FUSION REACTOR

also called thermonuclear reactor, a device to produce electrical power from the energy released in a nuclear fusion reaction. Since the 1930s, scientists have known that the Sun and other stars generate their energy by nuclear fusion. They realized that if fusion energy generation could be replicated in a controlled manner on Earth, it might very well provide a safe, clean, and inexhaustible source of energy. The 1950s saw the beginning of a worldwide research effort to develop a fusion reactor. The substantial accomplishments and prospects of this continuing endeavour are described here. also called Thermonuclear Reactor, a device that produces electrical power from the energy released in a nuclear fusion reaction. A fusion reaction occurs when two lighter atomic nuclei combine to form the nucleus of a heavier element. When the two nuclei fuse, a small amount of mass is converted into a large amount of energy. For a fusion reaction to occur, the two nuclei must be moving at high speed in order for their mutual electrical repulsion to be overcome. Extremely high temperatures are used to impart these high speeds to the nuclei. In all current designs, the core of a fusion reactor is a hot, dense plasmai.e., an ionized gas consisting of free nuclei and free electrons. Deuterium and tritium nuclei (heavy hydrogen), being the easiest nuclei to fuse, are the fuel for the reactor. The chief engineering challenge in producing fusion energy is to produce and confine a plasma at the necessary 100,000,000 C. The most successful approach is to hold a doughnut-shaped plasma together with a magnetic field, a method called magnetic confinement. The fusion of deuterium and tritium produces a helium nucleus (alpha particle) and a neutron. The neutron bombards and heats up the surrounding structure; this heat is then converted to electricity by conventional means (such as a turbine). An alternative approach is to focus an array of powerful lasers on a tiny frozen pellet made of deuterium and tritium. The pellet is compressed by laser energy to very high density; the resultant high temperature is sufficient to produce fusion reactions, and the pellet, now a dense plasma, is burned up in a microexplosion. This method is called inertial confinement, since the plasma is confined only by its own inertia. Fusion research began in the 1950s. Since that time the efficiency of magnetic confinement fusion reactors has increased a million-fold. In 1994 the Tokamak Fusion Test Reactor at Princeton University generated 10 million watts of fusion power for one second. In the late 20th century an international experiment called the International Thermonuclear Experimental Reactor, or ITER, was designed to generate 1.5 billion watts of fusion power continuously. This experiment, with an estimated cost of $10 billion, was a partnership between the European Union, Japan, Russia, and the United States and was scheduled to begin construction in 1998. Substantial research is still needed to achieve commercial fusion power, which is not expected to be realized earlier than the middle of the 21st century. A primary motivation for research in this field lies in the fact that fusion is environmentally clean, generating no pollutants or greenhouse gases and little radioactivity by comparison with fission-reaction nuclear power plants. A fusion reactor would also be safe, with no analog to the fission meltdown. If one of the reactor control systems fails, the plasma simply cools down and the reactions cease. And because deuterium is abundant in the oceans and tritium can be bred in the reactor, fusion reactors could prove a virtually inexhaustible source of energy for humanity. Additional reading Stanley W. Angrist, Direct Energy Conversion, 4th ed. (1987), provides a historical introduction and overview. Articles written for the lay reader include two from Scientific American: Robert W. Conn, The Engineering of Magnetic Fusion Reactors, 249(4):6071 (October 1983); and R. Stephen Craxton, Robert L. McCrory, and John M. Soures, Progress in Laser Fusion, 255(2):6879 (August 1986). The following books assume that the reader has a science background. Concepts of fusion in general are examined by Thomas James Dolan, Fusion Research: Principles, Experiments, and Technology (1982). Francis F. Chen, Introduction to Plasma Physics and Controlled Fusion, vol. 1, Plasma Physics, 2nd ed. (1984); and Weston M. Stacey, Jr., Fusion Plasma Analysis (1981), provide introductions to plasma physics. Particular approaches to fusion are analyzed in James J. Duderstadt and Gregory A. Moses, Inertial Confinement Fusion (1982); two articles from Physics Today, vol. 45, no. 9 (September 1992): John D. Lindl, Robert L. McCrory, and E. Michael Campbell, Progress Toward Ignition and Burn Propagation in Inertial Confinement Fusion, pp. 3240; and William J. Hogan, Roger Bangerter, and Gerald L. Kulcinski, Energy from Inertial Fusion, pp. 4250; Weston M. Stacey, Jr., Fusion: An Introduction to the Physics and Technology of Magnetic Confinement Fusion (1984); and two articles from Physics Today, vol. 45, no. 1 (January 1992): J. Geoffrey Cordey, Robert J. Goldston, and Ronald R. Parker, Progress Toward a Tokamak Fusion Reactor, pp. 2230; and James D. Callen, Benjamin A. Carreras, and Ronald D. Stambaugh, Stability and Transport Processes in Tokamak Plasmas, pp. 3442. Stewart C. Prager