Alex McColgan

Thank you to ZocDoc for sponsoring today's video.

With your medical problems hopefully now sorted, or at least on the way to being so, let's get back to the world's energy crisis.

As I was saying, fusion reactors take huge amounts of energy to get going.

Now you might think, surely once you have surpassed this feat, you've got more energy out than you put in.

That would be it.

A huge win.

After all, the fuel, hydrogen, is both cheap and abundant.

But alas, it is not that simple.

And to understand why, we have to look back once again at our star and the fusion reactions that only take place deep inside the stellar core.

It is here in the Sun's core that hydrogen is fused into helium via multi-step reaction.

Firstly, two hydrogen nuclei, or single protons, combine, with one undergoing a process called beta decay to transform it into a neutron.

The resulting neutron-proton pair is a nucleus of a heavy isotope of hydrogen known as deuterium.

In the next step, another proton combines with the deuterium nucleus to generate a helium-3 nucleus.

In the final step, two of these helium-3 nuclei fuse to produce a helium-4 nucleus containing two protons and two neutrons, also known as an alpha particle, as well as two protons.

Now, this may sound fairly straightforward, but it's not.

The reason being, that even in the sun, getting two protons to react to form deuterium, the first step of the reaction, is far from easy.

Firstly, in order to fuse, the protons must get extremely close to each other, around

10 to the power minus 15 meters apart.

That is so, the strong nuclear force, the force that holds protons and neutrons together in the nucleus of an atom, kicks in, and they are drawn together.

However, for two positively charged protons, getting this close means overcoming an immense amount of electrostatic propulsion.