Nuclear fusion
American chemist William Draper Harkins proposed the concept of nuclear fusion in 1915. Francis William Aston invented the mass spectrometer in 1919 to measure atomic masses. This invention revealed that four hydrogen atoms weigh more than one helium atom. Arthur Eddington correctly predicted in 1920 that hydrogen fusing into helium powered stars. Friedrich Hund discovered quantum tunneling in 1927 regarding electron levels. Soviet scientists Grigory Samuilovich Landsberg and Leonid Isaakovich Mandelstam independently observed this phenomenon. George Gamow applied tunneling to the nucleus in 1928 for alpha decay and then fusion. Robert Atkinson and Fritz Houtermans made the first estimates for stellar fusion rates in 1929. Hans Bethe worked with Charles Critchfield in 1938 to enumerate the proton, proton chain dominating Sun-type stars. Bethe published the discovery of the CNO cycle common to higher-mass stars in 1939. Patrick Blackett conducted the first conclusive experiments in artificial nuclear transmutation at the Cavendish Laboratory during the 1920s. John Cockcroft and Ernest Walton built their generator on the inspiration of Gamow's paper. They published experiments on a specific reaction in April 1932 involving beryllium-8 as an intermediary nuclide. Ernest Lawrence and his team accidentally produced the first deuterium, deuterium fusion reactions in July and November 1933 papers from the University of California Radiation Laboratory. Mark Oliphant, Paul Harteck, and Ernest Rutherford published an intentional deuterium fusion experiment in May 1934 at the Cavendish Laboratory. This experiment discovered both tritium and helium-3. A researcher at the University of Michigan made the first observation of deuterium, tritium fusion in 1938.
Research into fusion for military purposes began in the early 1940s as part of the Manhattan Project. Enrico Fermi and Edward Teller had a conversation about the possibility of a fission bomb creating conditions for thermonuclear fusion in 1941. Emil Konopinski brought Ruhlig's work on the deuterium, tritium reaction to the project's attention in 1942. J. Robert Oppenheimer commissioned physicists at Chicago and Cornell to use the Harvard University cyclotron to secretly investigate its cross-section. Measurements were obtained at Purdue, Chicago, and Los Alamos from 1942 to 1946. Egon Bretscher discovered a resonance enhancement giving the DT reaction a cross-section approximately one hundred times larger than previously thought in 1946. John von Neumann, Teller, and other Los Alamos scientists used ENIAC to simulate thermonuclear weapon detonations starting from 1945. The first artificial thermonuclear fusion reaction occurred during the 1951 US Greenhouse George nuclear test using a small amount of deuterium, tritium gas. This produced the largest yield to date at 225 kilotons, fifteen times that of Little Boy. The first true thermonuclear weapon detonation was the 1952 Ivy Mike test of a liquid deuterium-fusing device yielding over ten megatons. The key to this jump was the full utilization of the fission blast by the Teller, Ulam design. The Soviet Union began their focus on a hydrogen bomb program earlier and carried out the RDS-6s test in 1953. This had international impacts as the first air-deliverable bomb using fusion but yielded 400 kilotons and was limited by its single-stage design. The first Soviet two-stage test was RDS-37 in 1955 yielding 1.5 megatons using an independently reached version of the Teller, Ulam design. Modern devices benefit from the usage of solid lithium deuteride with an enrichment of lithium-6 due to the Jetter cycle involving an exothermic reaction.
Fusion powers stars and produces most elements lighter than cobalt in a process called nucleosynthesis. The Sun fuses 620 million metric tons of hydrogen and makes 616 million metric tons of helium each second. In its core, the Sun generates its energy by nuclear fusion of hydrogen nuclei into helium at a solar-core temperature of 14 million kelvin. The net result is the fusion of four protons into one alpha particle with the release of two positrons and two neutrinos. In heavier stars, the CNO cycle and other processes are more important. As a star uses up a substantial fraction of its hydrogen, it begins to fuse heavier elements. In massive cores, silicon-burning is the final fusion cycle leading to a build-up of iron and nickel nuclei. Nuclear binding energy makes the production of elements heavier than nickel via fusion energetically unfavorable. These elements are produced in non-fusion processes like the s-process, r-process, and supernova nucleosynthesis. Brown dwarfs fuse deuterium and in very high mass cases also fuse lithium. Carbon-oxygen white dwarfs approach the Chandrasekhar limit of 1.44 solar masses before carbon burning fusion destroys them within one second. Some neutron stars accrete hydrogen and helium from an active stellar companion. Periodically, the helium accretion reaches a critical level causing a thermonuclear burn wave to propagate across the surface on a timescale of one second. From approximately 10 seconds to 20 minutes after the Big Bang, the universe cooled allowing the combination of protons and neutrons in deuterium
nuclei. This began a rapid fusion chain ending in predominantly helium-4 with minimal fractions of lithium, beryllium, and boron nuclei.
To be a useful energy source, a fusion reaction must satisfy several criteria including being exothermic and involving low atomic number nuclei. The electrostatic repulsion that must be overcome is directly related to the number of protons it contains. Using deuterium, tritium fuel, the resulting energy barrier is about 0.1 megaelectronvolts. In comparison, the energy needed to remove an electron from hydrogen is 13.6 electronvolts. The reaction cross section measures the probability of a fusion reaction as a function of relative velocity. If reactants have a distribution of velocities, it is useful to perform an average over the distributions of product of cross-section and velocity. This average is called reactivity denoted by sigma v. The significance of reactivity as a function of temperature in a device with a particular energy confinement time is found by considering the Lawson criterion. An exception to this general trend is the helium-4 nucleus whose binding energy is higher than that of lithium. Helium-4 has an anomalously large binding energy because its nucleus consists of two protons and two neutrons which are all in the ground state. Any additional nucleons would have to go into higher energy states. The Coulomb barrier is smallest for isotopes of hydrogen as their nuclei contain only a single positive charge. A diproton is not stable so neutrons must also be involved ideally in such a way that a helium nucleus is one of the products.
The US has been counting on private industry to lead in fusion
power while more recently China's government has made fusion a national priority. In 2025, 2.1 billion dollars was poured into a single Chinese state-owned fusion company. This amount is two and a half times the U.S. Energy Department's annual fusion budget. Private companies pursuing the commercialization of nuclear fusion received 2.6 billion dollars in private funding in 2021 alone. This funding went to many notable startups including Commonwealth Fusion Systems, Helion Energy Inc., General Fusion, TAE Technologies Inc., and Zap Energy Inc. The ITER facility is currently expected to initiate plasma experiments in 2034 but is not expected to begin full deuterium, tritium fusion until 2039. One of the most recent breakthroughs occurred in France's WEST fusion reactor maintaining a 90 million degree plasma for a record time of six minutes. This is a tokamak-style reactor which is the same style as the upcoming ITER reactor. The US National Ignition Facility uses laser-driven inertial confinement fusion designed with a goal of achieving a fusion energy gain factor larger than one. Large-scale laser target experiments were performed in June 2009 and ignition experiments began in early 2011. On the 13th of December 2022, the United States Department of Energy announced they had successfully accomplished break-even fusion on the 5th of December 2022.
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Common questions
Who proposed the concept of nuclear fusion and when?
American chemist William Draper Harkins proposed the concept of nuclear fusion in 1915. This proposal laid the groundwork for future discoveries regarding atomic nuclei combining to release energy.
When did scientists first observe artificial deuterium tritium fusion experiments?
Mark Oliphant, Paul Harteck, and Ernest Rutherford published an intentional deuterium fusion experiment in May 1934 at the Cavendish Laboratory. A researcher at the University of Michigan made the first observation of deuterium tritium fusion in 1938.
What was the yield of the first true thermonuclear weapon detonation during the Ivy Mike test?
The first true thermonuclear weapon detonation was the 1952 Ivy Mike test of a liquid deuterium-fusing device yielding over ten megatons. This event utilized the full power of a fission blast through the Teller Ulam design.
How much hydrogen does the Sun fuse into helium each second?
The Sun fuses 620 million metric tons of hydrogen and makes 616 million metric tons of helium each second. This process occurs in its core at a solar-core temperature of 14 million kelvin.
Which country poured 2.1 billion dollars into a state-owned fusion company in 2025?
China's government made fusion a national priority and poured 2.1 billion dollars into a single Chinese state-owned fusion company in 2025. This amount is two and a half times the U.S. Energy Department's annual fusion budget.