Nuclear fusion is a nuclear process in which two light atomic nuclei combine to form a heavier nucleus, with the reaction releasing energy when the products have a lower total mass than the original reactants. The lost mass corresponds to released energy through the mass energy relationship. Fusion is the fundamental energy producing process operating in stars, including the Sun, where hydrogen nuclei ultimately participate in reactions that produce helium and other products.
Fusion differs from fission because fission splits a heavy nucleus whereas fusion combines light nuclei. The most extensively studied fusion reaction for terrestrial energy research involves deuterium and tritium, two isotopes of hydrogen, producing a helium 4 nucleus and a high energy neutron. Fusion reactions require extremely high temperatures because positively charged nuclei repel one another through the electromagnetic force.
At sufficiently high temperature, matter becomes plasma, a state in which electrons are separated from atomic nuclei and charged particles move collectively. Researchers seek to confine and heat this plasma sufficiently for fusion reactions to occur at useful rates while preventing the extremely hot material from damaging surrounding structures.
The fundamental difficulty of fusion is the electrostatic repulsion between positively charged nuclei, often described as the Coulomb barrier. Two nuclei must approach closely enough for the strong nuclear force to become significant. The strong nuclear force is attractive at nuclear distances but has a very short effective range, so nuclei must come extremely close together before it can overcome their electromagnetic repulsion. Increasing temperature raises the kinetic energy of the nuclei and therefore increases the probability of close encounters.
Quantum mechanical tunnelling also allows some nuclei to penetrate the Coulomb barrier even when their classical kinetic energy is insufficient to pass directly over it. This tunnelling effect is essential to fusion in stars, where temperatures and densities are enormous but not sufficient for every proton to classically overcome the electrostatic barrier. The fusion reaction rate depends strongly on temperature, density and the probability distribution of particle energies. Consequently, creating a useful terrestrial fusion system requires careful control of plasma temperature, density and confinement time rather than simply reaching one fixed temperature.
The deuterium tritium reaction is especially attractive for fusion research because it has a comparatively high reaction probability at temperatures achievable in experimental fusion devices. Deuterium is a stable hydrogen isotope containing one proton and one neutron, while tritium contains one proton and two neutrons and is radioactive. When deuterium and tritium fuse, they produce helium 4 and a neutron with substantial kinetic energy.
The helium nucleus is electrically charged and can remain within the plasma long enough to transfer energy back to it, a process known as alpha particle heating. The neutron is electrically neutral and therefore is not confined by magnetic fields in the same way as charged plasma particles. It travels into surrounding structures where its kinetic energy can be converted into heat.
In a future power plant, that heat could be transferred to a coolant and used to generate electricity through a thermal cycle. Tritium supply and breeding are therefore major engineering considerations because naturally occurring tritium is scarce and fusion plants based on deuterium tritium fuel would need to produce tritium from lithium through neutron interactions.
Fusion can occur in several different physical environments and is not limited to laboratory reactors. In stars, gravitational compression produces enormous pressures and temperatures, allowing nuclear reactions to proceed in dense stellar plasma. The Sun primarily converts hydrogen into helium through the proton proton chain, a sequence of reactions in which the net effect is the conversion of hydrogen into helium with the release of energy and other particles.
More massive stars can obtain much of their energy from the carbon nitrogen oxygen cycle, which acts as a catalytic sequence for hydrogen fusion. At later stages of stellar evolution, sufficiently massive stars can fuse progressively heavier nuclei, although the energy balance eventually changes. Fusion beyond iron does not generally provide the same energy release because iron group nuclei are near the maximum of nuclear binding energy per nucleon. Instead, forming nuclei heavier than the iron group through fusion generally requires energy input. Stellar nucleosynthesis therefore involves both energy releasing and energy consuming nuclear processes depending on the nuclei involved and the astrophysical environment.
Terrestrial fusion research has developed several major confinement approaches. Magnetic confinement uses magnetic fields to control charged plasma particles and includes tokamaks and stellarators. A tokamak confines plasma in a toroidal chamber using a combination of magnetic field components, while a stellarator uses complex external magnetic coils to create a three dimensional confinement geometry that can operate without relying on the same plasma current configuration.
Inertial confinement uses powerful lasers or other drivers to compress a small fuel capsule rapidly so that fusion reactions occur before the material can expand significantly. Other approaches use different magnetic, electrostatic, pulsed or hybrid concepts. Each approach faces challenges involving plasma stability, energy confinement, material durability, heating efficiency, fuel handling and conversion of fusion energy into useful electricity. Demonstrating fusion reactions is not by itself equivalent to demonstrating a practical power plant. A commercial system must account for the complete energy balance of the facility, including plasma heating, magnets, pumps, cooling, fuel processing, power conversion and other auxiliary systems.
Fusion has potentially important characteristics as an energy source, but it also has substantial technical challenges. Deuterium is abundant in ordinary water, while tritium can potentially be generated from lithium within a reactor blanket. Fusion reactions do not involve the same chain reaction mechanism as conventional fission reactors, because stopping the heating or confinement conditions causes the fusion plasma to cool and the reaction rate to fall rapidly.
Nevertheless, fusion facilities can produce intense neutron radiation and activate structural materials, particularly when using deuterium tritium reactions. The materials must therefore withstand high heat loads, neutron bombardment and mechanical stresses. Tritium also requires careful containment because it is radioactive and chemically behaves as hydrogen, allowing it to move through materials under some conditions. Fusion does not mean that all radioactive waste disappears. The quantity and characteristics of radioactive materials depend strongly on reactor design, materials selection and fuel cycle. Research therefore considers both plasma performance and the full nuclear, thermal, structural and environmental behaviour of a future plant.
The scientific development of fusion research followed the recognition during the twentieth century that stellar energy could be explained by nuclear reactions. Controlled fusion research accelerated after the Second World War, with multiple countries investigating magnetic confinement, inertial confinement and related approaches. The term fusion describes the physical process of joining nuclei and is related linguistically to the Latin root meaning to melt or join together.
Important milestones have included increasing plasma temperatures, extending confinement times, achieving high fusion yields in inertial confinement experiments and developing increasingly sophisticated superconducting magnets and plasma control systems. International programmes such as ITER have been designed to investigate sustained burning plasma behaviour at large scale, while numerous national laboratories and private companies have pursued alternative concepts. The field remains an active area of nuclear physics, plasma physics, materials science, engineering and computational science. Current understanding supports the physical feasibility of fusion reactions while leaving major engineering and economic questions concerning reliable commercial electricity generation.
Fusion should not be confused with ordinary high temperature combustion or with chemical joining processes. The energy released in fusion originates from changes in nuclear binding energy, not from chemical bonds between atoms. Fusion also does not automatically mean a reactor will explode in the manner of a conventional nuclear weapon or that a power reactor can sustain an uncontrolled fission style chain reaction.
A fusion plasma requires carefully maintained physical conditions, and loss of those conditions strongly suppresses the reaction. Nevertheless, fusion facilities must be designed with appropriate nuclear safety principles because radioactive tritium, activated materials and energetic neutrons create genuine hazards. Fusion research also illustrates an important distinction between physical possibility and technological practicality.
A laboratory can demonstrate a fusion reaction without establishing that a machine can operate continuously, maintain its components for many years, breed sufficient tritium, efficiently convert heat to electricity and produce more useful electrical energy than the entire facility consumes. Understanding these distinctions is essential when evaluating claims about the present or future capabilities of fusion technology.
