Fusion involves placing hydrogen atoms under high heat and pressure until they fuse into helium atoms. In stellarators, plasma is contained by external magnetic coils which create twisted field lines around the inside of the vacuum chamber (illustrated)

'The speed with which W7-X became operational is a testament to the care and quality of the fabrication of the device and makes a very positive statement about the stellarator concept itself.

'W7-X is a truly remarkable achievement and the worldwide fusion community looks forward to many exciting results.'

While critics have said the pursuit of nuclear fusion is an expensive waste of money that could be better spent on other projects, Germany has forged ahead in funding the Greifswald project.

Chancellor Angela Merkel, who holds a doctorate in physics, attended today's event, which took place in her constituency.

The massive microwave ovens that will turn hydrogen into plasma, eventually reaching 100 million°C. This has been designed to mimic the conditions seen inside the sun (stock image)