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But since then, there's been a big question - is the device working the way it's supposed to? That's pretty crucial when you're talking about a machine that could potentially maintain controlled nuclear fusion reactions one day, and thankfully, the answer is yes.

A team of researchers from the US and Germany have now confirmed that the Wendelstein 7-X (W 7-X) stellerator is producing the super-strong, twisty, 3D magnetic fields that its design predicted, with "unprecedented accuracy". The researchers found an error rate less than one in 100,000.

"To our knowledge, this is an unprecedented accuracy, both in terms of the as-built engineering of a fusion device, as well as in the measurement of magnetic topology," the researchers write in Nature Communications.

That might not sound exciting, but it's crucial, because that magnetic field is the only thing that will trap hot balls of plasma long enough for nuclear fusion to occur.

Nuclear fusion is one of the most promising sources of clean energy out there - with little more than salt water, it offers limitless energy using the same reaction that powers our Sun.

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