However effective a fusion bomb may be in liberating vast quantities of
energy, it is not what one has in mind when speaking 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.
[Illustration: _A supernova photographed on March 10, 1935._]
[Illustration: _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 laser 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 keep 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.
[Illustration: _Plasma in a magnetic field._]
[Illustration: _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”.
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]