Nuclear fusion

Nuclear fusion is a fundamental process in nuclear physics and astrophysics in which light atomic nuclei combine to form heavier nuclei, releasing energy in the process. It underpins the energy output of all active stars, provides the basis for thermonuclear weapons, and serves as the foundation for ongoing research into fusion power as a future energy source.
Fusion occurs when two nuclei overcome their mutual electrostatic repulsion and approach sufficiently closely for the strong nuclear force to bind them together. The mass of the resulting nucleus is slightly less than the total mass of the original particles; this mass difference is released as energy in accordance with the mass–energy relationship. The degree to which a reaction is energetically favourable depends on nuclear binding energies, which rise steeply for light nuclei up to the region around nickel-62.

Physical principles of fusion

Fusion reactions require extreme conditions of temperature, pressure, and confinement, expressed collectively through the Lawson criterion. These conditions naturally occur in stellar cores, where enormous gravitational pressures and temperatures exceeding ten million kelvin enable hydrogen nuclei to fuse. On Earth such conditions must be created artificially using magnetic confinement devices such as tokamaks or by inertial confinement driven by powerful lasers.
Nuclear fusion can occur through several pathways. The proton–proton chain, dominant in stars similar to the Sun, proceeds through a sequence of reactions beginning with the fusion of two protons to form deuterium. More massive stars utilise the carbon–nitrogen–oxygen (CNO) cycle, in which carbon acts as a catalyst. At extremely high temperatures, advanced burning stages allow fusion of heavier elements, with massive stars such as Antares capable of fusing silicon in the final stages of their evolution.
The most favourable fusion reactions for controlled fusion on Earth involve the light isotopes deuterium (D) and tritium (T). The D–T reaction has a low ignition temperature and produces a large energy yield, driven in part by its high fusion cross-section.

Historical foundations

The concept of nuclear fusion was first proposed in 1915 by the American chemist William Draper Harkins, suggesting that the fusion of hydrogen into helium could be the source of stellar energy. Key experimental and theoretical developments followed. Francis William Aston’s work with the mass spectrometer in 1919 demonstrated that four hydrogen atoms had a greater total mass than a helium atom, providing strong evidence for the energy potential of mass–energy conversion.
Arthur Eddington correctly surmised in 1920 that the Sun must obtain its energy from the fusion of hydrogen into helium. Subsequent breakthroughs in quantum mechanics provided the mechanism for how nuclei could fuse: in 1927 Friedrich Hund described quantum tunnelling, a process later applied by George Gamow to explain how nuclei could penetrate the Coulomb barrier. In 1929 Robert Atkinson and Fritz Houtermans used Gamow’s framework to provide the first quantitative estimates of stellar fusion rates.
During the 1930s numerous laboratory experiments explored fusion phenomena. Patrick Blackett demonstrated artificial nuclear transmutation, while John Cockcroft and Ernest Walton constructed a high-voltage generator that enabled the first artificial fusion reaction in 1932 through proton bombardment of lithium. Ernest Lawrence’s cyclotrons were also instrumental in probing fusion processes, although early observations were sometimes misinterpreted due to limited diagnostic capabilities.
A major advance came in 1934 when Mark Oliphant, Paul Harteck and Ernest Rutherford achieved intentional deuterium fusion, discovering both tritium and helium-3 in the process. By 1938 Arthur Ruhlig identified the characteristic 14 MeV neutrons associated with D–T fusion, confirming its exceptional favourability.

Fusion in thermonuclear weapon development

Research into thermonuclear weapon design began during the Second World War under the Manhattan Project. Edward Teller and Enrico Fermi discussed the possibility of using a fission bomb to trigger fusion, and experimental studies were undertaken to measure cross-sections for D–T and lithium-based reactions. A crucial discovery in 1946 by Egon Bretscher identified a strong resonance that greatly increased the D–T fusion cross-section, strengthening the feasibility of thermonuclear designs.
Computational modelling using early electronic computers such as ENIAC enabled detailed theoretical simulations of staged thermonuclear devices. The first partial demonstration of thermonuclear fusion occurred in 1951 during the Greenhouse Item test, which used a small quantity of fusion fuel to boost yield. The first true thermonuclear explosion was the Ivy Mike test of 1952, employing a cryogenic liquid-deuterium device and yielding over 10 megatonnes.
Parallel developments unfolded in the Soviet Union, where the 1953 RDS-6s device marked the first air-deliverable fusion bomb, followed by the two-stage RDS-37 test in 1955. Modern thermonuclear weapons employ solid lithium deuteride as the main fusion fuel. Enrichment with lithium-6 enables in-situ production of tritium during detonation, sustaining the highly energetic D–T reaction and supporting neutron-driven secondary reactions.

Controlled fusion research

While thermonuclear weapons provided the first practical fusion reactions, efforts to achieve controlled, peaceful fusion began much earlier. Experimental work in the 1930s and 1940s established the foundations for magnetic and inertial confinement approaches. In 1958 the Scylla I device at Los Alamos achieved the first laboratory thermonuclear fusion through a theta-pinch configuration.
Significant progress was made in the 1970s and 1980s with the development of tokamak devices, particularly in the Soviet Union. Tokamaks use powerful magnetic fields to confine hot plasma in a toroidal chamber, allowing sustained high-temperature conditions. The first large-scale successes involved D–T fuel mixtures. At Princeton’s Tokamak Fusion Test Reactor (TFTR), experiments from 1993 to 1996 produced 16 gigajoules of fusion energy, achieving peak powers exceeding 100 megawatts in single discharges.
In Europe, the Joint European Torus (JET) achieved a peak fusion power of 16 megawatts in 1997. Further advances included measurements of the core power gain, with experiments showing a central Q value exceeding one. In 2024 JET produced 69 megajoules of fusion energy using only 0.2 milligrams of fuel, a landmark result indicating steady progress towards net-energy fusion.

Inertial confinement and ignition milestones

Alongside magnetic confinement, inertial confinement fusion (ICF) has been pursued as an alternative route to break-even. The National Ignition Facility (NIF) in the United States, the world’s most powerful laser system, was designed specifically to demonstrate ignition by compressing tiny fuel pellets to immense densities. Early experiments began in 2009, and in December 2022 NIF achieved the world’s first laboratory fusion ignition event, producing 315 megajoules of fusion energy from 205 megajoules of laser input. Although the total system energy required remains much larger, this result marked the first instance of scientific breakeven.
ICF systems rely on rapid compression and heating of fuel targets by extremely intense laser pulses. The implosion generates the necessary temperature and density conditions for D–T fusion and may offer pathways to high-energy applications such as neutron sources and advanced materials testing.

Modern fusion technologies and prospects

Contemporary fusion research focuses on two main approaches: magnetic confinement, exemplified by tokamaks and stellarators; and inertial confinement using lasers or particle beams. Future devices aim to increase the energy gain factor significantly. The international ITER project seeks to create a tokamak capable of producing ten times more fusion power than the external heating power supplied. Designed as the largest magnetic confinement experiment ever built, ITER is expected to demonstrate integrated plasma scenarios supporting commercial reactor development.
Advances in superconducting magnets, high-temperature plasma stability, materials tolerances, and high-repetition-rate lasers continue to shape the feasibility of future reactors. Several national and private-sector initiatives are exploring compact reactor designs, high-field tokamaks, and innovative confinement systems.
Fusion’s potential applications extend beyond electricity generation. These include neutron sources for scientific research, production of superheavy elements, and enhancement of fission weapon designs through boosted fusion. However, the primary motivation remains the promise of a clean, virtually limitless energy source with no long-lived radioactive waste and minimal environmental impact.