Nuclear fusion, although it was known theoretically in the 1930s as the process by which the sun and most other stars radiate their great output of energy, was not achieved by scientists until the 1950s. Fusion reactions are also known as thermonuclear reactions because the temperatures required to initiate them are more than 1,000,000°C. In the hydrogen bomb, such temperatures are provided by the detonation of a fission bomb. The energy released during fusion is even greater than that released during fission. Moreover, the fuel for fusion reactions, isotopes of hydrogen, is readily available in large amounts, and there is no release of radioactive byproducts.
In stars ordinary hydrogen, whose nucleus consists of a single proton, is the fuel for the reaction and is fused to form helium through a complex cycle of reactions (see nucleosynthesis). This reaction takes place too slowly, however, to be of practical use on the earth. The heavier isotopes of hydrogen—deuterium and tritium—have much faster fusion reactions.
For sustained, controlled fusion reactions, a fission bomb obviously cannot be used to trigger the reaction. The difficulties of controlled fusion center on the containment of the nuclear fuel at the extremely high temperatures necessary for fusion for a time long enough to allow the reaction to take place. For deuterium-tritium fusion, this time is about 0.1 sec. At such temperatures the fuel is no longer in one of the ordinary states of matter but is instead a plasma, consisting of a mixture of electrons and charged atoms. Obviously, no solid container could hold such a hot mixture; therefore, containment attempts have been based on the electrical and magnetic properties of a plasma, using magnetic fields to form a "magnetic bottle." In 1994 U.S. researchers achieved a fusion reaction that lasted about a second and generated 10.7 million watts, using deuterium and tritium in a magnetically confined plasma. The use of tritium lowers the temperature required and increases the rate of the reaction, but it also increases the release of radioactive neutrons. Another method uses laser beams aimed at tiny pellets of fusion fuel to create the necessary heat and pressure to initiate fusion.
If practical controlled fusion is achieved, it could have great advantages over fission as a source of energy. Deuterium is relatively easy to obtain, since it constitutes a small percentage of the hydrogen in water and can be separated by electrolysis, in contrast to the complex and expensive methods required to extract uranium-235 from its sources. In 2007 China, the European Union, India, Japan, Russia, South Korea, and the United States formally established the International Thermonuclear Experimental Reactor (ITER) Organization to build an experimental fusion reactor at Cadarache in S France that would use the "magnetic bottle" approach. The Lawrence Livermore National Laboratory's National Ignition Facility, based in Livermore, Calif., and dedicated in 2009, is exploring the use of high-energy lasers focused on hydrogen fuel to achieve nuclear fusion.