Nuclear fusion is the excess energy resulting from the combination of two light nuclei that unite to form a heavier nucleus. These two light nuclei participate in a process that releases energy, which is inhibited by the Coulomb barrier between two particles of the same charge, which prevents two nuclei of equal charge from getting close enough for the strong interaction to predominate.
Large-scale fusion of atoms only occurs if the temperature is high enough for the nuclei to tunnel through the barrier. This can happen by increasing the temperature of the material until the nuclei have enough energy, a fact caused by thermal agitation.
Thermonuclear Fusion of the Sun
The fusion reaction that takes place in the Sun is a multi-step process in which the sun’s main source of energy is the thermonuclear burning of hydrogen to form helium gas in the proton-proton cycle . Elements up to A = 56 (peak of the binding energy curve) can be produced by other fusion processes after a star’s supply of hydrogen is exhausted.
The abundance of elements heavier than hydrogen and helium on Earth suggests that our solar system condensed from an interstellar cloud that contained the rest of these explosions.
Thermonuclear fusion is controlled, it could become an important source of energy in the future. The dd and dt reactions are the most promising. A fusion reactor based on the dt reaction must satisfy Lawson’s criterion, in addition to keeping the plasma at a sufficient temperature.
In tokamaks the plasma is confined by magnetic fields, in laser fusion inertial confinement is used.