What is fusion energy?

To understand what fusion is, and what it has to do with plasma physics, we need to take a small detour into some nuclear physics.

 A brief summary of nuclear physics

The nucleus is the dense core of protons and neutrons, which we collectively call nucleons, that sits at the heart of every atom. Each chemical element has a different number of protons; for example, hydrogen has one and helium has two. The nucleus is also home to a similar number of neutrons.

There are three types of hydrogen:

• Normal hydrogen (or protium) has only a single proton and no neutrons in its nucleus
• Deuterium adds a neutron, but still only has one proton
• Tritium again has one proton (as all forms of hydrogen must), but two neutrons. We call these elements with different numbers of neutrons isotopes.

If we add up the total number of protons and neutrons in a nucleus we get the so-called mass number of the nucleus, a measure of the total number of nucleons hiding within.

The ‘strong face’

What is holding this nucleus together? The answer is called the ‘strong force’. This fundamental force of nature binds together nucleons, but only when they are exceptionally close to one another (within one-millionth of a millionth of a metre). At larger distances, it has no effect at all.

But there is something strange happening with this strong force, which shows itself when we look at the mass of nuclei: the actual mass of a nucleus is slightly less than we get if we just add up the masses of the protons and neutrons within! To understand why we need to return to the most famous equation in physics – (E=mc^2).

This tells us that mass ((m)) is just another form of energy ((E)). (c^2) is just a number (the speed of light squared) equal to 9×10¹⁶ m²/s², so a small mass contains a huge amount of energy.

Missing energy

So we can consider our missing mass as missing energy. In fact, it is just the energy we would need to provide to split our nucleus back up into separate protons and neutrons.

In other words, it is a measure of how well stuck together the nucleus is. We can therefore call our missing mass the binding energy of the nucleus.

Some calculations

Let’s do some calculations using the nuclear binding energies in the table below. Energies are given in mega electron volts – a unit of energy used in nuclear physics, and equal to about 0.000000000000016 of a joule.

Let’s assume that we take one deuterium and one tritium nucleus and rip them apart to make 2 protons and 3 neutrons. How much energy do we need to provide? We can just add up the binding energies of deuterium and tritium (2.22 MeV + 8.48 MeV) to get 10.7 MeV.

Now, let’s rearrange these nucleons to make a helium-4 nucleus, which has 2 protons and 2 neutrons, leaving us one spare neutron.

We can ignore the spare neutron as it isn’t bound to anything, but when we form the Helium-4 nucleus we release the binding energy of Helium-4, which is 28.3 MeV. So we get out more energy than we have put in, 28.3-10.7 = 17.6 MeV to be precise.

Unlocked energy

This is the concept of nuclear fusion. When we combine light elements together to form heavier ones, we can free up some of the energy locked in the nuclei, seemingly creating energy from nowhere. Of course, we have really just unlocked energy that was stored in the deuterium and tritium.

You might want to use the table above to calculate how much energy is gained when we fuse together two deuterium nuclei to make one tritium nucleus and one protium nucleus.