Nuclear fusion

Nuclear fusion is a reaction in which two or more atomic nuclei combine to form a larger nucleus. The difference in mass between the reactants and products is manifested as either the release or the absorption of energy. This difference in mass arises as a result of the difference in nuclear binding energy between the atomic nuclei before and after the fusion reaction. Active stellar cores are powered by fusion. Nucleosynthesis via fusion, in the Big Bang and in stars, creates all elements lighter than nickel (atomic number 28).

Fusion typically occurs via thermonuclear fusion, an isotropic process requiring a triple product of very high temperature (in the kiloelectronvolt or hundred million Kelvin range), density, and confinement time. These conditions occur only in , thermonuclear weapons and boosted fission weapons, and fusion power experiments.

A nuclear fusion process that produces atomic nuclei lighter than nickel-62 is generally exothermic, due to the positive gradient of the nuclear binding energy curve. The most fusible nuclei are among the lightest, especially deuterium, tritium, and helium-3. The opposite process, nuclear fission, is most energetic for very heavy nuclei, especially the actinides.

Fusion power seeks to use fusion for energy development; tokamaks and stellarators are the dominant designs in magnetic confinement fusion research. Fusion devices can also be used as neutron sources and in superheavy element production.