In the world of fusion energy, bigger is simply better. Decades of experience with tokamaks shows that if you want more power from your plasma (hot, charged fuel) than you put in, you need to have a bigger machine. Which is why the international fusion project ITER in France, the most advanced ever designed, is twice the size of its European predecessor JET in the UK.
Where JET was never quite able to break-even and produce more fusion power than the heat needed to keep the reaction going in its plasma, ITER's 500 megawatts of fusion power will be ten times more than the heat sent into the artificial star's plasma. A world record power multiplication, although as an experimental facility ITER will not convert the heat it produces into electricity. Factoring in all power requirements on site, ITER would about break even if it did so.
Commonwealth Fusion Systems' SPARC, in contrast, will be four times smaller than ITER's 12 meter diameter; at 3 meters across, it will be just half the size of JET while still aiming to beat it by generating twice the fusion power than the heat going into the plasma. You might assume that the fusion community thinks that SPARC – like ITER, not intended to produce electricity – is a pipe dream, but researchers are actually cautiously optimistic. The key to the magic performance claim: a more concentrated plasma thanks to stronger, more advanced superconducting magnets than were available for either competitor.
Before we dig into the SPARC design, how do tokamaks work in general?
"A tokamak is a donut-shaped magnetic cage in which one can confine a very hot hydrogen plasma (a gas of 100 million degrees Celsius or more) so that it does not touch the wall of the container. At these temperatures, hydrogen nuclei can fuse to form helium and energy is released, just like in the heart of the sun."
"The magnetic cage is provided by strong electromagnets, which need to be superconducting so that the current flowing in them does not experience any electric resistance. Even the small resistance of copper magnets would consume too much energy for fusion reactors to be efficient."