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. 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. Fusion processes require fuel and a confined environment with sufficient temperature, pressure and confinement interval, to create a plasma in which fusion can occur. In stars, the most common fuel is hydrogen, and gravity creates the conditions needed for fusion energy production. Fusion reactors generally use hydrogen isotopes such as deuterium and tritium, which react more easily than hydrogen. The designs aim to heat their fuel to tens of millions of degrees using a wide variety of methods. The major challenge in realising fusion power is to engineer a system that can confine the plasma long enough at high enough temperature and density for many reactions to occur. 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. As a source of power, nuclear fusion is expected to have several theoretical advantages over fission. These include reduced radioactivity in operation and little high-level nuclear waste, ample fuel supplies, and increased safety. However, achieving the necessary temperature/pressure/duration combination has proven to be difficult to produce in a practical and economical manner. Research into fusion reactors began in the 1940s, but to date, no design has produced more fusion power output than the electrical power input, defeating the purpose. 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 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 and inertial electrostatic confinement, and new variations of the stellarator. 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 lighter than iron-56, the reaction is exothermic, releasing energy. For nuclei heavier than iron-56, the reaction is endothermic, requiring an external source of energy. Hence, nuclei smaller than iron-56 are more likely to fuse while those heavier than iron-56 are more likely to break apart. The strong force acts only over short distances. The repulsive electrostatic force acts over longer distances. In order to undergo fusion, the fuel atoms need to be given enough energy to approach each other close enough for the strong force to become active. The amount of kinetic energy needed to bring the fuel atoms close enough is known as the 'Coulomb barrier'. Ways of providing this energy include speeding up atoms in a particle accelerator, or heating them to high temperatures. Once an atom is heated above its ionization energy, its electrons are stripped away (it is ionized), leaving just the bare nucleus (the ion). The result is a hot cloud of ions and the electrons formerly attached to them. This cloud is known as plasma. Because the charges are separated, plasmas are electrically conductive and magnetically controllable. Many fusion devices take advantage of this to control the particles as they are heated. A reaction's cross section, denoted σ, is the measure of 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. Cross sections for many fusion reactions were measured (mainly in the 1970s) using particle beams.