To obtain a fusion reaction, we must bring two nuclei sufficiently close together for them to repel each other, as they are both charged positively. A certain amount of energy is therefore vital to cross this barrier and arrive in the zone, extremely close to the nucleus, where there are the nuclear forces capable of getting the better of electrostatic repulsion. The probability of crossing this barrier may be quantified by the " effective cross section". The variation against interaction energy expressed in keV of effective cross sections of several fusion reactions is shown on the graph opposite.
The most accessible fusion reaction is the reaction involving deuterium and tritium. It is on this reaction that research on controlled fusion is conducted.
The temperatures (which measure interaction energy) required for thermonuclear fusion are greater than a hundred million degrees! At such temperatures, electrons are completely detached from the nucleus; we say that the atom ionises and that we then enter the fourth state of matter, the state of plasma. Plasma exists in the universe in various forms and with very variable characteristics. Thus temperatures go from one to ten thousand electronvolts and densities range even more widely, going from a few particles per m³ in interstellar gas, up to 1030 particules per m³ in the centre of certain stars. Plasma is the most widespread form of matter in the universe.