Volume 3: Controlled Fusion

Worlds Within Worlds: The Story of Nuclear Energy, Volume 3 (of 3), by Isaac Asimov is part of HackerNoon’s Book Blog Post series. You can jump to any chapter in this book here. Volume III, NUCLEAR FUSION: Controlled Fusion

Controlled Fusion

However effective a fusion bomb may be in liberating vast quantities of energy, it is not what one has in mind when 151speaking of a fusion power station. The energy of a fusion bomb is released all at once and its only function is that of utter destruction. What is wanted is the production of fusion energy at a low and steady rate—a rate that is under the control of human operators.

The sun, for instance, is a vast fusion furnace 866,000 miles across, but it is a controlled one—even though that control is exerted by the impersonal laws of nature. It releases energy at a very steady and very slow rate. (The rate is not slow in human terms, of course, but stars sometimes do release their energy in a much more cataclysmic fashion. The result is a “supernova” in which for a short time a single star will increase its radiation to as much as a trillion times its normal level.)

The sun (or any star) going at its normal rate is controlled and steady in its output because of the advantage of huge mass. An enormous mass, composed mainly of hydrogen, compresses itself, through its equally enormous gravitational field, into huge densities and temperatures at its center, thus igniting the fusion reaction—while the same gravitational field keeps the sun together against its tendency to expand.

There is, as far as scientists know, no conceivable way of concentrating a high gravitational field in the absence of the required mass, and the creation of controlled fusion on earth must therefore be done without the aid of gravity. Without a huge gravitational force we cannot simultaneously bring about sun-center densities and sun-center temperatures; one or the other must go.

On the whole, it would take much less energy to aim at the temperatures than at the densities and would be much more feasible. For this reason, physicists have been attempting, all through the nuclear age, to heat thin wisps of hydrogen to enormous temperature. Since the gas is thin, the nuclei are farther apart and collide with each other far fewer times per second. To achieve fusion ignition, therefore, temperatures must be considerably higher than those at the center of the sun. In 1944 Fermi calculated that it might take a temperature of 50,000,000° to ignite a hydrogen-3 fusion with hydrogen-2 under earthly conditions, and 400,000,000° to ignite hydrogen-2 fusion alone. To ignite hydrogen-1 fusion, which is what goes on in the sun (at a mere 15,000,000°), physicists would have to raise their sights to beyond the billion-degree mark.

A supernova photographed on March 10, 1935.

The same star on May 6.

This would make it seem almost essential to use hydrogen-3 in one fashion or another. Even if it can’t be prepared in quantity to begin with, it might be formed by neutron bombardment of lithium, with the neutrons being formed by the fusion reaction. In this way, you would start with lithium and hydrogen-2 plus a little hydrogen-3. The hydrogen-3 is formed as fast as it is used up. Although in the end hydrogen is converted to helium in a controlled fusion reaction as in the sun, the individual steps in the reaction under human control are quite different from those in the sun.

Still, even the temperatures required for hydrogen-3 represent an enormous problem, particularly since the temperature must not only be reached, but must be held for a period of time. (You can pass a piece of paper rapidly through a candle flame without lighting it. It must be held in the flame for a short period to give it a chance to heat and ignite.)

The English physicist John David Lawson (1923- ) worked out the requirements in 1957. The time depended on the density of the gas. The denser the gas, the shorter the period over which the temperature had to be maintained. If the gas is about one hundred-thousand times as dense as air, the proper temperature must be held, under the most favorable conditions, for about one thousandth of a second.

There are a number of different ways in which a quantity of hydrogen can be heated to very high temperatures—through electric currents, through magnetic fields, through 154laser beams and so on. As the temperature goes up into the tens of thousands of degrees, the hydrogen atoms (or any atoms) are broken up into free electrons and bare nuclei. Such a mixture of charged particles is called a “plasma”. Ever since physicists have begun to try to work with very hot gases, with fusion energy in mind, they have had to study the properties of such “plasma”, and a whole new science of “plasma physics” has come into existence.

But if you do heat a gas to very high temperatures, it will tend to expand and thin out to uselessness. How can such a super-hot gas be confined in a fixed volume without an enormous gravitational field to hold it together?

An obvious answer would be to place it in a container, but no ordinary container of matter will serve to hold the hot gas. You may think this is because the temperature of the gas will simply melt or vaporize whatever matter encloses it. This is not so. Although the gas is at a very high temperature, it is so thin that it has very little total heat. It does not have enough heat to melt the solid walls of a container. What happens instead is that the hot plasma cools down the moment it touches the solid walls and the entire attempt to heat it is ruined.

What’s more, if you try to invest the enormous energies required to keep the plasma hot despite the cooling effect of the container walls, then the walls will gradually heat and melt. Nor must one wait for the walls to melt and the plasma to escape before finding the attempt at fusion ruined. Even as the walls heat up they liberate some of their own atoms into the plasma and introduce impurities that will prevent the fusion reaction.

Any material container is therefore out of the question.

Fortunately, there is a nonmaterial way of confining plasma. Since plasma consists of a mixture of electrically charged particles, it can experience electromagnetic interactions. Instead of keeping the plasma in a material container, you can surround it by a magnetic field that is designed to 155keep it in place. Such a magnetic field is not affected by any heat, however great, and cannot be a source of material impurity.

In 1934, the American physicist Willard Harrison Bennett (1903- ) had worked out a theory dealing with the behavior of magnetic fields enclosing plasma. It came to be called the “pinch effect” because the magnetic field pinched the gas together and held it in place.

The first attempt to make use of the pinch effect for confining plasma, with eventual ignition of fusion in mind, was in 1951 by the English physicist Alan Alfred Ware (1924- ). Other physicists followed, not only in Great Britain, but in the United States and the Soviet Union as well.

The first use of the pinch effect was to confine the plasma in a cylinder. This, however, could not be made to work. The situation was too unstable. The plasma was held momentarily, then writhed and broke up.

Plasma in a magnetic field.

Enormous machines and complex equipment, such as the Scyllac machine shown above, are required for nuclear fusion research.

Attempts were made to remove the instability. The field was so designed as to be stronger at the ends of the cylinder than elsewhere. The particles in the plasma would stream toward one end or another and would then bounce back producing a so-called “magnetic mirror”.

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In 1951 the American physicist Lyman Spitzer, Jr. (1914- ) had worked out the theoretical benefits to be derived from a container twisted into a figure-eight shape. Eventually, such devices were built and called “stellarators” from the Latin word for “star”, because it was hoped that it would produce the conditions that would allow the sort of fusion reactions that went on in stars.

All through the 1950s and 1960s, physicists have been slowly inching toward their goal, reaching higher and higher temperatures and holding them for longer and longer periods in denser and denser gases.

In 1969 the Soviet Union used a device called “Tokamak-3” (a Russian abbreviation for their phrase for “electric-magnetic”) to keep a supply of hydrogen-2, a millionth as dense as air, in place while heating it to tens of millions of degrees for a hundredth of a second.

A little denser, a little hotter, a little longer—and controlled fusion might become possible.[5]

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Isaac Asimov. 2015. Worlds Within Worlds: The Story of Nuclear Energy, Volume 3 (of 3). Urbana, Illinois: Project Gutenberg. Retrieved May 2022 from https://www.gutenberg.org/files/49821/49821-h/49821-h.htm#c38

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