Fusion Power (1966)

Naturally, it is not only for the sake of its destructive potentialities that fusion processes are of interest. If nuclear fusion could be made to proceed at a controlled pace, the energy requirements of mankind would be solved for the foreseeable future.

The advantage of fusion over fission involves first the matter of fuel. Where the fission fuels are comparatively rare metals, uranium and thorium, the fusion fuel is a much more common and readily available element, hydrogen. It would be most convenient if it were hydrogen-1 that were the specific isotope suitable for man-made fusion since that is the most common form of hydrogen. Unfortunately, the temperatures required for hydrogen-1 fusion, at a rate fast enough to be useful, are prohibitively high. Even at the temperatures of the solar interior, hydrogen-1 undergoes fusion slowly. It is only beacause of the vast quantity of hydrogen-1 available in the sun that the small percentage that does fuse is sufficient to keep the sun radiating as it does. (To be sure, if hydrogen-1 were more readily fusible than it is, the sun--and other stars--would explode.)

Hydrogen-2 (deuterium) can be made to undergo fusion at a lower temperature, and hydrogen-3 at a lower temperature still. However, hydrogen-3 is unstable and would be extremely difficult to collect in reasonable quantities. That leaves hydrogen-2 as the best possible fuel.

The atoms of deuterium can fuse in one of two ways with equal probability:

H2 + H2 -------> He3 + n1
H2 + H2 -------> H3 + H1
In the latter case, the H3 formed reacts quickly with another H2, thus:
H3 + H2 -------> He4 + n1
The overall reaction, then, would be:
5 H2 -------> He3 + He4 + H1 + 2 n1
The energy produced from such a fusion of five deuterium atoms (let's call it a "deuterium quintet") is 24.8 Mev. Since 1 Mev is equivalent to 1.6 X 10-6 ergs, the deuterium quintet, on fusion, yields 4.0 X 10-5 ergs.

A gram-moelcular wieght of hydrogen-2 contains 6.023 X 1023 atoms. Since a gram-molecular weight of hydrogen-2 is two grams, one] gram of hydrogen-2 contains 3.012 X 1022 deuterim quintets. The total energy produced by the complete fusion of one gram of hydrogen-2 is therefore 2.4 X 1018 ergs. Since there are 4.186 X 1010 ergs to a kilocalorie, we can say that the complete fusion of one gram of hydrogen-2 produces 5.7 X 107 kilocalories.

To be sure, only 1 out of every 7000 hydrogen atoms is hydrogen-2. Allowing for the fact that that one atom is twice as massive as the remaining 6999, we can say that one liter of water weighs 1000 grams, that 125 grams of it are hydrogen, and that of that hydrogen 43 milligrams are hydrogen-2. We can therefore say that the complete fusion of the hydrogen-2 contained in a liter of water will yield about 2.5 X 106 kilocalories.

This means that by the fusion of the hydrogen-2 contained in a liter of ordinary water, we could obtain as much energy as we would get through the combustion of 300 liters of gasoline.

Considering the vastness of the earth's ocean (from all of which hydrogen-2 is easily obtainable) we can see that the earth's supply of hydrogen-2 is something like 50,000 cubic miles. The energy that could be derived from this vast volume of hydrogen-2 is equivalent to the burning of a quantity of gasoline some 450 times the volume of the entire earth.

Obviously, if fusion power could be safely and practically tapped, mankind would have at its disposal an energy supply that should last for many millinos of years. And to top off that joyful prospect, the products of the fusion reaction are hydrogen-1, helium-3, and helium-4, all of which are stable and safe, plus some neutrons with could be easily absorbed.

There is one catch to this prospect of paradise. In order to ignite a hydrogen-2 fusion reaction, a temperature of the order of 100,000,000° C must be reached. This is far higher than the temperature of the solar interior, which is only 15,000,000° C, but then the sun has the advantage of keeping its hydrogen under enormous pressures, pressures unattainable on earth.

Any gas at such a temperature on earth would, if left to itself, simply expand to an excessively thin vapor and cool almost instantaneously. That this does not happen to the sun is due to the sun's mass, which produces a gravitational field capable of holding gases together even at the temperature reached in the solar interior.

Such gravitational fields cannot be produced on earth, of course, and the hot gas must be kept in place some other way. Material confinement would seem to be out of the question, for a hot gas making contact with a cool container would cool off at once--or heat the container itself to a thin gas. A gas connot be both hot enough for fusion and contained within a solid substance.

Fortunately, another method offers itself. As the temperature rises, all atoms are progressively stripped of their electrons and all that then exists are charges particles, negatively-charged electrons plus positively-charges nuclei. Substances made up of electrically charged atom-fragments, rather than intact atoms, are called plasma.

Investigators grew interested in plasma physics chiefly as a result of interst in controlled fusion, but, by hindsight, we now see that most of the univers is plasma. The stars are plasma, and here on earth phenomena such as ball lightning are isolated bits of plasma that have achieved temporary stability. Plasma even exists in man-made devices--for instance, within neon light tubes.

Plasma, consisting as it does of charged particles, can be confined by a nonmaterial container, a properly shaped magnetic field. The effort of physicists is now engaged in attempting to design magnetic fields that will keep plasma stably confined for periods long enough to initiate a fusion reaction--and to make the plasma hot enough for the fusion reaction to ignite. It is estimated that at the critical point, using gas, which at ordinary temperatures would be only 1/100 or less the density of the atmosphere, the pressures which would have to be withstood by the magnetic field at the point of fusino ignition would be something like 1500 pounds per square inch, or 100 atmospheres.

The requirements are stringent, and after a decade of research, success still lies frustratingly beyond the fingertips. Temperatures of about 20,000,000° C have been attained. Magnetic fields capable of containing the necessary pressures have been produced. Unfortunately, the combined temperature and pressure can be maintained only for millionths of a second, and it is estimated that at least a tenth of a second duration must be obtained in order for the first man-made controlled fusion reaction to be produced.

There is nothing (as far as we know) but time and effort standing in the way.

From Isaac Asimov's Understanding Physics

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