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Fusion energy research rests directly on our scientific knowledge of nuclear reactions—nuclear physics. Unlike electromagnetism, special relativity and quantum mechanism—theories that are fairly complete and describe phenomena in terms of a few basic equations—nuclear physics is still developing. Scientists do not have anywhere near the level of understanding of nuclear forces—the forces that hold the nucleus together, as they do of electromagnetic forces. Instead, nuclear physics is what is called a “phenomenological” field. It rests on many thousands of experiments that determine important quantities observationally, rather than deriving them from a simple theory. For example, we know how fast deuterium, the fuel we use for test, or hydrogen-boron, our ultimate fuel, will react or burn at a given temperature because experiments in accelerators have measured how fast these reactions take place. This is still very firm science, since we know that all protons—hydrogen nuclei—are exactly the same, as all are boron-11 nuclei. So we can still be confident of our predictions.
Since 1930, when the neutron was discovered, scientists have known that all nuclei consist of neutrons—electrically neutral particles—and protons—positively charged particles. The nuclei make up 99.9% of the mass of all atoms—the rest being in the electrons that orbit around the nucleus, attracted to it by its electric charge. Even though the protons in the nucleus repel each other with the electric forces, they are bound to each other and to the neutrons by the much stronger nuclear force. But the nuclear force only operates at very short ranges—of the order of the diameter of nuclei—around 10-12 cm. That is only one ten-thousandth the diameter of an atom, which is defined by the electrons’ orbits.
Fusion reactions occur because some nuclei are bound together more tightly than others. Helium, for example is bound very tightly. If a hydrogen and a boron nucleus pass within about 10-12 cm of each other, they will be drawn by their nuclear forces tighter to form a carbon nucleus. But that nucleus will have too much energy—will be vibrating too violently to hold together, so three helium nuclei will form from it. Since the three helium nuclei are bound together more tightly than the boron nucleus was, their formation releases energy, just like the coming together of two magnets releases energy. Since the nuclear forces are extremely strong, the energy released is huge.
Since the nuclei always repel each other electrically at long distances, very great velocity is needed to push them close enough for the nuclear attractive force to take hold. That’s why fusion reactions take place only at high temperatures—in other words, when the nuclei have high velocities.
For more on nuclear physics, go here.