Fusion is the joining together of small atomic nuclei to form larger ones which releases energy. This is the same process that powers the sun – and what we are working to recreate in our compact spherical tokamak.

The fusion process involves forcing together positively-charged particles that ordinarily repel each other, so it can only happen at very high temperatures – more than 100 million degrees Celsius.

At these high temperatures, the electrons of atoms break away from their nuclei to create a soup of very fast moving charged particles – an electrically-charged gas called plasma.

Ultimately, the fuels of the fusion reaction will be two isotopes of hydrogen, deuterium and tritium, but for initial experiments on ST40 we will be using hydrogen and helium. The deuterium-tritium (DT) reaction is considered the best starting point for fusion power generation because it requires the lowest amount of energy to achieve and is the fastest.

Fusion is the ideal energy source because it is safe, the fuels are inexhaustible, and the reaction doesn’t produce any carbon dioxide or long-lived radioactive waste. Additionally, a fusion plant wouldn’t take up much space compared to renewables, which require a large surface area.

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Deuterium is a heavier-than-normal hydrogen atom that contains two nucleons in its nucleus – one proton and one neutron – rather than the usual one neutron.


Tritum is a type of hydrogen that is even heavier than deuterium. It contains one proton and two neutrons in its nucleus.


When deuterium and tritium come together during fusion, helium is produced…


…and a neutron is also produced in the fusion process. The neutron ends up with most of the energy of the reaction because it is much lighter than helium and so can fly out of the reaction faster.

Further information



As our library of videos shows, our research builds on decades of work on tokamaks and applies new technologies to make improvements.


An introductory leaflet produced for the Royal Society Summer Exhibition 2015

Fusion is the energy of the stars. It is the joining together of small atomic nuclei to make larger ones, and it is the mechanism by which all the heavier elements in the universe are made.