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How does a fusion reactor produce energy from light atomic nuclei?

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3D illustration of the interior reaction chamber of a tokamak fusion reactor, showing a large, cylindrical, metallic structure with curved walls and bright light panels. — fusion reactor A tokamak is a device used in nuclear-fusion research for magnetic confinement of plasma.

A fusion reactor produces energy by joining two light atomic nuclei, typically isotopes of hydrogen known as deuterium and tritium, to form a heavier nucleus, such as helium.12

Because atomic nuclei are positively charged, they naturally repel each other via the Coulomb force. To overcome this repulsion, the fuel must be heated to extreme temperatures—often exceeding 75,000,000 K—creating a plasma where nuclei possess enough kinetic energy to approach each other closely enough for the strong force to dominate and trigger fusion.2

When these nuclei fuse, a small amount of mass is converted into a large quantity of energy, as described by Einstein’s relation, $E = m c^{2}$ . In a deuterium-tritium reaction, this process releases an alpha particle and a high-energy neutron. The neutron escapes the plasma to deposit its energy in a surrounding material, such as lithium, where it is converted into heat and subsequently into electricity via turbines.2