One of the greatest puzzles to be solved by physicists arose from the
work of geologists. When it became clear that coal and other fossil
remains of living things date from many hundreds of millions of years
ago, it was obvious that the earth’s sun had been shining at a quite
steady rate for an extremely long time.
How does it manage to do it? What is its source of energy? Chemical
energy supplied by combustion and gravitational potential energy
supplied by contraction are thousands of times too small to have kept
the sun going for such a long time.
The principle illustrated by Figure 4 suggests the most probable source
of energy for the sun and all the other stars as well. It is known that
the sun consists chiefly of hydrogen and that it has a temperature of
about 40,000,000 degrees Fahrenheit near its center. Several kinds of
nuclear reactions produced in atom smashers have demonstrated that
hydrogen nuclei, if energized by being heated to a very high
temperature, can actually combine, or fuse, to form helium nuclei.
The accompanying loss of weight per particle indicated by Figure 4 must
result in the appearance of sufficient energy to balance Einstein’s
famous equation. In fact, calculations by the German-born American
physicist Hans A. Bethe and others show that, based on reasonable
estimates of the conditions within the sun, familiar nuclear reactions
account for its energy. The calculations predict, furthermore, that the
sun can continue to operate at its present level for many billions of
years.
[Illustration: _Large loop prominences on the sun, caused by a
locally intense magnetic field. Project Sherwood, the U. S. program
in controlled fusion, is devoted to research on fusion reactions
similar to those from which the sun derives its energy._
Courtesy Sacramento Peak Observatory, AFCRL]
Since fusion of light nuclei is produced by extremely high temperatures,
fusion events are called _thermonuclear reactions_. The possibility of
bringing about thermonuclear reactions on earth to serve as a source of
energy has naturally attracted much attention.
In spite of the fact that fusion of ordinary hydrogen atoms (each of
which has one proton as its nucleus) supports the activity of the sun,
this particular reaction seems to occur much too slowly to be usable on
earth. Other isotopes of hydrogen, called deuterium and tritium,
however, which contain one and two neutrons in their nuclei,
respectively, fuse much more rapidly and seem to be potential earthly
sources of controlled thermonuclear energy.
[Illustration: _An early phase of a nuclear detonation at Eniwetok
Atoll during the 1951 tests._
Courtesy Joint Task Force Three]
The first large-scale application of thermonuclear energy was the
so-called hydrogen bomb, or “H-bomb.” For a brief time an exploding
fission bomb develops a temperature of hundreds of millions of degrees
Fahrenheit, hot enough to cause some light nuclei to fuse. In the
hydrogen bomb, light nuclei of deuterium and/or tritium are exposed to
this temperature during such a fission explosion. The resulting fusion
of these nuclei causes the explosion to be hundreds of times more
powerful than that of the fission device alone. In 1952 the Atomic
Energy Commission test-fired such a thermonuclear device at Eniwetok
Atoll in the Pacific Ocean. The energy released by the highly efficient
device produced an explosion that completely destroyed the coral islet
where it was detonated.
At such extreme temperatures all atoms are stripped of electrons; the
resulting mixture of nuclei and free electrons is called a _plasma_.
Several laboratories are now working on the problems connected with
creating and containing plasma. Ordinary solid containers cannot be
used. On contact with plasma they would instantly vaporize and would
cool the plasma below the temperature necessary for fusion to occur.
Fortunately, however, the particles that make up a plasma, being charged
electrically, respond to forces in a magnetic field. A strong magnetic
field of proper shape exerts a large confining pressure on a body of
plasma in a high-vacuum chamber. Thus plasma can be contained in a small
volume well removed from the walls of the chamber by surrounding the
chamber with suitably designed large magnets or solenoids to create a
“magnetic bottle.” In addition, a sudden increase in the intensity of
the field can compress the plasma; this compression raises the
temperature of the plasma to near that required for fusion.
[Illustration: _This plasma is being pushed outward by an internal
magnetic field as instabilities grow on its internal surface. The
photo was taken by means of fast-shutter photography permitting
photo sequences at intervals of 3 to 5 millionths of a second._
Courtesy General Atomic Division, General Dynamics Corporation]
Fusion of light nuclei would be a much “cleaner” source of energy for
peaceful purposes than fission of heavy ones, because the “ashes” of
fission reactions are radioactive while those of fusion (helium atoms)
are not. Great technical difficulties must be overcome, however, before
a controlled thermonuclear reaction is possible. Fusionable material
must be heated to a temperature of over 100 million degrees Fahrenheit
and must be contained long enough for an appreciable amount of fusion to
occur.
The greatest problem encountered to date is the extreme instability of
the plasma and the corresponding difficulty of maintaining it at the
proper temperature longer than a few millionths of a second. Many
physicists now think that the successful exploitation of thermonuclear
energy will not occur for many years. When and if it is achieved,
however, the deuterium present in the oceans of the earth will represent
an almost inexhaustible source of energy.