Fusion power

Fusion power is a potential method of electric power generation from heat released by nuclear fusion reactions. In fusion, two light atomic nuclei combine to form a heavier nucleus and release energy. Devices that use this process are known as fusion reactors. Research on fusion reactors began in the 1940s. As of 2025, the National Ignition Facility (NIF) in the United States is the only laboratory to have demonstrated a fusion energy gain factor above one, but efficiencies orders of magnitude higher are required to reach engineering breakeven (a net electricity-producing plant) or economic breakeven (where the net electricity pays for the plant's whole-life cost). Thermonuclear fusion reactions require fuel in a plasma state and a confined environment with high temperature, pressure, and sufficient confinement time. The relationship between these parameters is expressed by the Lawson criterion. In stars, gravity provides the conditions for fusing hydrogen isotopes. Experimental reactors use deuterium and tritium, heavier isotopes of hydrogen, in a process known as DT fusion. This reaction forms a helium nucleus and an energetic neutron. Fusion fuel is extremely energy-dense, but tritium is scarce on Earth and decays with a half-life of about 12.3 years. Future reactors plan to use lithium breeding blankets that generate tritium when exposed to neutron radiation. Fusion offers advantages compared with nuclear fission. It produces minimal high-level radioactive waste and involves lower inherent safety risks. However, the process generates intense neutron radiation that gradually damages the inner walls of a reactor. Achieving sustained energy gain beyond breakeven and converting it efficiently into electricity remain major technical challenges. Research focuses mainly on two methods: magnetic confinement fusion (MCF) and inertial confinement fusion (ICF). MCF devices use magnetic fields to contain plasma. Early concepts included the z-pinch, stellarator, and magnetic mirror, with the tokamak design becoming dominant after Soviet experiments in the 1960s. ICF compresses and heats small fuel pellets using high-energy lasers, developed primarily since the 1970s. The largest active projects are ITER in France and the National Ignition Facility in the United States. Commercial and academic teams are also studying alternatives such as magnetized target fusion and modern stellarator designs.

Terminology The terms "fusion experiment" and "fusion device" refer to the collection of technologies used for scientific investigation of plasma, and technical advancement. Not all are capable of, or routinely used for, producing thermonuclear reactions i.e. fusion. The term "fusion reactor" is used interchangeably to mean the above experiments, or to mean a hypothetical power-producing version, at the center of a commercial power plant, requiring additions such as a breeding blanket and heat engine.

Background

Mechanism Fusion reactions occur when two or more atomic nuclei come close enough for long enough that the nuclear force pulling them together exceeds the electrostatic force pushing them apart, fusing them into heavier nuclei. For nuclei heavier than iron-56, the reaction is endothermic, requiring an input of energy. The heavy nuclei bigger than iron have many more protons resulting in a greater repulsive force. For nuclei lighter than iron-56, the reaction is exothermic, releasing energy when they fuse. Since hydrogen has a single proton in its nucleus, it requires the least effort to attain fusion, and yields the most net energy output. Also, since it has one electron, hydrogen is the easiest fuel to fully ionize. The repulsive electrostatic interaction between nuclei operates across larger distances than the strong force, which has a range of roughly one femtometer—the diameter of a proton or neutron. The fuel atoms must be supplied enough kinetic energy to approach one another closely enough for the strong force to overcome the electrostatic repulsion in order to initiate fusion. The "Coulomb barrier" is the quantity of kinetic energy required to move the fuel atoms near enough. Atoms can be heated to extremely high temperatures or accelerated in a particle accelerator to produce this energy. An atom loses its electrons once it is heated past its ionization energy. The resultant bare nucleus is a type of ion. The result of this ionization is plasma, which is a heated cloud of bare nuclei and free electrons that were formerly bound to them. Plasmas are electrically conducting and magnetically controlled because the charges are separated. This is used by several fusion devices to confine the hot particles.

Cross section

A reaction's cross section, denoted σ, measures the probability that a fusion reaction will happen. This depends on the relative velocity of the two nuclei. Higher relative velocities generally increase the probability, but the probability begins to decrease again at very high energies. In a plasma, particle velocity can be characterized using a probability distribution. If the plasma is thermalized, the distribution looks like a Gaussian curve, or Maxwell–Boltzmann distribution. In this case, it is useful to use the average particle cross section over the velocity distribution. This is entered into the volumetric fusion rate:

P fusion = n A n B ⟨ σ v A , B ⟩ E fusion {\displaystyle P_{\text{fusion}}=n_{A}n_{B}\langle \sigma v_{A,B}\rangle E_{\text{fusion}}}

where:

P fusion {\displaystyle P_{\text{fusion}}} is the energy made by fusion, per time and volume n is the number density of species A or B, of the particles in the volume

⟨ σ v A , B ⟩ {\displaystyle \langle \sigma v_{A,B}\rangle } is the reactivity of that reaction, defined as the average of the cross section σ over all the velocities of the two species v

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