The fusion of two atoms occurs when the nuclei are brought within a very short range of each other. Here the strong force attracts nuclei together into one low-energy nucleus.

Problematically, nuclei also repel each other due to their electrostatic repulsion.

To fuse then, particles must have enough kinetic energy to overcome the repulsive region (greater than 3 fm) and reach the region where the strong force is dominant. This energy 'barrier' is known as the 'Coulomb barrier', the energy needed for two nuclei to become close enough to fuse.

The energy needed to begin fusion is difficult to reproduce on Earth as it is extremely high. The only environments where this amount of energy is found is in the center of stars, their explosive supernova, and the big bang!

Graph of the potential energy between two protons varying with separation. The left side shows the strong force dominated region. The particles have their lowest energy and fusion can take place. The right side shows the electrostatic region where particles are repelled. The two are separated by a peak, the Coulomb barrier.

Figure 1. Graph of the potential energy between two protons varying with separation. The left side shows the strong force dominated region. The particles have their lowest energy and fusion can take place. The right side shows the electrostatic region where particles are repelled. The two are separated by a peak, the Coulomb barrier.