Fusion power

Fusion power is a proposed form of power generation that would generate electricity by using heat from
nuclear fusion reactions. In a fusion process, two lighter atomic nuclei combine to form a heavier
nucleus, while releasing energy. Devices designed to harness this energy are known as fusion reactors.
Research into fusion reactors began in the 1940s, but as of 2024, no device has reached net power,
although net positive reactions have been achieved.[1][2][3][4]
Fusion processes require fuel and a confined environment with sufficient temperature, pressure, and
confinement time to create a plasma in which fusion can occur. The combination of these figures that
results in a power-producing system is known as the Lawson criterion. In stars the most common fuel is
hydrogen, and gravity provides extremely long confinement times that reach the conditions needed for
fusion energy production. Proposed fusion reactors generally use heavy hydrogen isotopes such as
deuterium and tritium (and especially a mixture of the two), which react more easily than protium (the
most common hydrogen isotope) and produce a helium nucleus and an energized neutron,[5] to allow
them to reach the Lawson criterion requirements with less extreme conditions. Most designs aim to
heat their fuel to around 100 million kelvins, which presents a major challenge in producing a successful
design. Tritium is extremely rare on Earth, having a half life of only ~12.3 years. Consequently, during
the operation of envisioned fusion reactors, known as breeder reactors, helium cooled pebble beds
(HCPBs) are subjected to neutron fluxes to generate tritium to complete the fuel cycle.[6]
As a source of power, nuclear fusion has a number of potential advantages compared to fission. These
include reduced radioactivity in operation, little high-level nuclear waste, ample fuel supplies (assuming
tritium breeding or some forms of aneutronic fuels), and increased safety. However, the necessary
combination of temperature, pressure, and duration has proven to be difficult to produce in a practical
and economical manner. A second issue that affects common reactions is managing neutrons that are
released during the reaction, which over time degrade many common materials used within the reaction
chamber.
Fusion researchers have investigated various confinement concepts. The early emphasis was on three
main systems: z-pinch, stellarator, and magnetic mirror. The current leading designs are the tokamak
and inertial confinement (ICF) by laser. Both designs are under research at very large scales, most
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 less expensive approaches. Among these
alternatives, there is increasing interest in magnetized target fusion and inertial electrostatic
confinement, and new variations of the stellarator.
Background
Main article: Nuclear fusion
The Sun, like other stars, is a natural fusion reactor, where stellar nucleosynthesis transforms lighter
elements into heavier elements with the release of energy.
Binding energy for different atomic nuclei. Iron-56 has the highest, making it the most stable. Nuclei to
the left are likely to release energy when they fuse (fusion); those to the far right are likely to be
unstable and release energy when they split (fission).
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.
[7] 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. An ion is the name for the
resultant bare nucleus. The result of this ionization is plasma, which is a heated cloud of ions 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
The fusion reaction rate increases rapidly with temperature until it maximizes and then gradually drops
off. The deuterium-tritium (D-T) fusion rate peaks at a lower temperature (about 70 keV, or 800 million
kelvin) and at a higher value than other reactions commonly considered for fusion energy.
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.[8]
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:[9]
fusion
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Fusion power is a proposed form of power generation that would generate electricity by using heat from

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