What is nuclear fusion and what have scientists achieved?

Physicists are a-buzz over new results from nuclear fusion experiments. But what have they found, and how big a breakthrough is it?

To do this ourselves takes some serious engineering. At the National Ignition Facility (NIF) at Lawrence Livermore National Laboratory in California, a weak laser beam is split and the energy amplified to give 192 laser beams. These are used to heat the walls of a small gold can, called a hohlraum, to more than 3m degrees Celsius, resulting in the emission of X-rays.

The upshot is that, in the hottest part of the fuel, fusion occurs, resulting in the formation of helium nuclei. As one helium nucleus has slightly less mass than the combination of one deuterium and one tritium nucleus, the difference in mass is released as a burst of energy. In the right conditions the helium nuclei produced in this process can transfer their kinetic energy to the remaining fuel, heating it up and triggering yet more fusion.

Should this happen, it becomes possible to release more energy than was put into the experiment by the lasers, a condition known as ignition.

Researchers at the NIF have announced that, for the first time, they have managed to do just that. The team used 2.05MJ of energy to heat the fuel with lasers, releasing 3.15MJ of energy.