Fusion, << FYOO zhuhn, >> in physics, is the combining of two atomic nuclei to form the nucleus of a heavier element. Fusion reactions between light nuclei release a great amount of energy. Fusion produces the energy of the sun and other stars and the explosive force of thermonuclear weapons.
Nuclei of hydrogen and other light elements fuse more readily than nuclei of heavier elements do. Scientists conduct most fusion experiments using deuterium and tritium, two isotopes (forms) of hydrogen. A deuterium nucleus and a tritium nucleus fuse to form a helium nucleus. This reaction releases a neutron.
When two light nuclei fuse, a small amount of mass is lost. This mass is converted to energy. Fusion reactions typically release millions of times more energy than chemical reactions do. For example, the fusion of 1 pound (0.45 kilogram) of a mixture of deuterium and tritium releases as much energy as burning 9,000 tons (8,200 metric tons) of coal.
Normally, nuclei repel each other. But if the nuclei are heated many millions of degrees, they begin moving so fast that they may collide and fuse. Fusion reactions typically occur at temperatures higher than 90,000,000 °F (50,000,000 °C).
Scientists often use a device called a particle accelerator to give nuclei enough energy to fuse. A particle accelerator uses electric or magnetic force to accelerate the nuclei, causing them to collide at high speeds. Scientists can also use a device called a magnetic bottle to achieve fusion. A magnetic bottle confines the nuclei in a strong magnetic field. Scientists then bombard the nuclei with electromagnetic waves or charged particles to increase their temperature. Another device, called an inertial confinement-system, rapidly compresses the nuclei to a small point, raising their temperature.
Fusion reactions also occur among nuclei of other elements. For example, nuclei of helium and boron isotopes can fuse with nuclei of hydrogen isotopes. These reactions require even higher temperatures. In the extremely hot cores of stars, light nuclei fuse to form nuclei of such elements as carbon, oxygen, and even iron.
Many scientists believe that fusion occurred on a large scale during a hot, explosive event called the big bang at the beginning of the universe. This theory holds that most of the universe’s helium was created by the fusion of hydrogen nuclei in the first few minutes of the big bang.
See also ITER ; National Ignition Facility ; Nuclear energy (Nuclear fusion) ; Particle accelerator ; Star (Fusion in stars) .
