Nuclear fusion is the process by which two light atomic nuclei combine to form a heavier atomic nucleus. As an example, a proton (the nucleus of a hydrogen atom) and a neutron will, under the proper circumstances, combine to form a deuteron (the nucleus of an atom of deuterium, an isotope of hydrogen). In general, the mass of the heavier nucleus thus produced is less than the total mass of the two lighter nuclei. This missing mass corresponds to energy released in the reaction.

When a proton and neutron combine, for example, the mass of the resulting deuteron is 0.00239 atomic mass unit less than the total mass of the proton and neutron combined. This “loss” of mass is expressed in the form of 2.23 MeV (million electron volts) of kinetic energy of the deuteron and other particles and as other forms of energy produced during the reaction. Nuclear fusion reactions are like nuclear fission reactions, therefore, in the respect that some quantity of mass is transformed into energy.

Some typical fusion reactions

The particles most commonly involved in nuclear fusion reactions include the proton, neutron, deuteron, a triton (a proton combined with two neutrons), a helium–3 nucleus (two protons combined with a neutron), and a helium–4 nucleus (two protons combined with two neutrons). Except for the neutron, all of these particles carry at least one positive electrical charge. That means that fusion reactions always require very large amounts of energy in order to overcome the force of repulsion between two like–charged particles. For example, in order to fuse two protons with each other, enough energy must be provided to overcome the force of repulsion between the two positively charged particles.

Naturally occurring fusion

As early as the 1930s, a number of physicists had considered the possibility that nuclear fusion reactions might be the mechanism by which energy is generated in stars. Certainly no familiar type of chemical reaction, such as oxidation, could possibly explain the vast amounts of energy released by even the smallest star over billions of years. In 1939, the German–American physicist Hans Bethe worked out the mathematics of fusion energy release, in which a proton first fuses with a carbon atom to form a nitrogen atom. The reaction then continues through a series of five more steps, the

net result of which is that four protons disappear and are replaced by one helium atom.

Bethe chose this sequence of reactions because it requires less energy than does the direct fusion of four protons and, thus, is more likely to take place in a star. Bethe was able to show that the total amount of energy released by this sequence of reactions was comparable to that which is actually observed in stars.

The Bethe carbon–cycle is not the only nuclear fusion reaction possible. Another approach, for example, would be one in which two protons fuse to form a deuteron. That deuteron could, then, fuse with a third proton to form a helium–3 nucleus. Finally, the helium–3 nucleus could fuse with a fourth proton to form a helium–4 nucleus. The net result of this sequence of reactions would be the combining of four protons (hydrogen nuclei) to form a single helium–4 nucleus. The only net difference between this reaction and Bethe’s carbon cycle is the amount of energy involved in the overall set of reactions.

Thermonuclear reactions

The reason that fusion reactions can occur in stars is that when they first form by gravitational compression of drifting gas, the temperatures in their centers are raised high enough to bring about fusion. Thereafter, gravity keeps a star together and fusion keeps it hot. Since the temperatures needed for fusion are in the millions of degrees, fusion reactions are also known as thermonuclear (“thermo” for heat) reactions.

Artificial fusion reactions

The understanding that fusion reactions are responsible for energy production in stars brought the accompanying realization that such reactions might be a source of energy for human needs. The practical problems with building a fusion power plant are great, however: it is extremely difficult to keep a fusion reaction going at all, much less drawing more energy out of the reaction than one puts into it (the “break–even point”). Scientists are still at least half a century from achieving affordable fusion power—by optimistic estimates. A simpler challenge, unfortunately, is to construct a fusion device that does not need to be controlled, that is, a fusion bomb.

The hydrogen bomb

Scientists who worked on the first fission (atomic) bomb during World War II were aware of the possibility of building an even more powerful bomb that operated on fusion. Here is how a basic fusion bomb works:

The core of the fusion bomb consists of a fission bomb, such as the one they were then developing. That core is then be surrounded by a casing filled with isotopes of hydrogen. Isotopes of hydrogen are forms of hydrogen that all have a single proton in their nucleus, but may have zero, one, or two neutrons. The nuclei of the hydrogen isotopes are, respectively, the proton, the deuteron, and the triton.

In the first fraction of a second of the detonation of such a device, the fission bomb would explode, releasing huge amounts of energy. In fact, the temperature at the heart of the fission bomb would reach several million degrees.

That temperature would not last very long, but in the microseconds that it did exist, it would provide the energy for fusion to begin to occur within the casing surrounding the fission bomb. Protons, deuterons, and tritons would begin fusing with each other, releasing more energy and triggering further fusion. The original explosion of the fission bomb would have ignited a small, starlike reaction in the casing surrounding it.

From a military standpoint, the fusion bomb had one powerful advantage over the fission bomb: there is no technical upper limit on the size of a fusion bomb. One simply makes the casing surrounding the fission bomb larger and larger, until (in theory) there is no longer a way to lift the bomb into the air so that it can be dropped on an enemy.

On November 1, 1952, the United States the world’s first fusion bomb, annihilating Eniwetok Atoll in the Pacific and leaving an underwater crater over 6,000 feet across. This bomb was about 1,000 times more powerful than the fission bombs with which the U.S. had destroyed the Japanese cities of Hiroshima and Nagasaki in 1945. The Soviet Union exploded its first nuclear device in 1953. The two superpowers soon became proficient at manufacturing fusion bombs in an assembly–line manner, producing tens of thousands of the devices.

Peaceful applications of nuclear fusion

As research on fusion weapons was going on, attempts were also being made to develop peaceful uses for nuclear fusion. The most obvious challenge is simply to find a way to hold the nuclear fusion reaction in place as it occurs. One cannot build a machine made out of metal, plastic, glass, or any other common kind of material. At the temperatures at which fusion occurs, any one of these materials would vaporize instantly. Traditionally, two general approaches have been developed to solve this problem, namely, magnetic confinement and inertial confinement. To understand the first technique, imagine that a mixture of hydrogen isotopes has been heated to a very high temperature. At a sufficiently high temperature, the nature of the mixture begins to change. Atoms totally lose their electrons, and the mixture consists of a swirling mass of positively charged nuclei and electrons. Such a mixture is known as a plasma.

One way to control that plasma is with a magnetic field. One can design such a field so that a swirling hot mass of plasma within it can be held in any kind of a shape one chooses. The best–known example of this approach is a doughnut–shaped Russian machine known as a tokamak. In the tokamak, two powerful electromagnets create fields that are so powerful that they can hold a hot plasma in place as readily as a person can hold an orange in his hand.

The technique, then, is to heat the hydrogen isotopes to higher and higher temperatures while containing them within a confined space by means of the magnetic fields. At some critical temperatures, nuclear fusion will begin to occur. At that point, the tokamak is producing energy by means of fusion while the fuel is being held in suspension by the magnetic field.

Inertial confinement

A second method for creating controlled nuclear fusion makes use of a laser beam or a beam of electrons or atoms. In this approach, hydrogen isotopes are suspended at the middle of the machine in tiny hollow glass spheres known as microballoons. The microballoons are then bombarded by the laser, electron, or atomic beam and caused to implode. During implosion, enough energy is produced to initiate fusion among the hydrogen isotopes within the pellet. The plasma thus produced is then confined and controlled by means of the external beam.

The production of useful nuclear fusion energy by either of these methods depends on three factors: temperature, containment time, and energy release. That is, it is first necessary to raise the temperature of the fuel (the hydrogen isotopes) to a temperature of about 100 million degrees. Then, it is necessary to keep the fuel suspended at that temperature long enough for fusion to begin. Finally, some method must be found for tapping off the energy produced by fusion.

A measure of the success of a machine in producing useful fusion energy is known as the Lawson confinement parameter, the product of the density of particles in the plasma and the time the particles are confined. That is, in order for controlled fusion to occur, particles in the plasma must be brought close together and they must be kept together for some critical period of time. All of this must take place, of course, at a temperature at which fusion can occur.

D–D and D–T reactions

The two nuclear reactions now most commonly used for power research purposes are designated as DD and D–T reactions. The former stands for deuterium–deuterium and involves the combination of two deuterium nuclei to form a helium–3 nucleus and a free neutron. The second reaction stands for deuterium–tritium and involves the combination of a deuterium nucleus and a tritium nucleus to produce a helium–4 nucleus and a free neutron. The most common form of an inertial confinement machine, for example, uses a fuel that consists of equal parts of deuterium and tritium.

Hope for the future

Research on controlled fusion power has now been going on for a half century with disappointing results. Some experts believe that success is “just around the corner”—just a few billion dollars more!—but others argue that the problems of an economically feasible fusion power plant will never be solved.

In late twentieth century, scientists began to explore approaches to fusion power that departed from the more traditional magnetic and inertial confinement techniques. One such approach was called the PBFA process. In this machine, electric charge is allowed to accumulate in capacitors and then discharged in 40–nanosecond micropulses. Lithium ions are accelerated by means of these pulses and forced to collide with deuterium and tritium targets. Fusion among the lithium and hydrogen nuclei takes place, and energy is released. However, the PBFA approach to nuclear fusion has been no more successful than has that of more traditional methods.

KEY TERMS

Cold fusion —A form of fusion that some researchers believe can occur at or near room temperatures as the result of the combination of deuterons with palladium metal as a catalyst.

Deuteron —The nucleus of the deuterium atom, consisting of one proton combined with one neutron.

Isotopes —Two or more forms of the same element, whose atoms differ from each other in the number of neutrons contained in their nuclei.

Neutron —A subatomic particle with a mass of about one atomic mass unit and no electrical charge.

Nuclear fission —A nuclear reaction in which one large atomic nucleus breaks apart into at least two smaller particles.

Nucleus —The core of an atom consisting of one or more protons and, usually, one or more neutrons.

Plasma —A form of matter that consists of positively charged particles and electrons completely independent of each other.

Proton —A subatomic particle with a mass of about one atomic mass unit and a single positive charge.

Thermonuclear reaction —A nuclear reaction that takes place only at very high temperatures, usually of the order of a few million degrees.

Cold fusion

The scientific world was stunned in March, 1989 when two electrochemists, Stanley Pons and Martin Fleischmann, reported that they had obtained evidence for the occurrence of nuclear fusion at room temperatures. During the electrolysis of heavy water (deuterium oxide), it appeared that the fusion of deuterons was made possible by the presence of palladium electrodes used in the reaction. If such an observation could have been confirmed by other scientists, it would have been truly revolutionary. It would have meant that energy could be obtained from fusion reactions at moderate temperatures. The Pons–Fleischmann discovery was the subject of immediate and intense scrutiny by other scientists around the world.

It soon became apparent, however, that evidence for cold fusion could not consistently be obtained by other researchers. A number of alternative explanations were developed by scientists for the fusion results that Pons and Fleischmann believed they had obtained. Today, few scientists are still convinced that Pons and Fleischmann had made a breakthrough. In what has so far been a replay of the Pons–Fleischmann story, nuclear engineer Rusi Taleyarkhan of Purdue University announced the detection of desktop cold fusion in March, 2005. The fusion was supposedly triggered by the collapse of tiny bubbles in heavy water caused by ultrasonic waves. Taleyarkhan’s findings were published in a reputable peer–reviewed journal and, since the collapse of such bubbles is known to generate extraordinarily high temperatures, his work struck many scientists as more plausible than Pons and Fleischmann’s. However, as of late 2006 all attempts to duplicate Taleyarkhan’s results had failed, and most physicists believed that his apparent breakthrough was another case of honest measurement error.