Fusion requires extremely high temperatures.

Under normal conditions, atomic nuclei do not fuse spontaneously. Nuclei are positively charged and therefore repel each other through the electrostatic force long before they come close enough for the strong nuclear force to bind them together. The electrostatic force is long-range, while the strong nuclear force acts only over very short distances of about 1 femtometre (10⁻¹⁵ m). When two positively charged nuclei are more than a few femtometres apart, the strong nuclear force is negligible and the nuclei repel each other through the electrostatic force.

Basically, fusion requires nuclei to approach closely enough for the strong nuclear force to act. High particle energies greatly increase the probability of this happening, while quantum tunnelling allows nuclei to penetrate the Coulomb barrier even when their kinetic energy is lower than the barrier itself. High pressure or other forms of confinement can further increase the probability of collisions.

High Temperature

At the particle level, temperature is related to the kinetic energy of particles. The higher the temperature, the faster the particles move on average and the greater the probability of fusion collisions. The required temperature depends strongly on the nuclei involved. Proton-proton fusion takes place in the core of the Sun at a temperature of about 15 million kelvin, aided by quantum tunnelling. Because proton-proton fusion has an extremely low reaction rate, terrestrial fusion research mainly focuses on the deuterium-tritium reaction, which requires plasma temperatures of about 150 million kelvin. Other fusion reactions generally require even higher temperatures. For example, proton-boron-11 fusion requires temperatures of the order of a billion kelvin or more.

Compression of Matter

There are many ways to increase the density of fusion fuel. Stars use gravity to compress their enormous masses until conditions for fusion are reached in their cores. Shock waves can rapidly compress fusion fuel to very high densities, as used in inertial confinement fusion. Magnetic fields can also confine plasma, while in some devices strong electric currents generate magnetic forces that compress the plasma column. Even highly improbable approaches have been examined. One of the “cold fusion” ideas proposed that deuterium absorbed in palladium during electrolysis could come close enough to undergo fusion. However, experimentally reproducible evidence for such fusion has never been established.

Heating and compression are not entirely independent. Compressing matter generally increases its temperature, while high temperature alone is not sufficient if the particle density is too low or the particles are not confined for long enough. Successful fusion therefore requires a suitable combination of temperature, density and confinement time.