What is fusion?

Nuclear fusion involves two light atomic nuclei combining to form a heavier nucleus. It is the process that powers the sun and other stars. Fusion is possible because the heavier nucleus has slightly lower mass than the sum of the masses of the initial two nuclei. This results in the release of energy in the form of kinetic energy of the products that can be calculated from Einstein’s famous equation, E=mc².

Before a fusion reaction can occur, it is necessary for the two nuclei to be sufficiently close for the strong nuclear force (which is very short range) to make them coalesce. At these short distances the electrostatic repulsion of the two positively charged nuclei is very high. Therefore, it is necessary to heat the fusion fuel to very high temperature (millions of degrees kelvin, MK) to provide the necessary kinetic energy for the nuclei to overcome their electrostatic repulsion, known as the Coulomb barrier, so that the strong nuclear force becomes dominant and they fuse together.

However, only a few of the possible fusion reactions can form the basis of a power production system on earth because the rate at which the reaction occurs needs to be high enough at the densities and temperatures that can be achieved. For example, the fusion of protons, which is the energy source that powers the stars, occurs very slowly even at the very high pressures in the cores of stars – for example it takes ~10 billion years before a proton in the centre of the sun fuses. Therefore, fusing protons in a plasma cannot be the basis for a terrestrial power production system.

The reactions that have the most potential are D-T, D-D, D-³He and p-¹¹B.  Although all these reactions have similar rates at very high temperatures, at temperatures of a few 100MK, which appears to be the highest feasible in near-term fusion systems on earth, the D-T reaction rate is ~100 times higher than the other reactions. Therefore, the D-T reaction has been the focus of most fusion concepts although the other fuels have been or are being considered in some concepts.