Fusion power is an experimental form of power generation that generates electricity by using nuclear fusion reactions. In a fusion process, two atomic nuclei combine to form a heavier nucleus, while releasing energy. Devices that produce energy in this way are known as fusion reactors.
Fusion occurs in a plasma confined at sufficient temperature and pressure for a sufficient interval. The combination of these constraints is known as the Lawson criterion. Higher values for one element permit lower values in the others.
In stars, the most common fuel is hydrogen. Gravity provides long confinement times and high pressure. The power produced by the fused nuclei sustain the necessary temperature to keep the reaction going. Proposed reactors generally use hydrogen isotopes such as deuterium and tritium (or a mixture of the two), which react more easily than individual protons. This allows them to reach the Lawson criterion without extreme values of the other constraints.
As a source of power, nuclear fusion is expected to have many advantages over fission. These include reduced radioactivity in operation and little high-level nuclear waste, ample fuel supplies, and increased safety. However, the Lawson criterion has not been met in a practical system.
Research into fusion reactors began in the 1940s, but to date, no design has produced more fusion power output than the power input. Most fusion designs produce a stream of energetic neutrons that over time degrade the materials used within the reaction chamber.
Fusion researchers have investigated many confinement concepts. The early emphasis was on the z-pinch, stellarator, and magnetic mirror. Later the tokamak and inertial confinement took the lead. Both designs are under research at large scales, notably the ITER tokamak in France, and the National Ignition Facility (NIF) laser in the United States. Researchers are also studying other designs that may offer cheaper approaches. Among these alternatives, there is increasing interest in magnetized target fusion, inertial electrostatic confinement, and new variations of the stellarator.