The hydrogen was heated in the doughnut-shaped Wendelstein 7-X machine (illustrated). Called a stellarator, the device uses a complicated system of magnetic coils to trap plasma long enough for fusion to take place

The Wendelstein 7-X machine in Germany, which cost €1billion to build, creates conditions similar to those inside stars (illustrated). It's part of a worldwide effort to harness nuclear fusion, a process in which atoms join at extremely high temperatures and release large amounts of energy

The nuclear fusion research centre at the Max Planck Institute for Plasma Physics is pictured. The device was first fired up in December using helium, which is easier to heat