A team of researchers from Germany has completed the construction of a new experimental fusion reactor, which they hope will surpass all the other models in having the longest sustained reaction, according to Science.

Nuclear fusion reactors aim to replicate the processes that go on inside stars to get electricity, fusing lighter atoms (like hydrogen or helium) together to form heavier elements. Nuclear fusion is still experimental, and it is unlikely we will have commercial power stations before 2050, but it is being pursued (including projects such as ITER) because it produces a lot of energy using a tiny amount of fuel.

It requires having the atoms in an extremely hot plasma (over 100 million kelvins), though, something that can't be done on a large scale at the moment. Scientists at the Max Planck Institute for Plasma Physics in Germany, however, think they may have a solution.

To achieve the scalding plasma, scientists typically use lasers to heat up gas and the plasma is trapped by a magnetic field. Keeping the plasma hot and confined is the main technological challenge when it comes to reactors.

The most common model of nuclear reactors is called a tokamak, a device that confines the plasma in a donut shape. To maintain the plasma in this shape, strong magnets surround the reactor, and an electrical current is induced in the plasma. This current severely limits the tokamak, as it can make the plasma break free of the magnetic confinement and seriously damage the reactor. The current can only be provided in short pulses, limiting the fusion time to only a few minutes.

The other model is called a stellarator. Stellarators were invented by American physicist Lyman Spitzer in 1950, but they fell from favor in the 1970s as the tokamaks were giving much better results. They force the plasma in a weird twisted donut, with no need for an induced electric current inside but requiring a lot more magnets and a more complicated design to keep the plasma confined.

The new machine from the Max Planck Institute, called Wendelstein 7-X (W7X), is a stellarator. It’s the largest in the world at 16 meters (52 feet) in diameter and the team behind it hopes to assess the suitability of its design for commercial fusion reactors.

^{Shown is the interior of the reactor in 2011. IPP/Wolfgang Filser.}

The €1 billion ($1.1 billion/£715 million) German reactor took 19 years to complete; it has 425 metric tons (470 U.S. tons) of superconducting magnets, which need to be cooled down to almost absolute zero, and between those magnets there are 250 ports to supply and remove fuel, to position diagnostic instruments and to be able to heat the plasma inside using microwaves. The construction of such a complex machine could only be done using advanced 3D software. “It can only be done on computer,” Thomas Klinger, leader of the project, told Science. “You can't adapt anything on site.”

W7X should be able to operate with up to 30 minutes of plasma discharge. If the machine ends up doing so, it will become a serious contender to the technology employed in fusion reactors in the future. The current record holder is the French tokamak “Tore Supra” with six minutes and 30 seconds.

The reactor should be turned on by the end of November, and it is currently awaiting approval for the go-ahead from the German nuclear authority. 

You can check out a time-lapse video of WX7's construction and an animation of how it works in the video below: