t of "advanced" fusion-fuel cycles that do not produce neutrons, such as the fusion of deuterons with helium-3 nuclei. Nearly neutron-free fusion systems, which require higher temperatures than D-T fusion, might make up a "second generation" of fusion reactors). Finally, a fusion reactor would not release the gaseous pollutants that accompany the combustion of fossil fuels; hence, fusion would not produce a greenhouse effect. The fusion process has been studied as part of nuclear physics for much of the 20th century. In the late 1930s the German-born physicist Hans A. Bethe first recognized that the fusion of hydrogen nuclei to form deuterium is exoergic (there is release of energy) and, together with subsequent reactions, accounts for the energy source in stars. Work proceeded over the next two decades, motivated by the need to understand nuclear matter and forces, to learn more about the nuclear physics of stellar objects, and to develop thermonuclear weapons (the hydrogen bomb) and predict their performance. During the late 1940s and early 1950s, research programs in the United States, United Kingdom, and Soviet Union began to yield a better understanding of nuclear fusion, and investigators embarked on ways of exploiting the process for practical energy production. This work focused on the use of magnetic fields and electromagnetic forces to contain extremely hot gases called plasmas. A plasma consists of unbound electrons and positive ions whose motion is dominated by electromagnetic interactions. It is the only state of matter in which thermonuclear reactions can occur in a self-sustaining manner. Astrophysics and magnetic fusion research, among other fields, require extensive knowledge of how gases behave in the plasma state. The inadequacy of the then-existent knowledge became clearly apparent in the 1950s as the behavior of plasma in many of the early magnetic confinement systems proved too complex to understand. Moreover, rese...
