View of Princeton University's Tokamak Fusion Test Reactor (TFTR). The TFTR is being used to research nuclear fusion, a process in which atoms of deuterium ('heavy hydrogen') are combined, giving off an immense amount of energy. Such processes are thought to power stars, and many think that nuclear fusion will be the power source of the future. Fusion requires incredible temperatures, around 100 million Celsius, where the reaction gas is in a plasma state. The aim of TFTR is to achieve fusion break-even, where the power input is balanced by the fusion energy output.