Nuclear fusion

• 1. Introduction

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  Nuclear fusion is the reaction that lights every star in the sky, yet on Earth it has taken more than a century of science to approach anything resembling a controlled version of that same fire. The difference in mass between the raw ingredients and the finished products of a fusion reaction is tiny. But according to the mass-energy relationship that Albert Einstein described, that sliver of lost mass escapes as an enormous release of energy. The most fusible nuclei are among the lightest: deuterium, tritium, and helium-3. A fusion process producing nuclei lighter than nickel-62 releases energy rather than absorbing it, which is why the lightest elements are the best fuel. The opposite is true for nuclear fission, which is most energetic at the heavy end of the periodic table. Two questions have driven researchers for decades: what does it actually take to ignite fusion, and can it ever be made practical on Earth?

• 2. The Theorists Who Looked at Stars

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  American chemist William Draper Harkins first proposed the concept of nuclear fusion in 1915. Five years later, Francis William Aston's mass spectrometer showed that four hydrogen atoms are measurably heavier than a single helium atom, opening the door for Arthur Eddington to correctly predict in 1920 that hydrogen fusing into helium was the primary source of stellar energy. His reasoning drew on three independent threads: observations of Cepheid variable stars failing to show the rotational speedup the competing contraction hypothesis required; Einstein's demonstration that a small amount of matter equates to a large amount of energy; and Aston's finding that helium mass was about 0.8 percent less than the four hydrogen atoms that would form it. Eddington noted that if a star contained just 5 percent fusible hydrogen, that alone would explain stellar longevity. It is now known that most ordinary stars are around 70 to 75 percent hydrogen. The quantum piece arrived in 1927 when Friedrich Hund discovered tunneling in relation to electron levels. George Gamow applied tunneling to the nucleus itself in 1928, first to alpha decay and then to fusion as an inverse process. From Gamow's paper, Robert Atkinson and Fritz Houtermans made the first estimates for stellar fusion rates in 1929. Hans Bethe then worked with Charles Critchfield in 1938 to enumerate the proton-proton chain that dominates Sun-type stars. In 1939, Bethe published the discovery of the CNO cycle common to higher-mass stars.

• 3. First Experiments

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  Patrick Blackett made the first conclusive experiments in artificial nuclear transmutation at the Cavendish Laboratory during the 1920s. Building on Gamow's paper, John Cockcroft and Ernest Walton constructed their particle generator at the same laboratory. In April 1932, they published experiments showing a lithium nucleus hit by a proton producing two helium nuclei by way of an extremely short-lived beryllium-8 intermediate, giving that reaction a claim to the first artificial fusion. Ernest Lawrence and colleagues at the University of California Radiation Laboratory accidentally produced the first deuterium-deuterium fusion reactions in papers from July and November 1933, during some of the earliest cyclotron experiments. Detecting only the resulting energized protons and neutrons, the team misread the source as an exothermic disintegration of the deuterons, a process now known to be impossible. The first intentional deuterium fusion came in May 1934, when Mark Oliphant, Paul Harteck, and Ernest Rutherford published their Cavendish experiments and in doing so discovered both tritium and helium-3. That work is widely considered the first experimental demonstration of fusion. By 1938, researchers at the University of Michigan recorded the first observation of deuterium-tritium fusion and its characteristic 14 MeV neutrons, a reaction now recognised as the most favourable of all.

• 4. Weapons and the Bomb

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  Research into fusion for military purposes began in the early 1940s as part of the Manhattan Project. In 1941, Enrico Fermi and Edward Teller had a conversation about whether a fission bomb could create conditions suitable for thermonuclear fusion. The following year, Emil Konopinski drew the project's attention to Ruhlig's work on the deuterium-tritium reaction, and J. Robert Oppenheimer quietly commissioned physicists at Chicago and Cornell to use the Harvard University cyclotron to measure its cross-section, along with that of the lithium reaction. Measurements were gathered at Purdue, Chicago, and Los Alamos from 1942 to 1946. Theoretical assumptions had given the DT reaction a cross-section similar to DD, but in 1946 Egon Bretscher discovered a resonance enhancement making the DT reaction roughly 100 times larger. From 1945, John von Neumann, Teller, and other Los Alamos scientists ran thermonuclear weapon simulations on ENIAC, one of the first electronic computers. The first artificial thermonuclear fusion reaction occurred during the 1951 Greenhouse George nuclear test, using a small amount of deuterium-tritium gas and yielding 225 kt, fifteen times the yield of Little Boy. The first true two-stage thermonuclear weapon was the 1952 Ivy Mike test, a liquid deuterium-fusing device yielding over 10 Mt, driven by the Teller-Ulam design's full utilization of the fission blast. The Soviet RDS-6s test in 1953 produced 400 kt and was the first air-deliverable bomb using fusion, but was limited by its single-stage design. The first Soviet two-stage test was RDS-37 in 1955, yielding 1.5 Mt via an independently reached version of the Teller-Ulam design. A later physics surprise with weapons came during the 1954 Castle Bravo test: designers understood the usefulness of lithium-6 in tritium production but failed to account for lithium-7 fission at higher neutron energies, pushing the yield to 15 Mt, 150 percent greater than the predicted 6 Mt.

• 5. The Physics of Ignition

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  Achieving fusion on Earth demands an extremely large triple product of temperature, density, and confinement time. These conditions appear naturally only in stellar cores and advanced nuclear weapons, and are approached in laboratory experiments. The Coulomb barrier describes the electrostatic repulsion that positively charged nuclei must overcome before the short-range nuclear force can pull them together. For deuterium-tritium fuel, that energy barrier is about 0.1 MeV. Once overcome, the fusion produces an unstable helium-5 nucleus that immediately ejects a neutron carrying 14.1 MeV, while the remaining helium-4 carries away 3.5 MeV, for a total of 17.6 MeV released. That is many times more energy than was needed to clear the barrier. Quantum tunneling helps: nuclei do not need to fully surmount the Coulomb barrier; they can tunnel through the remaining portion if they are close enough in energy. The fusion energy density of the DT reaction far exceeds nuclear fission, and fission itself is millions of times more energetic than chemical reactions. Via the mass-energy equivalence, fusion converts 0.7 percent of the reactant mass into energy. Only neutron star or black hole accretion, approaching 40 percent efficiency, and antimatter annihilation at 100 percent surpass it. At solar core temperature and density, fusion reactions release only about 276 microwatts per cubic centimeter, roughly a quarter of the heat a resting human body generates per unit volume. Terrestrial reactors therefore need temperatures 10 to 100 times higher than stellar interiors to compensate for lower densities.

• 6. Fusion in the Cosmos

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  At a solar-core temperature of 14 million kelvin, the Sun fuses 620 million metric tons of hydrogen per second and produces 616 million metric tons of helium. The 4 million metric ton shortfall escapes as energy. In the process, 0.645 percent of the mass of each fused hydrogen pair leaves as kinetic energy of an alpha particle or electromagnetic radiation. The primary mechanism in Sun-type stars is the proton-proton chain; in more massive stars the CNO cycle takes over. As a massive star exhausts its hydrogen, it begins fusing heavier elements. In the most massive stars, at least 8 to 11 solar masses, this continues through silicon-burning, the final fusion cycle, which builds up iron and nickel. Because iron has one of the highest binding energies per nucleon, reactions producing heavier elements are generally endothermic, and significant amounts of elements heavier than iron form in supernova explosions rather than during stable stellar burning. Brown dwarfs fuse deuterium, and in very high mass cases also lithium. Carbon-oxygen white dwarfs that accrete matter and approach the Chandrasekhar limit of 1.44 solar masses ignite carbon-burning fusion that destroys the Earth-sized dwarf within one second in a Type Ia supernova. Some neutron stars periodically accumulate enough accreted helium that a thermonuclear burn wave propagates across the surface on the timescale of one second. In the extreme environment around lower stellar-mass black holes, below 10 solar masses, fusion of nitrogen, oxygen, neon, and magnesium can occur close to the event horizon. Big Bang nucleosynthesis began roughly 10 seconds after the Big Bang and lasted about 20 minutes, producing predominantly helium-4 with trace lithium, beryllium, and boron. The first stars formed around 13.6 billion years ago as pockets of gas grew dense enough to collapse under their own gravity.

• 7. Controlled Power on Earth

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  Los Alamos National Laboratory's Scylla I device produced the first laboratory thermonuclear fusion in 1958, but the technology has remained in its developmental phase ever since. The first large-scale controlled fusion power came from tokamak experiments mixing deuterium and tritium. The TFTR at Princeton University produced 1.6 GJ of fusion energy during experiments from 1993 to 1996, with a peak fusion power of 10.3 MW from 3.7 reactions per second. Subsequent work at the JET in 1997 achieved a peak fusion power of 16 MW. A JET experiment in 2024 produced 69 MJ of fusion power from just 0.2 milligrams of deuterium and tritium combined, with a central Q of 1.3. The US National Ignition Facility, using laser-driven inertial confinement, announced on the 13th of December 2022 that on the 5th of December 2022 it had achieved break-even fusion, delivering 2.05 MJ to the target and receiving 3.15 MJ of fusion energy output. The rate of supplying power to the experimental cell is hundreds of times larger than the power delivered to the target, so true net energy gain remains a further challenge. The two most advanced confinement approaches are magnetic confinement using toroidal designs such as tokamaks, and inertial confinement using lasers. ITER, a workable toroidal design projected to deliver ten times more fusion energy than is needed to heat the plasma, is currently expected to begin plasma experiments in 2034, with full deuterium-tritium fusion not expected until 2039. France's WEST fusion reactor, a tokamak of the same style as ITER, maintained a plasma at 90 million degrees for a record six minutes. Private companies pursuing commercial fusion received $2.6 billion in private funding in 2021 alone, going to ventures including Commonwealth Fusion Systems, Helion Energy, General Fusion, TAE Technologies, and Zap Energy.