Nuclear fusion occurs when two nuclei slam together to form a heavier nucleus, like two hydrogen atoms fusing to form one helium atom. This is the same process that powers the sun and the stars and creates huge amounts of energy.
Since the 1950s, scientists have tried to design devices that can harness nuclear fusion on Earth. Using these devices, they have studied the behavior of matter at very high temperature and produced small amounts of controlled fusion energy. However, no device has ever produced more fusion power than the amount of power needed to run it.