to lose its plasma could be controlled. The issue current. Shown is the
was how "twisty" the fields were; fields that caused the particles to transit toroidal field and the coils
inside and out more than once per orbit around the long axis torus were (blue) that produce it, the
much more stable than devices that had less twist. This ratio of twists to plasma current (red) and
orbits became known as the safety factor, denoted q. Previous devices the poloidal field created by
it, and the resulting twisted
operated at q about 1⁄3, while the tokamak operates at q ≫ 1. This
field when these are
increases stability by orders of magnitude. overlaid.

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
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.[94]

Another instability is tearing instability. In 2024 researchers used reinforcement learning 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.[95][96]

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 and tritium, is in the form of alpha particles. 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.[97]

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".[98]
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.[99] 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.[100]
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 that would cause the heavier
elements to be flung out of the reactor. This was initially part of the stellarator 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 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.[101]

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.[101] 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.[102]

For modern high-energy devices, where plasma currents are on the order of 15 megaamperes 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.[102]

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.[101] 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.[102]

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.[103][104][105]
The development of systems to counter the effects of runaway electrons is considered a must-have piece
of technology for the operational level ITER.[102]

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.[106]
Densities over the Greenwald limit, a bound depending on the plasma current and the minor radius,
typically leads to disruptions.[107][108] It has been exceeded up to factors of 10,[109] but it remains an
important concept describing the phenomenology of the transition of the plasma flow, which still needs to
be understood.[110]

Plasma heating
In an operating fusion reactor, part of the energy generated will serve to maintain the plasma temperature
as fresh deuterium and tritium 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 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.[111]

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.
Set of hyperfrequency tubes (84
GHz and 118 GHz) for plasma
heating by electron cyclotron waves
Particle inventory on the Tokamak à Configuration
Variable (TCV). Courtesy of SPC-
EPFL.
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 neutrons. 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[112]) in Prague, Czech Republic. In operation in
Kurchatov Institute since the early 1960s but renamed to
Castor in 1977 and moved to IPP CAS,[113] Prague. In
2007 moved to FNSPE, Czech Technical University in
Prague and renamed to Golem.[114]
1975: T-10, in Kurchatov Institute, Moscow, Russia
(formerly Soviet Union); 2 MW
1986: DIII-D,[115] in San Diego, 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 The Tokamak à Configuration
tokamak.[116] Variable
1988: Tore Supra,[117] but renamed to WEST in 2016, at the CEA, Cadarache, France
1989: Aditya, at Institute for Plasma Research (IPR) in Gujarat, India
1989: COMPASS,[113] in Prague, Czech Republic; in operation since 2008, previously
operated from 1989 to 1999 in Culham, United Kingdom
1990: FTU,[118] in Frascati, Italy
1991: ISTTOK,[119] at the Instituto de Plasmas e Fusão Nuclear, Lisbon, Portugal
1991: ASDEX Upgrade, in Garching, Germany
1992: H-1NF (H-1 National Plasma Fusion Research
Facility)[120] 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 (https://www.epfl.ch/research/dom
ains/swiss-plasma-center/), EPFL, Switzerland
1993: HBT-EP Tokamak, at Columbia University in New
York City[121] Outside view of the NSTX reactor
1994: TCABR, at the University of São Paulo, São
Paulo, Brazil; this tokamak was transferred from CRPP
(now Swiss Plasma Center (https://www.epfl.ch/research/domains/swiss-plasma-center/)) in
Switzerland
1996: Pegasus Toroidal Experiment[122] at the University of Wisconsin–Madison; in
operation since the late 1990s
1999: NSTX in Princeton, New Jersey
1999: Globus-M (http://globus.rinno.ru/) in Ioffe Institute, Saint Petersburg, Russia
2000: ETE at the National Institute for Space Research, São Paulo, Brazil
2002: HL-2A, in Chengdu, China
2006: EAST (HT-7U), in Hefei, at The Hefei Institutes of Physical Science, China (ITER
member)
2007: QUEST, in Fukuoka, JAPAN https://www.triam.kyushu-
u.ac.jp/QUEST_HP/suben/history.html
2008: KSTAR, in Daejon, South Korea (ITER member)
2010: JT-60SA, in Naka, Japan (ITER member); upgraded from the JT-60.
2012: Medusa CR, in Cartago, at the Costa Rica Institute of Technology, Costa Rica
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, Iran[123]
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 Oxford, United Kingdom
2020: HL-2M China National Nuclear Corporation and the Southwestern Institute of Physics,
China
 2020: MAST Upgrade, in Culham, United Kingdom

Previously operated
1960s: T-3 and T-4, in Kurchatov Institute, 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, University of
Texas at Austin, US The control room of the Alcator C
1972: The Adiabatic Toroidal Compressor begins tokamak at the MIT Plasma Science
operation at PPPL and Fusion Center, in about 1982–
1973–1976: Tokamak de Fontenay aux Roses (TFR), 1983.
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[124]
1980–2004: TEXT/TEXT-U, University of Texas at Austin, US
1982–1997: TFTR, Princeton University, US
1983–2023: Joint European Torus (JET), in Culham, United Kingdom[125]
1983–2000: Novillo Tokamak,[126] at the Instituto Nacional de Investigaciones Nucleares, in
Mexico City, Mexico
1984–1992: HL-1 Tokamak, in Chengdu, China
1985–2010: JT-60, in Naka, Ibaraki Prefecture, Japan; (Being upgraded 2015–2018 to
Super, Advanced model)
1987–1999: Tokamak de Varennes; Varennes, Canada; operated by Hydro-Québec 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, Moscow, Russia (formerly Soviet Union); 10 MW
1991–1998: START, in Culham, United Kingdom
1990s–2001: COMPASS, in Culham, United Kingdom
1994–2001: HL-1M Tokamak, in Chengdu, China
1999–2006: UCLA Electric Tokamak, in Los Angeles, US
1999–2014: MAST, in Culham, United Kingdom
1992–2016: Alcator C-Mod,[127] MIT, Cambridge, US
 1995–2013: HT-7, at the Institute of Plasma Physics, Hefei, China[128]

Planned
ITER, international project in Cadarache, France; 500
MW; construction began in 2010, first plasma expected
in 2025. Expected fully operational by 2035.[129]
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
ITER, currently under construction,
reactor, is a new tokamak device.[130][131][132][133]
will be the largest tokamak by far.
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.[134]
SPARC a development of Commonwealth Fusion Systems (CFS) in collaboration with the
Massachusetts Institute of Technology (MIT) Plasma Science and Fusion Center (PSFC) in
Devens, Massachusetts.[135][136] Expected to achieve energy gain in 2026 with a fraction of
ITERs size by utilizing high magnetic fields.

See also
Nuclear
technology portal
Energy portal

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
a. D–T fusion occurs at even lower energies, but tritium was unknown at the time. Their work
created tritium, but they did not separate it chemically to demonstrate its existence. This was
performed by Luis Alvarez and Robert Cornog in 1939.[18]
b. The system Lavrentiev described is very similar to the concept now known as the fusor.
 c. Although one source says "late 1957".[40]

References

Citations
1. "DOE Explains...Tokamaks" (https://www.energy.gov/science/doe-explainstokamaks).
Energy.gov. Retrieved 15 December 2023.
2. Greenwald, John (24 August 2016). "Major next steps for fusion energy based on the
spherical tokamak design" (https://web.archive.org/web/20210919050707/https://www.pppl.
gov/news/2016/08/major-next-steps-fusion-energy-based-spherical-tokamak-design).
Princeton Plasma Physics Laboratory. United States Department of Energy. Archived from
the original (https://www.pppl.gov/news/2016/08/major-next-steps-fusion-energy-based-sph
erical-tokamak-design) on 19 September 2021. Retrieved 16 May 2018.
3. B.D.Bondarenko The role of O. A. Lavrentiev in raising the issue and initiating research on
controlled thermonuclear fusion in the USSR (https://ufn.ru/ru/articles/2001/8/q/) Archived (h
ttps://web.archive.org/web/20170912101849/https://ufn.ru/ru/articles/2001/8/q/) 12
September 2017 at the Wayback Machine // UFN 171, 886 (2001).
4. "The Soviet Magnetic Confinement Fusion Program: An International future (SW 90-" (http
s://web.archive.org/web/20101105091945/http://www.foia.cia.gov/docs/DOC_0000498695/D
OC_0000498695.pdf) (PDF). Archived from the original (https://www.cia.gov/library/readingr
oom/document/0000498695) on 5 November 2010. Retrieved 27 June 2019.
5. V.Reshetov "An ocean of energy" (http://www.vokrugsveta.ru/vs/article/429/) Archived (http
s://web.archive.org/web/20131113224010/http://www.vokrugsveta.ru/vs/article/429/) 13
November 2013 at the Wayback Machine // Around the world
6. Garry McCracken, Peter Stott (2015). Fusion: The Energy of the Universe (https://books.goo
gle.com/books?id=XagVcVzaG5oC&pg=PA167). Elsevier Academic Press. p. 167.
ISBN 978-0-12-481851-4.
7. L.A.Artsimovich; et al. (1969). Experimental studies on Tokamak installations (CN-24/B-1) (h
ttps://inis.iaea.org/search/search.aspx?orig_q=RN:44064038). Proceedings of the Third
International Conference on Plasma Physics and Controlled Nuclear Fusion Research Held
by the International Atomic Energy Agency at Novosibirsk, 1–7 August 1968. Vol. 1 (Plasma
Physics and Controlled Nuclear Fusion Research. ed.). Vienna: IAEA. pp. 157–173.
8. Juho Miettunen (2015). Modelling of global impurity transport in tokamaks in the presence of
non-axisymmetric effects (https://aaltodoc.aalto.fi/server/api/core/bitstreams/92ee2a0e-769a
-4d9f-8f2b-3ec9a57640bc/content) (PhD thesis). Helsinki: Unigrafia Oy. p. 19. ISBN 978-
952-60-6189-4.
9. Robert Arnoux (9 October 2009). "Off to Russia with a thermometer" (https://web.archive.or
g/web/20190708181712/https://www.iter.org/newsline/102/1401). ITER Newsline 102.
Archived from the original (https://www.iter.org/newsline/102/1401) on 8 July 2019.
Retrieved 8 July 2019.
10. Peacock N. J.; et al. (1969). "Measurement of the Electron Temperature by Thomson
Scattering in Tokamak T3". Nature. 224 (5218): 488–490. Bibcode:1969Natur.224..488P (htt
ps://ui.adsabs.harvard.edu/abs/1969Natur.224..488P). doi:10.1038/224488a0 (https://doi.or
g/10.1038%2F224488a0). S2CID 4290094 (https://api.semanticscholar.org/CorpusID:42900
94).
11. Evgeny Velikhov (2004). "I didn't let my soul be lazy. To the 95th anniversary of the birth of
Academician L. A. Artsimovich" (https://web.archive.org/web/20201022150618/http://vivovoc
o.astronet.ru/VV/PAPERS/BIO/VELAR.HTM). Herald of the Russian Academy of Sciences.
74 (10): 940. Archived from the original (http://vivovoco.astronet.ru/VV/PAPERS/BIO/VELA
R.HTM) on 22 October 2020.
12. "Nuclear fusion: new record brings dream of clean energy closer" (https://www.bbc.co.uk/ne
ws/science-environment-68233330). www.bbc.co.uk. 8 February 2024. Retrieved
8 February 2024.
13. "Tokamak" (http://www.merriam-webster.com/dictionary/tokamak). Merriam-Webster. 6 July
2023.
14. V.D.Shafranov (1999). "Prospects of screw magnetic systems for TC". Achievements of the
Physical Sciences (in Russian). 169 (7). Russian Academy of Sciences: 808.
15. A.Y.Pogosov; V.A.Dubkovsky (2013). Pogosov A. Yu. (ed.). Ionizing radiation: radioecology,
physics, technology, protection: textbook for university students. Odessa: Science and
Technology. p. 343. ISBN 978-966-1552-27-1.
16. Oliphant, Mark; Harteck, Paul; Rutherford, Ernest (1934). "Transmutation Effects Observed
with Heavy Hydrogen" (http://www.chemteam.info/Chem-History/Rutherford-1934b/Rutherfo
rd-1934b.html). Proceedings of the Royal Society. 144 (853): 692–703.
Bibcode:1934RSPSA.144..692O (https://ui.adsabs.harvard.edu/abs/1934RSPSA.144..692
O). doi:10.1098/rspa.1934.0077 (https://doi.org/10.1098%2Frspa.1934.0077).
17. McCracken & Stott 2012, p. 35.
18. Alvarez, Luis; Cornog, Robert (1939). "Helium and Hydrogen of Mass 3". Physical Review.
56 (6): 613. Bibcode:1939PhRv...56..613A (https://ui.adsabs.harvard.edu/abs/1939PhRv...5
6..613A). doi:10.1103/PhysRev.56.613 (https://doi.org/10.1103%2FPhysRev.56.613).
19. McCracken & Stott 2012, pp. 36–38.
20. Bromberg 1982, p. 18.
21. "UTPhysicsHistorySite" (https://web.archive.org/web/20220529211017/https://web2.ph.utex
as.edu/utphysicshistory/AlanAWare.html). Archived from the original (https://web2.ph.utexa
s.edu/utphysicshistory/AlanAWare.html) on 29 May 2022. Retrieved 29 May 2022.
22. Herman 1990, p. 40 (https://archive.org/details/fusionsearchfore00herm/page/40).
23. Shafranov 2001, p. 873.
24. Bondarenko, B.D. (2001). "Role played by O. A. Lavrent'ev in the formulation of the problem
and the initiation of research into controlled nuclear fusion in the USSR" (http://ufn.ru/ufn01/
ufn01_8/Russian/r018m.pdf) (PDF). Phys. Usp. 44 (8): 844.
doi:10.1070/PU2001v044n08ABEH000910 (https://doi.org/10.1070%2FPU2001v044n08AB
EH000910). S2CID 250885028 (https://api.semanticscholar.org/CorpusID:250885028).
25. Shafranov 2001, p. 837.
26. Bromberg 1982, p. 15.
27. Shafranov 2001, p. 838.
28. Shafranov 2001, p. 839.
29. Bromberg 1982, p. 16.
30. Arnoux, Robert (26 October 2011). " 'Proyecto Huemul': the prank that started it all" (https://
www.iter.org/newsline/196/930). iter.
31. Bromberg 1982, p. 75.
32. Bromberg 1982, p. 14.
33. Bromberg 1982, p. 21.
34. Bromberg 1982, p. 25.
35. Shafranov 2001, p. 840.
36. Adams, John (31 January 1963). "Can we master the thermonuclear plasma?" (https://book
s.google.com/books?id=TF7wQYDRIXAC&pg=PA222). New Scientist. pp. 222–225.
37. Cowley, Steve. "Introduction to Kink Modes – the Kruskal- Shafranov Limit" (https://web.arch
ive.org/web/20180128001629/http://home.physics.ucla.edu/calendar/conferences/cmpd/talk
s/cowley.pdf) (PDF). UCLA. Archived from the original (http://home.physics.ucla.edu/calenda
r/conferences/cmpd/talks/cowley.pdf) (PDF) on 28 January 2018. Retrieved 9 April 2018.
38. Kadomtsev 1966.
39. Clery 2014, p. 48.
40. Arnoux, Robert (27 October 2008). "Which was the first 'tokamak' – or was it 'tokomag'?" (htt
ps://www.iter.org/newsline/55/1194). ITER.
41. Bromberg 1982, p. 70.
42. Shafranov 2001, p. 240.
43. Shafranov 2001, p. 841.
44. Kurchatov, Igor (26 April 1956). The possibility of producing thermonuclear reactions in a
gaseous discharge (https://www.iter.org/doc/www/content/com/Lists/Mag%20Stories/Attach
ments/64/kurchatov_1956.pdf) (PDF) (Speech). UKAEA Harwell.
45. McCracken & Stott 2012, p. 5.
46. Arnoux, Robert (27 October 2008). "Which was the first 'tokamak' – or was it 'tokomag'?" (htt
ps://www.iter.org/newsline/55/1194). ITER. Retrieved 6 November 2018.
47. Shafranov 2001.
48. "К столетию со дня рождения Н. А. Явлинского" (http://vant.iterru.ru/vant_2012_1/naj.pdf)
(PDF).
49. "В. Д. Шафранов "К истории исследований по управляемому термоядерному синтезу" "
(http://ufn.ru/ufn01/ufn01_8/Russian/r018l.pdf) (PDF). Успехи Физических Наук. 171 (8):
877. August 2001.
50. "ОТЦЫ И ДЕДЫ ТЕРМОЯДЕРНОЙ ЭПОХИ" (http://www.ras.ru/digest/showdnews.aspx?id
=578d7525-c224-4f92-9a57-ed7113cb2c75&print=1). Retrieved 6 November 2018.
51. Herman 1990, p. 53.
52. Smirnov 2009, p. 2.
53. Shafranov 2001, p. 842.
54. Bromberg 1982, p. 66.
55. Spitzer, L. (1960). "Particle Diffusion across a Magnetic Field". Physics of Fluids. 3 (4): 659.
Bibcode:1960PhFl....3..659S (https://ui.adsabs.harvard.edu/abs/1960PhFl....3..659S).
doi:10.1063/1.1706104 (https://doi.org/10.1063%2F1.1706104).
56. Bromberg 1982, p. 130.
57. Bromberg 1982, p. 153.
58. Bromberg 1982, p. 151.
59. Bromberg 1982, p. 166.
60. Bromberg 1982, p. 172.
61. "The Valleys boy who broached the Iron Curtain to convince the USA that Russian Cold War
nuclear fusion claims were true" (https://www.walesonline.co.uk/news/wales-news/valleys-b
oy-who-broached-iron-1794244). WalesOnline. 3 November 2011.
62. Arnoux, Robert (9 October 2009). "Off to Russia with a thermometer" (http://www.iter.org/ne
wsline/102/1401). ITER Newsline. No. 102.
63. Bromberg 1982, p. 167.
64. Peacock, N. J.; Robinson, D. C.; Forrest, M. J.; Wilcock, P. D.; Sannikov, V. V. (1969).
"Measurement of the Electron Temperature by Thomson Scattering in Tokamak T3". Nature.
224 (5218): 488–490. Bibcode:1969Natur.224..488P (https://ui.adsabs.harvard.edu/abs/196
9Natur.224..488P). doi:10.1038/224488a0 (https://doi.org/10.1038%2F224488a0).
S2CID 4290094 (https://api.semanticscholar.org/CorpusID:4290094).
65. Kenward, Michael (24 May 1979). "Fusion research - the temperature rises" (https://books.g
oogle.com/books?id=tbhTdnZsqMUC&pg=PA626). New Scientist.
66. Cohen, Robert S.; Spitzer, Lyman Jr.; McR. Routly, Paul (October 1950). "The Electrical
Conductivity of an Ionized Gas" (http://ayuba.fr/pdf/spitzer1950.pdf) (PDF). Physical Review.
80 (2): 230–238. Bibcode:1950PhRv...80..230C (https://ui.adsabs.harvard.edu/abs/1950Ph
Rv...80..230C). doi:10.1103/PhysRev.80.230 (https://doi.org/10.1103%2FPhysRev.80.230).
67. Bromberg 1982, p. 161.
68. Bromberg 1982, p. 152.
69. Bromberg 1982, p. 154.
70. Bromberg 1982, p. 158.
71. Bromberg 1982, p. 159.
72. Bromberg 1982, p. 164.
73. Bromberg 1982, p. 165.
74. Bromberg 1982, p. 168.
75. Bromberg 1982, p. 169.
76. Bromberg 1982, p. 171.
77. Bromberg 1982, p. 212.
78. "Timeline" (https://www.pppl.gov/about/history/timeline). PPPL.
79. Bromberg 1982, p. 173.
80. Bromberg 1982, p. 175.
81. Smirnov 2009, p. 5.
82. Bromberg 1982, p. 10.
83. Bromberg 1982, p. 215.
84. Arnoux, Robert (25 October 2010). "Penthouse founder had invested his fortune in fusion"
(https://www.iter.org/newsline/151/468). ITER.
85. Reagan, Ronald (19 April 1986). "Radio Address to the Nation on Oil Prices" (http://www.pre
sidency.ucsb.edu/ws/index.php?pid=37156). The American Presidency Project.
86. Arnoux, Robert (15 December 2008). "INTOR: The international fusion reactor that never
was" (https://www.iter.org/newsline/62/146). ITER.
87. Joint Soviet-United States Statement on the Summit Meeting in Geneva (http://www.reagan.
utexas.edu/archives/speeches/1985/112185a.htm) Archived (https://web.archive.org/web/20
160307091345/https://www.reagan.utexas.edu/archives/speeches/1985/112185a.htm) 7
March 2016 at the Wayback Machine Ronald Reagan. 21 November 1985
88. Educational Foundation for Nuclear Science, Inc. (October 1992). "Bulletin of the Atomic
Scientists" (https://archive.org/details/bub_gb_wQwAAAAAMBAJ). Bulletin of the Atomic
Scientists: Science and Public Affairs. Educational Foundation for Nuclear Science, Inc.: 9
(https://archive.org/details/bub_gb_wQwAAAAAMBAJ/page/n10)–. ISSN 0096-3402 (https://
search.worldcat.org/issn/0096-3402).
89. Braams & Stott 2002, pp. 250– (https://books.google.com/books?id=Zj4vx9O0T0YC&pg=PA
250).
90. "Inauguration" (https://www.jt60sa.org/wp/category/uncategorized/). 24 October 2023.
Retrieved 1 January 2024.
91. Szondy, David (5 December 2023). "World's largest tokamak fusion reactor powers up" (http
s://newatlas.com/energy/worlds-largest-tokamak-fusion-reactor-powers-up/). New Atlas.
Retrieved 1 January 2024.
92. Wesson 1999, p. 13.
93. Kenward 1979b, p. 627.
94. Wesson 1999, pp. 15–18.
 95. Seo, Jaemin; Kim, SangKyeun; Jalalvand, Azarakhsh; Conlin, Rory; Rothstein, Andrew;
Abbate, Joseph; Erickson, Keith; Wai, Josiah; Shousha, Ricardo; Kolemen, Egemen
(February 2024). "Avoiding fusion plasma tearing instability with deep reinforcement
learning" (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC10881383). Nature. 626 (8000):
746–751. Bibcode:2024Natur.626..746S (https://ui.adsabs.harvard.edu/abs/2024Natur.626..
746S). doi:10.1038/s41586-024-07024-9 (https://doi.org/10.1038%2Fs41586-024-07024-9).
ISSN 1476-4687 (https://search.worldcat.org/issn/1476-4687). PMC 10881383 (https://www.
ncbi.nlm.nih.gov/pmc/articles/PMC10881383). PMID 38383624 (https://pubmed.ncbi.nlm.ni
h.gov/38383624).
96. Ate-a-Pi (26 February 2024). "Deep learning fusion breakthrough" (https://x.com/8teAPi/stat
us/1762190250355658885?s=20). X.
97. Wesson 1999, p. 20.
98. Gray, W.H.; Stoddart, W.C.T.; Akin, J.E. (1977). Bending free toroidal shells for tokamak
fusion reactors (https://www.osti.gov/servlets/purl/5233082) (Technical report). Oak Ridge
National Laboratory.
99. Wesson 1999, p. 22.
100. Wesson 1999, p. 26.
101. Kruger, S. E.; Schnack, D. D.; Sovinec, C. R. (2005). "Dynamics of the Major Disruption of a
DIII-D Plasma" (https://web.archive.org/web/20130227121923/http://www.scidac.gov/FES/F
ES_FusionGrid/pubs/kruger-phys-plasma-2005.pdf) (PDF). Phys. Plasmas. 12 (5): 056113.
Bibcode:2005PhPl...12e6113K (https://ui.adsabs.harvard.edu/abs/2005PhPl...12e6113K).
doi:10.1063/1.1873872 (https://doi.org/10.1063%2F1.1873872). Archived from the original
(http://www.scidac.gov/FES/FES_FusionGrid/pubs/kruger-phys-plasma-2005.pdf) (PDF) on
27 February 2013. Retrieved 5 January 2012.
102. Runaway Electrons in Tokamaks and Their Mitigation in ITER (http://w3fusion.ph.utexas.ed
u/ifs/iaeaep/talks/s11-i11-putvinski-sergei-ep-talk.pdf) Archived (https://web.archive.org/web/
20210308000033/http://w3fusion.ph.utexas.edu/ifs/iaeaep/talks/s11-i11-putvinski-sergei-ep-
talk.pdf) 8 March 2021 at the Wayback Machine, S. Putvinski, ITER Organization
103. Wurden, G. A. (9 September 2011). Dealing with the Risk and Consequences of Disruptions
in Large Tokamaks (https://web.archive.org/web/20151105232903/http://advprojects.pppl.go
v/ROADMAPPING/presentations/MFE_POSTERS/WURDEN_Disruption_RiskPOSTER.pdf)
(PDF). MFE Roadmapping in the ITER Era. Archived from the original (http://advprojects.pp
pl.gov/ROADMAPPING/presentations/MFE_POSTERS/WURDEN_Disruption_RiskPOSTE
R.pdf) (PDF) on 5 November 2015.
104. Baylor, L. R.; Combs, S. K.; Foust, C. R.; Jernigan, T.C.; et al. (2009). "Pellet Fuelling, ELM
Pacing and Disruption Mitigation Technology Development for ITER" (http://www-pub.iaea.or
g/MTCD/Meetings/FEC2008/it_p6-19.pdf) (PDF). Nucl. Fusion. 49 (8): 085013.
Bibcode:2009NucFu..49h5013B (https://ui.adsabs.harvard.edu/abs/2009NucFu..49h5013B).
doi:10.1088/0029-5515/49/8/085013 (https://doi.org/10.1088%2F0029-5515%2F49%2F8%2
F085013). S2CID 17071617 (https://api.semanticscholar.org/CorpusID:17071617).
105. Thornton, A. J.; Gibsonb, K. J.; Harrisona, J. R.; Kirka, A.; et al. (2011). "Disruption
mitigation studies on the Mega Amp Spherical Tokamak (MAST)". J. Nucl. Mater. 415 (1):
S836 – S840. Bibcode:2011JNuM..415S.836M (https://ui.adsabs.harvard.edu/abs/2011JNu
M..415S.836M). doi:10.1016/j.jnucmat.2010.10.029 (https://doi.org/10.1016%2Fj.jnucmat.20
10.10.029).
106. von Goeler, S.; Stodiek, W.; Sauthoff, N. (1974). "Studies of internal disruptions and m= 1
oscillations in tokamak discharges with soft – x-ray techniques". Physical Review Letters. 33
(20): 1201. Bibcode:1974PhRvL..33.1201V (https://ui.adsabs.harvard.edu/abs/1974PhRvL..
33.1201V). doi:10.1103/physrevlett.33.1201 (https://doi.org/10.1103%2Fphysrevlett.33.120
1).
107. Greenwald, Martin (1 August 2002). "Density limits in toroidal plasmas" (https://iopscience.io
p.org/article/10.1088/0741-3335/44/8/201). Plasma Physics and Controlled Fusion. 44 (8):
R27 – R53. doi:10.1088/0741-3335/44/8/201 (https://doi.org/10.1088%2F0741-3335%2F4
4%2F8%2F201). hdl:1721.1/93996 (https://hdl.handle.net/1721.1%2F93996).
108. "Greenwald limit" (https://wiki.fusion.ciemat.es/wiki/Greenwald_limit). FusionWiki.
109. Hurst, N. C.; Chapman, B. E.; Sarff, J. S.; Almagri, A. F.; McCollam, K. J.; Den Hartog, D. J.;
Flahavan, J. B.; Forest, C. B. (29 July 2024). "Tokamak Plasmas with Density up to 10
Times the Greenwald Limit" (https://link.aps.org/doi/10.1103/PhysRevLett.133.055101).
Physical Review Letters. 133 (5): 055101. doi:10.1103/PhysRevLett.133.055101 (https://doi.
org/10.1103%2FPhysRevLett.133.055101). PMID 39159104 (https://pubmed.ncbi.nlm.nih.g
ov/39159104).
110. Gates, D. A.; Delgado-Aparicio, L. (20 April 2012). "Origin of Tokamak Density Limit
Scalings" (https://link.aps.org/doi/10.1103/PhysRevLett.108.165004). Physical Review
Letters. 108 (16): 165004. doi:10.1103/PhysRevLett.108.165004 (https://doi.org/10.1103%2
FPhysRevLett.108.165004). ISSN 0031-9007 (https://search.worldcat.org/issn/0031-9007).
PMID 22680727 (https://pubmed.ncbi.nlm.nih.gov/22680727).
111. "Neutral Beam Test Facility" (https://web.archive.org/web/20161010212050/https://www.igi.c
nr.it/www/sites/default/files/home201511/SchedaNBTF_MIUR_EN.pdf) (PDF). Archived from
the original (https://www.igi.cnr.it/www/sites/default/files/home201511/SchedaNBTF_MIUR_
EN.pdf) (PDF) on 10 October 2016. Retrieved 9 October 2016.
112. Vojtěch Kusý. "GOLEM @ FJFI.CVUT" (http://golem.fjfi.cvut.cz). cvut.cz.
113. "Tokamak Department, Institute of Plasma Physics" (https://web.archive.org/web/201509011
15841/http://www.ipp.cas.cz/Tokamak/). cas.cz. Archived from the original (http://www.ipp.ca
s.cz/Tokamak/) on 1 September 2015.
114. "History of Golem" (https://archive.today/20130217030814/http://golem.fjfi.cvut.cz:5001/Intro
duction/History/GOLEM%20History). Archived from the original (http://golem.fjfi.cvut.cz:500
1/Introduction/History/GOLEM%20History) on 17 February 2013. Retrieved 14 January
2013.
115. Fenstermacher, M.E.; et al. (2022). "DIII-D research advancing the physics basis for
optimizing the tokamak approach to fusion energy" (https://iopscience.iop.org/article/10.108
8/1741-4326/ac2ff2). Nuclear Fusion. 62 (4): 042024. Bibcode:2022NucFu..62d2024F (http
s://ui.adsabs.harvard.edu/abs/2022NucFu..62d2024F). doi:10.1088/1741-4326/ac2ff2 (http
s://doi.org/10.1088%2F1741-4326%2Fac2ff2). hdl:1721.1/147629 (https://hdl.handle.net/17
21.1%2F147629). S2CID 244608556 (https://api.semanticscholar.org/CorpusID:24460855
6).
116. Singh, A.K.; Morelli, J.; Xiao, C.; Mitarai, O.; Hirose, A. (2006). "Investigation of Plasma
Equilibrium in the Saskatchewan Torus-Modified (STOR-M) during Alternating Current
Operation". Contributions to Plasma Physics. 46 (10): 773. Bibcode:2006CoPP...46..773S (h
ttps://ui.adsabs.harvard.edu/abs/2006CoPP...46..773S). doi:10.1002/ctpp.200610077 (http
s://doi.org/10.1002%2Fctpp.200610077). S2CID 123466788 (https://api.semanticscholar.or
g/CorpusID:123466788).
117. Tore Supra (http://www-drfc.cea.fr/gb/cea/ts/ts.htm) Archived (https://web.archive.org/web/2
0121115112229/http://www-drfc.cea.fr/gb/cea/ts/ts.htm) 15 November 2012 at the Wayback
Machine
118. EMazzitelli, Giuseppe. "ENEA-Fusion: FTU" (https://web.archive.org/web/20190104175759/
http://www.fusione.enea.it/FTU/index.html.en). www.fusione.enea.it. Archived from the
original (http://www.fusione.enea.it/FTU/index.html.en) on 4 January 2019. Retrieved
31 January 2017.
119. "Centro de Fusão Nuclear" (https://web.archive.org/web/20100307154259/http://www.cfn.ist.
utl.pt/eng/Prj_Tokamak_main_1.html#intro). utl.pt. Archived from the original (http://www.cfn.
ist.utl.pt/eng/Prj_Tokamak_main_1.html#intro) on 7 March 2010. Retrieved 24 February
2008.
120. Fusion Research: Australian Connections, Past and Future (http://h1nf.anu.edu.au/media/pd
fs/Blackwell_AIP_fusion_article_draft_6-1.pdf) Archived (https://web.archive.org/web/20180
313171528/http://h1nf.anu.edu.au/media/pdfs/Blackwell_AIP_fusion_article_draft_6-1.pdf)
13 March 2018 at the Wayback Machine B. D. Blackwell, M.J. Hole, J. Howard and J.
O'Connor
121. Gates, David (1993). Passive-stabilization-of-MHD-instabilities at high βn in the HBT-EP
Tokamak (Thesis). doi:10.2172/10104897 (https://doi.org/10.2172%2F10104897).
S2CID 117710767 (https://api.semanticscholar.org/CorpusID:117710767).
122. "Pegasus Toroidal Experiment" (http://pegasus.ep.wisc.edu/). wisc.edu.
123. "Tokamak" (http://www.pprc.srbiau.ac.ir/index.php?option=com_content&view=article&id=2
7:tokamak&catid=5:research-advanced-labs&Itemid=20). Pprc.srbiau.ac.ir. Retrieved
28 June 2012.
124. De Villiers, J. A. M.; Hayzen, A. J.; Omahony, J. R.; Roberts, D. E.; Sherwell, D. (1979).
"Tokoloshe - the South African Tokamak". South African Journal of Science. 75: 155.
Bibcode:1979SAJSc..75..155D (https://ui.adsabs.harvard.edu/abs/1979SAJSc..75..155D).
125. Crepaz, Leah (20 December 2023). "Pioneering JET delivers final plasma" (https://ccfe.ukae
a.uk/pioneering-jet-delivers-final-plasma/). Culham Centre for Fusion Energy. Retrieved
6 July 2024.
126. Ramos J, de Urquijo J, Meléndez L, Muñoz C, et al. (1983). "Diseño del Tokamak Novillo" (h
ttps://web.archive.org/web/20160808040124/http://rmf.smf.mx/pdf/rmf/29/4/29_4_551.pdf)
(PDF). Rev. Mex. Fís. (in Spanish). 29 (4): 551–592. Archived from the original (http://rmf.sm
f.mx/pdf/rmf/29/4/29_4_551.pdf) (PDF) on 8 August 2016. Retrieved 7 June 2016.
127. "MIT Plasma Science & Fusion Center: research>alcator>" (https://web.archive.org/web/201
50709210155/http://www.psfc.mit.edu/research/alcator/). mit.edu. Archived from the original
(http://www.psfc.mit.edu/research/alcator/) on 9 July 2015.
128. "China's HT-7 retires after 11,800 plasma shots" (http://www.iter.org/newsline/270/1616).
ITER. 4 June 2013. Retrieved 6 July 2024.
129. "ITER & Beyond. The Phases of ITER" (https://web.archive.org/web/20120922162049/http://
www.iter.org/proj/iterandbeyond). Iter. Archived from the original (http://www.iter.org/proj/iter
andbeyond) on 22 September 2012. Retrieved 12 September 2012.
130. Gao, X.; et al. (CFETR team) (17–20 December 2013). Update on CFETR Concept Design
(https://web.archive.org/web/20190330120409/http://www-naweb.iaea.org/napc/physics/me
etings/TM45256/talks/Gao.pdf) (PDF). 2nd IAEA DEMO Programme Workshop. Vienna,
Austria. Archived from the original (http://www-naweb.iaea.org/napc/physics/meetings/TM45
256/talks/Gao.pdf) (PDF) on 30 March 2019. Retrieved 17 August 2015.
131. Zheng, Jinxing; Liu, Xufeng; Song, Yuntao; Wan, Yuanxi; et al. (2013). "Concept design of
CFETR superconducting magnet system based on different maintenance ports". Fusion
Engineering and Design. 88 (11): 2960–2966. Bibcode:2013FusED..88.2960Z (https://ui.ads
abs.harvard.edu/abs/2013FusED..88.2960Z). doi:10.1016/j.fusengdes.2013.06.008 (https://
doi.org/10.1016%2Fj.fusengdes.2013.06.008).
132. Song, Yun Tao; et al. (2014). "Concept Design of CFETR Tokamak Machine". IEEE
Transactions on Plasma Science. 42 (3): 503–509. Bibcode:2014ITPS...42..503S (https://ui.
adsabs.harvard.edu/abs/2014ITPS...42..503S). doi:10.1109/TPS.2014.2299277 (https://doi.
org/10.1109%2FTPS.2014.2299277). S2CID 24159256 (https://api.semanticscholar.org/Cor
pusID:24159256).
133. Ye, Minyou (26 March 2013). "Status of design and strategy for CFETR" (https://web.archiv
e.org/web/20151125075902/http://aries.ucsd.edu/LIB/MEETINGS/1302-USJ-PPS/Ye.pdf)
(PDF). Archived from the original (http://aries.ucsd.edu/LIB/MEETINGS/1302-USJ-PPS/Ye.p
df) (PDF) on 25 November 2015. Retrieved 17 August 2015.
134. Kim, K.; Im, K.; Kim, H.C.; Oh, S.; et al. (2015). "Design concept of K-DEMO for near-term
implementation" (https://doi.org/10.1088%2F0029-5515%2F55%2F5%2F053027). Nuclear
Fusion. 55 (5): 053027. Bibcode:2015NucFu..55e3027K (https://ui.adsabs.harvard.edu/abs/
2015NucFu..55e3027K). doi:10.1088/0029-5515/55/5/053027 (https://doi.org/10.1088%2F0
029-5515%2F55%2F5%2F053027). ISSN 0029-5515 (https://search.worldcat.org/issn/0029
-5515).
135. Chesto, Jon (3 March 2021). "MIT energy startup homes in on fusion, with plans for 47-acre
site in Devens" (https://www.bostonglobe.com/2021/03/03/business/mit-energy-startup-hom
es-fusion-with-plans-47-acre-site-devens/). BostonGlobe.com. Retrieved 3 March 2021.
136. Verma, Pranshu. Nuclear fusion power inches closer to reality. (https://www.washingtonpost.
com/technology/2022/08/26/nuclear-fusion-technology-climate-change/) Archived (https://we
b.archive.org/web/20220827165948/https://www.washingtonpost.com/technology/2022/08/2
6/nuclear-fusion-technology-climate-change/) 27 August 2022 at the Wayback Machine The
Washington Post, August 26, 2022.

Bibliography
Braams, C.M. & Stott, P.E. (2002). Nuclear Fusion: Half a Century of Magnetic Confinement
Research (https://books.google.com/books?id=Zj4vx9O0T0YC). Institute of Physics
Publishing. ISBN 978-0-7503-0705-5.
Bromberg, Joan Lisa (1982). Fusion: Science, Politics, and the Invention of a New Energy
Source (https://archive.org/details/fusionsciencepol0000brom). MIT Press. ISBN 978-0-262-
02180-7.
Clery, Daniel (2014). A Piece of the Sun: The Quest for Fusion Energy (https://books.googl
e.com/books?id=EGcjCQAAQBAJ). MIT Press. ISBN 978-1-4683-1041-2.
Dolan, Thomas J. (1982). Fusion Research, Volume 1 – Principles. Pergamon Press.
LCC QC791.D64 (https://catalog.loc.gov/vwebv/search?searchCode=CALL%2B&searchArg
=QC791.D64&searchType=1&recCount=25).
Herman, Robin (1990). Fusion: the search for endless energy (https://archive.org/details/fusi
onsearchfore00herm). Cambridge University Press. ISBN 978-0-521-38373-8.
Kadomtsev, B. (1966). "Hydrodynamic Stability of a Plasma" (https://www.astro.princeton.ed
u/~kunz/Site/KadomtsevR.pdf) (PDF). Reviews of Plasma Physics. pp. 153–199.
Kenward, Michael (24 May 1979b). "Fusion Research – the temperature rises" (https://book
s.google.com/books?id=tbhTdnZsqMUC&pg=PA626). New Scientist. Vol. 82, no. 1156.
p. 627.
McCracken, Garry; Stott, Peter (2012). Fusion: The Energy of the Universe (https://books.go
ogle.com/books?id=6Tud4RyMjlwC). Academic Press. ISBN 978-0-12-384657-0.
Nishikawa, K. & Wakatani, M. (2000). Plasma Physics. Springer-Verlag. ISBN 978-3-540-
65285-4.
Raeder, J.; et al. (1986). Controlled Nuclear Fusion. John Wiley & Sons. ISBN 978-0-471-
10312-7.
Shafranov, Vitaly (2001). "On the history of the research into controlled thermonuclear
fusion" (https://fire.pppl.gov/rf_shafranov.pdf) (PDF). Journal of the Russian Academy of
Sciences. 44 (8): 835–865.
Smirnov, Vladimir (30 December 2009). "Tokamak foundation in USSR/Russia 1950–1990"
(https://fire.pppl.gov/nf_50th_5_Smirnov.pdf) (PDF). Nuclear Fusion. 50 (1): 014003.
Bibcode:2010NucFu..50a4003S (https://ui.adsabs.harvard.edu/abs/2010NucFu..50a4003S).
CiteSeerX 10.1.1.361.8023 (https://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.361.
8023). doi:10.1088/0029-5515/50/1/014003 (https://doi.org/10.1088%2F0029-5515%2F50%
2F1%2F014003). S2CID 17487157 (https://api.semanticscholar.org/CorpusID:17487157).
Wesson, John; et al. (2004). Tokamaks. Oxford University Press. ISBN 978-0-19-850922-6.
 Wesson, John (November 1999). The Science of JET (https://scipub.euro-fusion.org/wp-con
tent/uploads/2014/11/JETR99013.pdf) (PDF). JET Joint Undertaking.

External links
CCFE (http://www.ccfe.ac.uk) Archived (https://web.archive.org/web/20201028103448/http://
ccfe.ac.uk/) 28 October 2020 at the Wayback Machine – site from the UK fusion research
centre CCFE.
Int'l Tokamak research (http://www.iter.org/sci/tkmkresearch) – various that relate to ITER
Plasma Science (http://www-fusion-magnetique.cea.fr/gb/fusion/physique/sommaire.htm) –
site on tokamaks from the French CEA.
Fusion Programs (http://www.ga.com/energy/) Archived (https://web.archive.org/web/20091
004202945/http://www.ga.com/energy/) 4 October 2009 at the Wayback Machine at General
Atomics, including the DIII-D National Fusion Facility, an experimental tokamak.
General Atomics DIII-D Program (https://web.archive.org/web/20170118171621/http://www.
ga.com/magnetic-fusion-energy)
Fusion and Plasma Physics Seminar (http://ocw.mit.edu/courses/nuclear-engineering/22-01
2-seminar-fusion-and-plasma-physics-spring-2006/) at MIT OCW
Unofficial ITER fan club (https://web.archive.org/web/20061116091253/http://www.iterfan.or
g/) – fans of the biggest tokamak planned to be built in near future.
All-the-Worlds-Tokamaks (http://www.tokamak.info) Extensive list of current and historic
tokamaks from around the world.
SSTC-1 (http://www.educatedearth.net/video.php?id=3753) Overview video of a small scale
tokamak concept.
SSTC-2 (https://www.youtube.com/watch?v=VBkIikDfWb8) on YouTube Section View Video
of a small scale tokamak concept.
SSTC-3 (https://www.youtube.com/watch?v=E2-Y8bYtvX4) on YouTube Fly Through Video
of a small scale tokamak concept.
LAP Tokamak Development (https://web.archive.org/web/20081221165349/http://www.plas
ma.inpe.br/LAP_Portal/LAP_Site/Text/Tokamak_Development.htm) Information on
conditions necessary for nuclear reaction in a tokamak reactor
A. P. Frass (1973). Engineering Problems In The Design Of Controlled Thermonuclear
Reactors (https://www.osti.gov/bridge/servlets/purl/4547512-RL4I3j/4547512.pdf) (PDF)
(Report). Oak Ridge National Laboratory. doi:10.2172/4547512 (https://doi.org/10.2172%2F
4547512). Retrieved 30 September 2013.
Observer Newspaper Article on Tokomak (https://www.theguardian.com/science/2015/jan/2
5/iter-nuclear-fusion-cadarache-international-thermonuclear-experimental-reactor-steven-co
wley) Nuclear fusion and the promise of a brighter tomorrow

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