Posted on: 27th August 2012

One of the most puzzling things about fusion is that two positively charged particles, deuterium and tritium, could possibly stick together. Surely they should repel each other and fly apart?

This question first arose with the discovery of the structure of the atom in the early twentieth century. Scientists postulated that there had to be another, stronger force holding nuclei together. It was not for fifty years that this imaginatively named “Strong Force” would be fully understood – by which point it was realised that nucleons (the generic term for protons and neutrons) were each made up of three smaller particles, quarks. This theory, named quantum chromodynamics, proposed that the charged quarks inside protons and neutrons are held together by “colour” – a similar mechanism to electrical charge except instead of the two charges of electricity (positive and negative), it is based around attraction between three colours, making it considerably more complex. In an analogy with quantum electrodynamics, which explained electrical force as the transmission of photons, the strong force is transmitted by particles called gluons, which – unlike photons – cannot exist outside subatomic particles. This same theory predicted the existence of the Higgs Boson, whose existence has been confirmed only this year at the Large Hadron Collider.

One question remains: if gluons, which transmit the strong force, cannot exist outside nucleons, then it would seem impossible for the strong force to bind a proton to a neutron, unless the particles actually overlap. However, although the attraction between nucleons is strong enough to exceed the electrical repulsion at distances less than 1.7 femtometers, the strong force actually becomes repulsive at distances less than 0.7 of a femtometer – inside a nucleus the protons and neutrons do not touch each other.

It is now understood that the attraction between nucleons is not the strong force proper, but a residual effect of the strong force, similar to the momentary electrical forces which bind non-polar molecules together (known as London forces, a type of Van Der Waals force.) This residual force is called the nuclear force and is described in quantum chromodynamics not as the exchange of gluons, but as the exchange of pairs of quarks (known as mesons) – this process is shown in the Feynman diagram above. Although the energies involved seem huge – fusion gives about a million times more energy than the chemical processes associated with burning coal for example – they are a mere fraction of the true strong force, which holds the quarks together inside protons or neutrons.

The experiments at CERN explore the structure within nucleons and the strong force which holds them together, which is why the particles inside the Large Hadron Collider need to be accelerated to such high energy – at 7 tera-electronvolts, nearly a billion times more energetic than the hot plasma particles inside JET!

- Thanks to Dr Bruce Yabsley of the Centre of Excellence for Particle Physics at the Terascale, University of Sydney, for technical advice.