Coulomb Barrier

The electrostatic repulsion two nuclei must overcome to fuse.

Nuclei are positively charged and repel each other; the Coulomb barrier is the energy hill they must climb (or quantum-tunnel through) to get close enough for the strong force to bind them. Higher temperature gives nuclei the speed to cross it, which is why fusion needs tens to hundreds of millions of degrees.

Nuclei mostly cross the Coulomb barrier by quantum tunnelling rather than going over it, which is why fusion proceeds at temperatures far below the classical barrier height and why the reaction rate climbs so steeply with temperature. Choosing a fuel is largely choosing how high that barrier is — D–T lowest, D–³He higher, proton–boron highest.