Fusion reactions can take place between two ions if the resultant nucleus is energetically stable. This is true for the light elements Hydrogen and Helium.

The Coulomb force means that two ions will normally repel one another. By contrast, the strong nuclear force attracts protons and neutrons to one another and binds the nucleus together. It exceeds the Coulomb force very close to the nucleus (in the femtometer range). Thus an energy barrier known as the Coulomb barrier must be surmounted for fusion to take place. Quantum mechanical tunnelling means ions have some probability of fusing even at energies less than that required to overcome Coulomb repulsion.

Deuterium 1D2and tritium 1T3 are isotopes of hydrogen. Their nuclei contain repectively two and three neutrons plus a proton. The hydrogen nucleus 1H1 contains just a single proton. The next element in the periodic table, helium 2H4, contains two protons and two neutrons.

When deuterium and tritium fuse they produce helium and a neutron (0n1). The helium ion is also known as an alpha particle. The mass of deuterium plus tritium is greater than the mass of helium and a neutron. This mass deficit Δm is converted into the kinetic energy E of the reaction products by Einstein's famous equation-

E=Δmc2=2.8 X 10-12Joules= 17.6 MeV

Thus, mass is being converted into kinetic energy. This is the basis of fusion power. Some fusion reactions are shown below with the kinetic energy released.

1D2+1T3 -> 2He4+0n1+17.6 MeV
1D2+1D2 -> 1T3+1H1+4.03 MeV
1D2+1D2 -> 2He3+0n1+3.27 MeV
1D2+2He3 -> 2He4+1H1+18.3 MeV
The energy required to overcome the Coulomb barrier means that the cross-section for these reactions is insignificant at low energies (cold fusion). The fusion of D and T has the lowest energy at which the maximum cross-section occurs (about 100 keV) and produce the most highly energised neutrons. As such, it is the reaction that will probably take place in a future fusion reactor.