A machine for creating controlled fusion reactions, where smaller atoms are combined to create larger atoms. In creating the elements up to iron, energy is liberated in this process. It is hoped that the energy generated by fusion could be used to generate power. Unfortunately fusion requires very high temperatures and pressures, which is why it requires advanced reactors. All fusion reactors at the moment use either deuterium and tritium or just deuterium as their fuel. Fusion reactors come in three main types:

Magnetic confinement reactors

Inertial confinement reactors

Cold fusion devices

Magnetic confinement reactors use strong magnetic fields to confine and compress the plasma as it is heated this category consists of tokamaks, stellerators and spherical tokamaks

Inertial confinement reactors, or laser fusion reactors, use a massively powerful array of lasers to heat the outside of a pellet of fusion fuel. The expansion of the outside surface of the pellet caused by the heating compresses the inside of the pellet, allowing it to reach the temperatures and pressures required for fusion.

Cold fusion devices are machines designed to create fusion reactions at room temperature. This type of fusion became infamous after the "discovery" of cold fusion in 1989 by two scientists became a worldwide fiasco, as it became apparent that their discovery was largely fiction. Investigation had effectively ceased into cold fusion, although recent developments in sonoluminescence have re-ignited interest in this avenue of research

None of these types of reactor has managed to create a fusion reactor with a positive energy output, ie. creates more energy than it needs to run. ITER, a planned next-generation fusion reactor, is hoped to be the first to have a positive energy output.