Field · Emerged 1934 – 1969

Fusion Energy

Can we release the energy that powers the stars in a controlled way on Earth?

5 chapters5 min read7 turning points 1 open problem

Branched from: Nuclear Structure + Stellar Astrophysics
Branched into: Not yet surveyed past here
Figures: Ernest Rutherford, Mark Oliphant, Paul Harteck, Edward Teller, Stanislaw Ulam, John D. Lawson, Andrei Sakharov, Lev Artsimovich, Martin Fleischmann, Stanley Pons

In brief

Fusion joins light nuclei into heavier ones, releasing energy from the steep side of the binding-energy curve. The Sun does it with hydrogen under the crushing weight of its own gravity. On Earth the most practical fuel is a mixture of two heavy forms of hydrogen, deuterium and tritium, which must be heated to over a hundred million degrees and held together long enough to burn.

Uncontrolled fusion was achieved quickly, in the hydrogen bomb of 1952. Controlling it has proved one of the hardest problems in physics. A hot gas of charged particles, a plasma, slips out of every magnetic trap built to hold it. The Soviet tokamak of the late 1960s set the course for most research since, and in 2022 a laser-driven capsule released more fusion energy than the lasers delivered to it. Whether fusion can become an economic source of electricity is still unknown.

Key ideas

Deuterium–tritium fusionEnters 1934: A deuterium nucleus and a tritium nucleus fuse into helium-4 and a neutron, releasing 17.6 MeV. It is the easiest fusion reaction to ignite.
PlasmaEnters 1968 – 1969: A gas so hot that its atoms are stripped of their electrons. Being made of charged particles, it can be steered and confined by magnetic fields, but it is prone to instabilities.
Lawson criterionEnters 1955 – 1957: A fusion plasma produces net power only if its density, temperature and energy confinement time together exceed a threshold. The product of the three, the triple product, is the standard measure of progress.
TokamakEnters 1968 – 1969: A Soviet design, from a Russian acronym, that confines plasma in a doughnut-shaped vessel with a strong magnetic field wound around the ring and a large current flowing through the plasma itself.
Ignition and gainEnters 2022: Ignition is when the fusion reactions themselves supply enough heat to keep the fuel burning. Gain is the ratio of fusion energy released to the energy put in.

Chapter I

The Stars' Fuel

In 1920 Eddington saw that turning hydrogen into helium would release the mass that Aston had found missing, and by 1939 Bethe had worked out how the Sun does it, as stellar astrophysics describes. The Sun has gravity to hold its fuel together and a core of 15 million degrees. Physicists wondered whether the reaction could be made on Earth.

In 1934 at Cambridge, Mark Oliphant, Paul Harteck and Ernest Rutherford fired deuterium, the heavy hydrogen isotope discovered two years earlier, at targets containing deuterium. The nuclei fused, producing tritium and helium-3, and each reaction released millions of electronvolts. But only a tiny fraction of the accelerated particles ever fused. The rest simply lost their energy. Fusion by particle beams could never pay its way. The fuel would have to be heated so hot that every particle moves fast enough, as in a star. That is called thermonuclear fusion.

Chapter II

The Bomb First

A fission bomb produces temperatures of tens of millions of degrees for a moment. Edward Teller pressed for a fusion bomb from 1942 onwards, but his early designs would not work. In 1951 Stanislaw Ulam and Teller found the arrangement that did: radiation from a fission explosion compresses and heats a separate charge of fusion fuel. On 1 November 1952 "Ivy Mike" released about 10 megatons, some 700 times the Hiroshima bomb. The Soviet Union followed within three years, with a design to which Andrei Sakharov was a leading contributor.

Chapter III

Bottling the Sun

A controlled reactor needs a plasma, a gas of bare nuclei and electrons, held at a hundred million degrees without touching any wall. Because a plasma is made of charged particles, magnetic fields can confine it. Programmes began in secret in Britain, the United States and the Soviet Union around 1951. In 1955 John Lawson at Harwell set out what a reactor must achieve. Burning fuel must be hot enough, and dense enough for long enough, to release more energy than it takes to heat. Every magnetic bottle leaked. In 1958 Britain's ZETA machine announced fusion neutrons, and had to withdraw the claim within months.

The breakthrough came from Moscow. In 1950 Sakharov and Igor Tamm had proposed a ring-shaped trap in which a current flows through the plasma itself. In 1968 Lev Artsimovich reported that the tokamak T-3 reached temperatures about ten times higher than any Western machine. Sceptical Western physicists sent a team from Culham with a laser to measure it. They confirmed the result in 1969, and tokamaks spread worldwide.

Twenty years later came an embarrassment. In 1989 Martin Fleischmann and Stanley Pons claimed fusion at room temperature in a jar of heavy water. Hundreds of laboratories tried to repeat it, and most found nothing. Real progress was slower. The Joint European Torus produced 16 megawatts of fusion power in 1997, and a record 69 megajoules of fusion energy in a single pulse in 2023. In December 2022 the National Ignition Facility in California, taking a different route, crushed a fuel capsule with 192 lasers and got more fusion energy out than the laser energy that went in.

Chapter IV

A Closer Look: The Triple Product

The fuel. A deuterium nucleus and a tritium nucleus fuse into helium-4 and a neutron. The masses are 2.014102 u and 3.016049 u in, and 4.002602 u and 1.008665 u out. The mass lost is 0.018884 u, and multiplying by 931.5 MeV per u gives 17.6 MeV per reaction. Per kilogram of fuel that is $3.4 \times 10^{14}$ J, four times the energy of fissioning a kilogram of uranium-235, and about ten million times the energy of burning a kilogram of carbon.

The barrier. Before they can fuse, the two nuclei must approach within about 3.2 femtometres against their electrical repulsion, an energy of about 0.44 MeV. A plasma at 15 keV, which is about 170 million degrees, has particles with a typical energy of a thirtieth of that. Fusion happens only because a few fast particles tunnel through the barrier, as in the Sun.

Lawson's condition. For deuterium–tritium fuel to ignite, the density of particles $n$ , the temperature $T$ and the energy confinement time $\tau$ , the time the plasma would take to lose its heat if not reheated, must satisfy roughly

n \, T \, \tau \gtrsim 3 \times 10^{21}\ \text{keV s m}^{-3} .

A magnetic plasma cannot be very dense. At $n = 10^{20}$ particles per cubic metre, a few millionths of the density of air, and $T = 15$ keV, the plasma must hold its heat for

\tau \gtrsim \frac{3 \times 10^{21}}{10^{20} \times 15} = 2\ \text{seconds} .

That is the design target of tokamaks such as ITER. Laser fusion takes the opposite route: it squeezes the fuel to many times the density of lead, and then a confinement time of well under a billionth of a second suffices.

The gain. At the National Ignition Facility on 5 December 2022, 2.05 MJ of laser light produced 3.15 MJ of fusion energy, a gain of $3.15 / 2.05 \approx 1.5$ . It was a real threshold. The best shot since, in April 2025, reached a gain of about 4.1. But a power plant would need a gain many times larger, since making the laser light takes more than a hundred times more electricity than the light carries.

Chapter V

Still Coming

Fusion has been said to be thirty years away for seventy years. The physics is now largely understood, and the problems left are engineering: materials that survive the neutrons, a way to breed tritium, and a machine cheap enough to run. ITER, a tokamak being built in France by a collaboration of seven members including China, the European Union, India, Japan, Korea, Russia and the United States, is meant to produce ten times more fusion power than the heating power put into its plasma. Private companies are racing it with smaller designs. Whether any of them will put fusion power on a grid is one of the open questions of the century.

Turning points7

1934EXPERIMENT
Fusion in the laboratory
At the Cavendish Laboratory, Mark Oliphant, Paul Harteck and Ernest Rutherford accelerate deuterium nuclei into targets containing deuterium. The nuclei fuse, producing two new isotopes, hydrogen-3 (tritium) and helium-3, and releasing far more energy than was put into each collision. It was the first fusion reaction made on Earth, though the accelerator used vastly more energy than the reactions gave back.
Ernest Rutherford, Mark Oliphant, Paul Harteck
› Sources (1)
- Oliphant, M. L. E., Harteck, P. & Rutherford, E. (1934). Transmutation effects observed with heavy hydrogen. Proceedings of the Royal Society A 144: 692–703.
1952EXPERIMENTContested
The hydrogen bomb
On 1 November 1952 the United States detonates "Ivy Mike" on Enewetak Atoll in the Pacific, the first device in which a fission explosion ignites fusion fuel. It yields about 10 megatons, some 700 times the Hiroshima bomb, and vaporises the island it stood on. The Soviet Union tested its own thermonuclear designs in 1953 and 1955.
What is disputed
The design that made it work, using radiation from a fission bomb to compress the fusion fuel, came from Stanislaw Ulam and Edward Teller in 1951. How the credit should be divided between them was disputed by Teller himself, and historians still weigh their accounts differently.
Edward Teller, Stanislaw Ulam
› Sources (1)
- Rhodes, R. (1995). Dark Sun: The Making of the Hydrogen Bomb. Simon & Schuster.
1955 – 1957DISCOVERY
The Lawson criterion
John Lawson, an engineer at the British atomic research centre at Harwell, works out what a fusion reactor must achieve to produce more energy than it consumes. The fuel must be hot enough, and the product of its density and the time it holds its heat must exceed a fixed threshold. Written in 1955 as a secret report, it was published in 1957 and gave the field its yardstick.
John D. Lawson
› Sources (1)
- Lawson, J. D. (1957). Some criteria for a power producing thermonuclear reactor. Proceedings of the Physical Society B 70: 6–10.
1968 – 1969EXPERIMENT
The tokamak works
At a conference in Novosibirsk in 1968, Lev Artsimovich reports that the Soviet tokamak T-3 holds plasma at around 10 million degrees, far better than any Western machine. Western physicists doubt the measurement. In 1969 a team from Culham in Britain takes laser equipment to Moscow, during the Cold War, and confirms the temperature. The design went back to a proposal by Andrei Sakharov and Igor Tamm in 1950. Laboratories around the world switch to tokamaks.
Andrei Sakharov, Lev Artsimovich
› Sources (1)
- 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: 488–490.
1989EXPERIMENTContested
Cold fusion
At a press conference in Utah, the electrochemists Martin Fleischmann and Stanley Pons announce that a tabletop cell of palladium in heavy water produces excess heat from fusion at room temperature. Laboratories worldwide try to repeat it within weeks. Most find nothing, and the expected neutrons are absent. Within a year the mainstream verdict was that the claim was mistaken.
What is disputed
A small community has continued to report anomalous heat under the name low-energy nuclear reactions, and disputes the dismissal. A large study funded by Google, published in 2019, found no evidence for the effect but called for further careful work on the materials involved.
Martin Fleischmann, Stanley Pons
› Sources (2)
- Fleischmann, M., Pons, S. & Hawkins, M. (1989). Electrochemically induced nuclear fusion of deuterium. Journal of Electroanalytical Chemistry 261(2A): 301–308.
- Berlinguette, C. P. et al. (2019). Revisiting the cold case of cold fusion. Nature 570: 45–51.
1997 – 2023EXPERIMENT
Deuterium–tritium in the Joint European Torus
JET, the largest tokamak of its day, at Culham, is one of only two magnetic machines, with the American TFTR, to burn real deuterium–tritium fuel. In 1997 it produces 16 megawatts of fusion power, about two-thirds of the heating power put in. In 2021 it sustains fusion for five seconds and releases 59 megajoules, a test of the conditions planned for ITER. In its final experiments, in October 2023, it raises the record to 69 megajoules. JET closed at the end of 2023.
› Sources (3)
- Keilhacker, M. et al. (1999). High fusion performance from deuterium-tritium plasmas in JET. Nuclear Fusion 39: 209–234.
- Mailloux, J. et al. (2022). Overview of JET results for optimising ITER operation. Nuclear Fusion 62: 042026.
- EUROfusion (2024). Breaking new ground: JET tokamak's latest fusion energy record shows mastery of fusion processes. Press release, 8 February 2024.
2022EXPERIMENT
Ignition with lasers
At the National Ignition Facility in California, 192 laser beams deliver 2.05 megajoules to a peppercorn-sized capsule of deuterium–tritium, crushing it in billionths of a second. On 5 December 2022 the capsule releases 3.15 megajoules, the first time a fusion experiment has produced more energy than the laser light delivered to its target. The lasers themselves drew more than a hundred times more energy from the grid. Later shots did better: in April 2025 one released 8.6 megajoules from 2.08, a gain of about four.
› Sources (2)
- Abu-Shawareb, H. et al. (2024). Achievement of target gain larger than unity in an inertial fusion experiment. Physical Review Letters 132(6): 065102.
- Lawrence Livermore National Laboratory. Achieving fusion ignition. National Ignition Facility & Photon Science.

Applications

Where it is used

Industry
Neutron generators
Small sealed tubes that fuse deuterium and tritium are used as compact neutron sources. They are lowered down oil wells to identify rock layers by the gamma rays that neutrons provoke, and used to scan cargo for explosives. They produce far less energy than they consume, but the neutrons are what is wanted.
Weapons science
Stockpile stewardship
Since the end of nuclear testing by the United States in 1992, laser fusion experiments like those at the National Ignition Facility have been a main source of data about thermonuclear conditions. Much of the funding for inertial fusion comes from this role, not from energy research.

Open problems

Where the map runs out

Open

Can fusion become a practical power source?

Open as of 2026. No fusion device has yet produced more electricity than it consumes.

Fusion burns an abundant fuel, emits no carbon dioxide and cannot run away like a fission reactor. A power plant would have to reach high gain continuously or many times a second, breed its own tritium, and survive years of bombardment by fast neutrons, all at a cost that competes with other sources.

Why it is hard: Plasmas are turbulent and prone to sudden disruptions. Fast neutrons from the reaction damage and activate the walls. Tritium does not occur naturally in useful amounts and must be made from lithium inside the reactor. ITER, under construction in France, does not plan deuterium–tritium operation before the late 2030s, and a wave of private companies is attempting faster routes.
What resolving it unlocks: A nearly inexhaustible source of low-carbon electricity, and a new kind of engineering at the limits of materials science.

› Sources (1)

Wurzel, S. E. & Hsu, S. C. (2022). Progress toward fusion energy breakeven and gain as measured against the Lawson criterion. Physics of Plasmas 29: 062103.