At the end of 2015, Germany switched on a new type of massive nuclear fusion reactor for the first time, and it was successfully able to contain a scorching hot blob of helium plasma.

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.

Unlike nuclear fission, which is achieved by our current nuclear plants, and involves splitting the nucleus of an atom into smaller neutrons and nuclei, nuclear fusion generates huge amounts of energy when atoms are fused together at incredibly high temperatures. And it produces no radioactive waste or other byproducts.

Based on the longevity of our Sun, nuclear fusion also has the potential to supply humanity with energy for as long as we need it - if we can figure out how to harness the reaction, that is.

And that's a pretty big 'if', because scientists have been working on the problem for more than 60 years, and we're still a fair way off our goal.

The main challenge is that, in order to achieve controlled nuclear fusion, we have to actually recreate conditions inside the Sun. That means building a machine that's capable of producing and controlling a 100-million-degree-Celsius (180 million degree Fahrenheit) ball of plasma gas.

As you can imagine, that's easier said than done. But there are several nuclear fusion reactor designs in operation around the world right now that are trying their best, and the W 7-X is one of the most promising attemps.

Instead of trying to control plasma with just a 2D magnetic field, which is the approach used by the more common tokamak reactors, the stellerator works by generating twisted, 3D magnetic fields. 

This allows stellerators to control plasma without the need for any electrical current - which tokamaks rely on - and as a result, it makes stellerators more stable, because they can keep going even if the internal current is interrupted.

Well, that was the idea of its design, at least.

Despite the fact that the machine successfully controlled helium plasma in December 2015, and then the more challenging hydrogen plasma in February 2016, no one had shown that the magnetic field was actually working as it should be.

To measure it, a team of researchers from the US Department of Energy and the Max Planck Institute of Plasma Physics in Germany sent an electron beam along the magnetic field lines in the reactor.

Using a fluorescent rod, they swept through those lines and created light in the shape of the fields. The result, which you can see in the image above, shows the exact type of twisted magnetic fields that it was supposed to make.

"We've confirmed that the magnetic cage that we've built works as designed," said one of the lead researchers, Sam Lazerson from the US Department of Energy's Princeton Plasma Physics Laboratory.

Despite this success, W 7-X isn't actually intended to generate electricity from nuclear fusion - it's simply a proof of concept to show that it could work.

In 2019, the reactor will begin to use deuterium instead of hydrogen to produce actual fusion reactions inside the machine, but it won't be capable of generating more energy than it current requires to run. 

That's something that the next-generation of stellerators will hopefully overcome. "The task has just started," explain the researchers in a press release.

It's not something that will happen tomorrow, but it's an incredibly exciting time for nuclear fusion, with W 7-X officially competing with France's ITER tokamak reactor - both of which have been able to trap plasma for long enough for fusion to occur.

The real question now is, which of these machines will be the first to bring us efficient power from nuclear fusion? We can't wait to find out.

The research has been published in Nature Communications.