Fusion is the process by which two or more atomic nuclei are fused together to form a heavier single nucleus. The energy is not conserved in this process and some of the mass of the atoms is turned into energy and then released. This process is what drives all stars and there is potential on Earth to harness this energy for the means of power. Some nuclear weapons can potentially be completed using fusion, but more often it is the process of fission that is used to make atomic bombs. However, it can be used to make the more powerful hydrogen bomb and it is also hydrogen which is thought to be the best material to develop sustainable fusion power with.

Process

Energy released in the fusion of light elements, such as hydrogen, is caused by two opposing forces, nuclear force and the Coulomb force. Nuclear force binds protons and neutrons together to make an atomic nucleus. In this process the mass of the combined particles is less than that of the two alone. due to the energy release during combination. The Coulomb force, however, causes protons to repel each other. Protons are positively charged thus repelling each other, however they stick together which indicates that nuclear attraction is at work, overcoming electric repulsion at close ranges.

The process is very dependent on distance. Light elements have nuclei which are small and closely packed, while heavier elements, such as iron, have much larger nuclei which prevents energy from being released because nuclear force cannot work at such distances. When dealing with heavier elements with larger distances, the process is actually reversed and energy is absorbed rather than released.

Fusion reactions in stars produce most elements by this process, called nucleosynthesis. For example, when hydrogen, which is present in all stars, undergoes this process, combining two nuclei to create a single helium nucleus, energy is released. This is experienced as things, such as kinetic energy or electromagnetic radiation. However, once a star begins producing iron, fusion reverses, and only through supernova nucleosynthesis can elements heavier than iron be produced.

• Fusion Power Glossary
• Fusion for Energy has a detailed discussion of how fusion power works.
• The Princeton Plasma Physics Laboratory has lots of educational material on the subject.

Applications

The process described above requires massive amounts of energy to fuse even two hydrogen nuclei and doing so has proved extremely difficult on earth. At this point there have been no successful attempts to create ‘break even’, or self-sustaining fusion reactions. This is because of the amount of energy required to force two nuclei together, thereby overcoming their electrostatic force, which keeps them apart, is contingent on creating conditions with thermonuclear heat.

Laser technology is seen as a potential way to create the proper conditions for a break even process. The first tests of this method were conducted at the National Ignition Facility, which is part of the Lawrence Livermore National Laboratory in California. Targeting experiments for the lasers were carried out in 2008 and 2009 and in early 2011 the first full experiments began.

To date the largest experiments conducted have been in England at the Joint European Torus (JET) labs. In 2010 JET produced 16 megawatts with fusion power. It was sustained for just over 0.5 seconds. Several months later its successor, ITER, was announced and construction began in France.

• The US Department of Energy has a fusion energy research department.
• Fusion Science and Technology is a leading journal on the subject
• There is a large database of information about potential fusion energy projects here.

Weapons

Besides these theoretical possibilities the only man-made nuclear fusion process has been carried out via hydrogen bomb. Using the Teller-Ulam design, a bomb can be ignited by fissile material, just like an atomic bomb. Most of the destructive power is still the result of this fission rather than fusion. However, the design is stipulated on the fact that different elements of a thermonuclear reaction can be chained together, resulting in stages. The energy in each subsequent stage of the detonation is used for the next stage. After the first two stages of fission enough heat will be created setting off a process of fusion.

The largest weapon ever detonated by man was a thermonuclear device using the Teller-Ulam design. However, there have been few recorded detonations of such massive weapons. They were deemed to be too large and inefficient to be delivered during the cold war, and the testing of such devices was banned in

• A technical description of thermonuclear weapons can be found here.
• Global Security has a long description of fusion in weapons.
• A bibliography of fusion in nuclear weapons can be found here.