Scientists are poised to flip the switch on an experiment that could take them a step closer to the goal of generating clean and cheap nuclear power.

Researchers at the Max Planck Institute in Greifswald, Germany, plan to inject hydrogen into a doughnut-shaped device to produce a super-hot gas known as plasma.

This is needed to achieve nuclear fusion, similar to how the sun's energy is produced.

Although harnessing fusion is likely still decades away, scientists hope it could someday replace the need for fossil fuels and conventional nuclear fission plants.

The Wendelstein 7-X stellarator was first fired up in December using helium, which is easier to heat.

Wednesday's test will show whether the 400-million-euro ($435-million) device can handle hydrogen, which would be the fuel in future fusion reactors.