Nuclear Fusion
How is energy produced in a star? That question has intrigued many for centuries. If scientists could learn about energy production in the stars, they might be able to apply that knowledge to the development of commercially efficient systems for energy production on Earth.
Today, scientists believe that energy is produced in stars by means of nuclear fusion, the process by which two nuclei are joined to make one nucleus, with the release of huge amounts of energy. The idea that nuclear fusion might explain stellar energy release goes back at least seventy years, to the writings of the American physicist William Draper Harkins (1873-1951), the Dutch physicists R. d'E. Atkinson and F. G. Houtermans, and the French physicist Jean Baptiste Perrin.
German physicist Hans Bethe presented the clearest development of this idea in about 1938. Bethe worked out a series of reactions by which six hydrogen nuclei might combine to form a single helium nucleus, with the release of two extra hydrogen nuclei. The production of two hydrogen nuclei in the reaction make it possible for a second fusion reaction to begin. The Bethe cycle, then, is a chain reaction that, once initiated, can proceed on its own as long as sufficient raw material (hydrogen) is available.
Since carbon isotopes occur in the Bethe scheme of reactions as catalysts, his theory is sometimes referred to as the carbon cycle of fusion. Bethe was able to calculate the amount of energy that would be released during such a set of reactions and found that his results closely matched values obtained by empirical studies of our own Sun's energy production.
Until the 1940s, fusion theory remained largely a matter of speculation. Actual experimentation on the phenomenon was limited by a practical problem inherent in all fusion reactions. In order for two protons, for example, to fuse, they must be brought very close to each other. But the electrostatic force of repulsion between two like-charged particles, such as protons, is very large indeed. In fact, the only way to achieve fusion of two protons is to provide them with enough energy to overcome this force of repulsion. In practice, this means heating the protons to very high temperatures, in excess of ten million degrees Celsius. Because of the very high temperatures required, such reactions are often referred to as thermonuclear reactions. Until the 1940s, the only place where such temperatures were known to exist was at the centers of stars. During the 1940s, however, a second, earth-bound source became available: the explosion of an atomic (fission) bomb. In the first fractions of a second after such a bomb explodes, its temperature rises well above ten million degrees. During that moment, conditions exist that permit a fusion reaction to occur.
Some scientists working on the Manhattan Project saw here the potential for an even more powerful weapon than the atomic bomb on which they were working. This "super bomb" would consist of a fission bomb surrounded by a large mass of hydrogen isotopes. When the fission bomb ignites, the heat it releases will initiate fusion reactions among the hydrogen isotopes around it. The first test of such a hydrogen (fusion) bomb occurred in 1952 when the United States exploded a simplified "thermonuclear device" (not quite a true fusion bomb) on Bikini Atoll in the Pacific Ocean.
As was the case with nuclear fission, scientists immediately began to imagine methods for producing controlled fusion reactions that could be used to meet human needs and desires for energy production. Their progress in this direction has been much slower, however, than it was with the development of nuclear fission power. The most important problem is the need to contain the hot plasma produced when hydrogen isotopes are heated to ten million degrees or more. Obviously, no standard building material can survive such temperatures.
It has been clear from the outset that one way to contain a fusion reaction is with a magnetic field. The study of plasma behavior within a magnetic field--magnetohydro-dynamics, or MHD--was being developed at about the same time the fusion bomb was being built. Many astrophysicists were giving thought to the ways in which their understanding of stellar phenomena might be utilized in controlled-fusion research. The Swedish physicist, Hannes Olof Göst Alfvén (1908-), for example, won the Nobel Prize for physics in 1970 for his pioneering work on MHD. Igor Evgenevich Tamm (1895-1971), of the former Soviet Union, and Lyman Spitzer, Jr. (1914-), of the United States, suggested two different mechanisms for entrapping plasma during fusion. The former concept led to the development of today's tokamak, while Spitzer's model became known as a stellerator.
A successful controlled fusion reaction involves bringing together a sufficiently high concentration of particles at a sufficiently high temperature for a sufficient period of time. The product of the first and third of these factors is known as the Lawson confinement factor. Progress continues to be made in the design and development of machines that can produce a controlled fusion reaction for an extended period of time, but no one can really predict how long it will be before such a goal is finally realized.
In 1989 Martin Fleischmann, of the University of Southampton in England, and B. Stanley Pons, of the University of Utah, announced that they had observed cold fusion in their laboratory in Salt Lake City. Cold fusion is the process by which hydrogen nuclei are caused to combine at or near room temperatures. In theory, cold fusion might be possible if an appropriate catalyst and other conditions can be found. Cold fusion obviously has enormous commercial significance since it would mean that a major source of energy--fusion--might be available at a very modest production cost.
Upon hearing of the Fleischmann-Pons results, scientists around the world set out to replicate their findings. After more than two years of research, most experts are convinced that these scientists did not observe cold fusion, but that other reactions produced their results. Most scientists doing research on fusion continue to concentrate their efforts, therefore, on developing methods for the control of thermonuclear reactions.
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