Nuclear fusion

Nuclear fusion is a reaction where two or more atomic nuclei combine to form a single, larger nucleus. This process is fundamental to the universe because it powers all active stars. When nuclei fuse, the mass of the resulting product is different from the mass of the original parts. This difference in mass is released as a massive amount of energy. This energy comes from a change in the nuclear binding energy of the atoms.

Fusion is generally exothermic, meaning it releases energy, when creating nuclei lighter than nickel-62. This happens because of the positive gradient in the nuclear binding energy curve.

The mechanism of fusion relies on a battle between two opposing physical forces. The first is the Coulomb force, which causes positively charged protons to repel each other. The second is the nuclear force, which acts as a strong attraction to hold protons and neutrons together.

For light nuclei, the short-range nuclear force can overcome the infinite-range Coulomb repulsion. To make this happen, the particles must reach an extremely large triple product of temperature, density, and confinement time. Sometimes, nuclei use a process called quantum tunneling to cross the Coulomb barrier.

This allows a nucleus to pass through a barrier it might not otherwise be able to cross.

Different pathways exist for fusion depending on the size and type of the stars involved. In Sun-type stars, the proton–proton chain is the dominant process.

However, in higher-mass stars, the CNO cycle is more common.

The most fusible nuclei are among the lightest elements. These include isotopes like deuterium, tritium, and helium-3. In contrast, the process of nuclear fission is most energetic when using very heavy nuclei, such as the actinides.

This distinction defines how energy is harvested from different parts of the periodic table.

Our understanding of fusion grew through many decades of scientific discovery. American chemist William Draper Harkins first proposed the concept in 1915. In 1919, Francis William Aston invented the mass spectrometer. This tool helped prove that four hydrogen atoms are heavier than one helium atom. Consequently, Arthur Eddington predicted in 1920 that hydrogen fusion powers stars. In 1934, Mark Oliphant, Paul Harteck, and Ernest Rutherford performed the first intentional deuterium fusion experiment. This experiment led to the discovery of tritium and helium-3. By 1938, researchers observed deuterium-tritium (DT) fusion, which is considered the most favorable reaction for energy.

Fusion has been applied to both peaceful and military technologies. During the Manhattan Project, scientists like Enrico Fermi and Edward Teller studied fusion for thermonuclear weapons. The first artificial thermonuclear fusion occurred during the 1951 US Greenhouse test. This test produced 225 kilotons of energy, which was 15 times the yield of the Little Boy bomb. Later, the 1952 Ivy Mike test used a two-stage Teller–Ulam design to yield over 10 megatons. Modern devices often use solid lithium deuteride to provide tritium for the reaction. This creates a closed neutron cycle that makes the reaction highly energetic.

Today, scientists aim to use fusion for clean, controlled power on Earth. There are two main approaches: magnetic confinement and inertial confinement. Magnetic confinement uses toroidal designs, like the tokamak, to hold hot plasma.

One major international project is ITER, which is expected to begin full deuterium-tritium fusion in 2039. The other approach, inertial confinement, uses lasers to drive the reaction. A massive milestone occurred on December 5, 2022, in the United States. Scientists achieved "break-even," producing 3.15 megajoules of fusion energy from only 2.05 megajoules of input energy.

The scale of fusion is truly massive when looking at the stars. In the core of the Sun, 620 million metric tons of hydrogen fuse every second. This process produces 616 million metric tons of helium every second. In the fusion of two hydrogen nuclei into helium, about 0.645% of the mass is converted into energy. This energy is carried away as kinetic energy or electromagnetic radiation. On Earth, the pursuit of this power is growing rapidly. In 2025, a single Chinese state-owned fusion company received $2.1 billion in funding. Private companies also invested $2.6 billion in fusion startups in 2021 alone.