Thermonuclear Fusion: beyond particle physics [1]

The process on which is base thermonuclear fusion is very simple, and concerns two light atoms, fusing in a heavier one. But how could this process take place?


If we consider two Hydrogen atoms, their nucleus consist of one proton. This means that they will be affected by a gaining repulsive electromagnetic force while getting closer. According to Coulomb law this force is:

If this was the only force holding together the nucleus, it couldn't even exist. Gravity on the other hand is attractive, but it is not strong enough. In fact if we compare it to electromagnetic force, we'll see that it is 10^39 times stronger.


The standard model of particle physics postulates the existence of two more forces:  weak and strong nuclear forces. Those kind of forces do not affect every particle, and their radius of effect is very little: 10^-15 for the strong one and 10^-18 for the weak one. This explains why those forces are not familiar to us, and don't have any noticeable effect on macroscopic world. 

The standard model proposed also that every force has a quantum mediator, represent by a certain particle. For instance electromagnetic force is mediated by photons, while gravity is supposed to be mediated by gravitons. We'll later focus on strong nuclear force, fundamental for thermonuclear fusion, and their mediators gluons, but before we need to explain what are those particles.


Particles are divided in several categories:

• Leptons: those particles are fundamental. It means that they aren't made up by other particles and aren't affected by strong nuclear force, but only by the weak one. Are divided in three other categories: electron, muon, tau. Everyone of them has a corresponding neutrino.
• Hadrons: those are massive particles, affected by all the 4 fundamental forces. Nowadays are known more than a hundred of those particles, even though proton is the only stable one. Are made up by quarks, and are divided in two further categories, Baryons an Mesons

If we take in consideration Pauli exclusion principle, according to which:

"Two fermions cannot occupy simultaneously the same quantum state"

We have to divide particles in other two categories:

• Fermions: particles with one half spin, obeying to Pauli principle;
• Bosons:  particles with integer spin, that do not obey to Pauli principle. Are also known as quantum mediator.

Strong force is based for instance on Bosons called Gluons, that obey according to the laws of quantum chromodynamics. Unlike the electrically neutral photon of quantum electrodynamics (QED), gluons themselves carry color charge and therefore participate in the strong interaction in addition to mediating it.

Barions are composed of 3 quarks with certain color charge. The various possible combinations determinate which particles gets formed. This color charge is not static, and changes continuously. This phenomena allows the creation of strong interactions between Hadrons, and therefore their coexistence inside the nucleus.


Strong nuclear force allows the creation of atomic nucleus. It is deeply related to binding energy, which is defined as:

"The energy required to break the bond between protons and neutrons inside the nucleus."

This energy is different for every nucleon, because depends on mass number. It increases with mass number for elements lighter than iron, and then decreases. The fusion of two lighter atoms will then lead to the formation of a heavier and more energetic atom, allowing the liberation of a certain amount of energy.

The mass of the resulting atom will be in fact lower than the sum of the two lighter ones. According to Einstein's special relativity this mass is transformed in energy.