s recombining with ions to form neutral particles, while photons are continuously being produced and absorbed. Scientists have estimated that more than 99 percent of the matter in the universe exists in the plasma state. All of the observed stars, including the Sun, consist of plasma, as do interstellar and interplanetary media and the outer atmospheres of the planets. Although most terrestrial matter exists in a solid, liquid or gaseous state, plasma is found in lightning bolts and auroras, in gaseous discharge lamps (neon lights), and in the crystal structure of metallic solids. Plasmas are currently being studied as an affordable source of clean electric power from thermonuclear fusion reactions. The scientific problem for fusion is thus the problem of producing and confining a hot, dense plasma. The core of a fusion reactor would consist of burning plasma. Fusion would occur between the nuclei, with electrons present only to maintain macroscopic charge neutrality. Stars, including the Sun, consist of plasma that generates energy by fusion reactions. In these “natural fusion reactors” the reacting, or burning, plasma is confirmed by its own gravity. It is not possible to assemble on Earth a plasma sufficiently massive to be gravitationally confined. The hydrogen bomb is an example of fusion reactions produced in an uncontrolled, unconfined manner in which the energy density is so high that the energy release is explosive. By contrast, the use of fusion for peaceful energy generating requires control and confinement of a plasma at high temperature and is often called controlled thermonuclear fusion. In the development of fusion power technology, demonstration of “ energy breakeven” is taken to signify the scientific feasibility of fusion. At breakeven, the fusion power produced by a plasma is equal to the power input to maintain the plasma. This requires a plasma that is hot, dense, and well confined. The tempe...
