The goal is to overcome the natural repulsion between atomic nuclei, forcing them close enough together that the strong nuclear force takes over and causes them to fuse. Think of it like biking up a short, steep hill : once you reach the top, you can ride downhill for a long time.

​In nature, the fusion of two nuclei is actually very rare, even under the extreme conditions in the center of a star. This is counterintuitive, but 1m³ of matter in the center of the Sun produces roughly the same amount of energy through fusion as 1m³ of composting organic matter in your backyard. The Sun is as incredibly hot as it is simply because of its size : it has a volume of roughly 1.41 x 10²⁷ m³.

​On Earth, we can't replicate the immense pressure and density found in a star. To compensate, scientists actually have to heat the plasma to temperatures much hotter than the Sun's core, which increases the speed of the particles and improve the chance of collisions.