The search for fusion energy used to be dominated by tokamaks. Just as ITER, the world's largest tokamak, nears completion in the hills of southern France, a much smaller testbed with a twistier geometry will be built in Germany.

fusion scientists might rethink their field if the stellarator can match or surpass similar-size tokamaks. The natural ability to keep the superhot gases stable is one of the main advantages of stellarators. Even more crucial for a fusion power plant is that tokamaks must stop periodically to reset their magnets.

The 1 billion German machine is already getting tokamak-like performance, thanks to its ability to prevent particles and heat from escaping. W7-X will be in the lead if it can achieve long runs. That's where stellarators excel. All of a sudden stellarators are back in the game. The encouraging prospects are inspiring a number of startup companies, including one that Gates is leaving to develop his own stellarators.

W7-X operates at relatively low power levels at the IPP in Greifswald, Germany. The creators of W7-X stripped it down and replaced it with water-cooled versions, opening the way to much longer, hotter runs. At a W7-X board meeting last week, the team reported that the new vessel has no leaks and is ready to leave. It is expected to restart later this month, on its way to showing whether or not it can get a fusion device to work.

inside Wendelstein 7-X
Wendelstein 7-X’s twisting inner surface is now water cooled, enabling longer runs.IPP/JAN HOSAN

The magnetic cages created by stellarators and tokamaks are so hot it would melt any metal container. High energy particle beams provide heating. The outrageous temperatures cause a roiling mix of separated nuclei and electrons to slam together and release energy. The fusion power plant would be powered by a mixture of hydrogen and tritium. W7-X is a research machine that doesn't try to generate energy by using radioactive tritium.

To make their magnetic fields, tokamaks and stellarators use coiling around the vessel. The field is closer to the hole than the outer edge.

The tokamaks make the plasma flow around the ring. The magnetic field generated by that streaming is similar to a candy cane. The twist is produced by the weirdly shaped magnetic coil. The tokamak scheme has proved to be more successful than before, but once the power of the computer made it possible to modify stellarator magnets to improve confinement, the scheme was no longer as successful.

The largest stellarator in the world, W7-X, contains 50 bizarrely twisted superconducting coils.

Researchers haven't been disappointed despite the wait W 7-X director Thomas Klinger says the machine worked immediately. The machine is very easy to use. It did what we said it would do. This is different from tokamaks, which are prone to "instabilities" such as bulging or wobbling in unpredictable ways. A whole branch of instabilities can be removed because stellarators don't rely onplasma current.

The geometry of the magnetic field caused some slower moving particles to follow a banana-shaped path until they collided with other particles and got knocked out of the plasma. Gates says that W7-X'soptimization worked as it was supposed to.

W7-X loses most of its heat through other forms of turbulence. Figuring out how to model turbulence takes a lot of computing power. W7-X will be testing ways to combat turbulence.

Wendelstein 7-X’s construction was long and complex (video produced in 2015).

The campaign should show a stellarator's ability to run continuously in comparison to a tokamak. W7-X is able to run for 100 seconds at low power. The device's microwave and particle heating systems were only able to deliver a small amount of power. The heating power will be increased. The real test of stellarators' ability to produce fusion power will be when they run W7-X at high temperature and high density. The goal is to get the ion temperature up to 50 million degrees Celsius. That would make W7-X one of the top machines in the world. The team will push it for more time. He says that they will step by step explore the unexplored territory.

Several startups are getting funding because of W7-X's achievements. The first thing the startup should do is find a simpler way to make magnets.

Gates and colleagues have secured $3 million to build a demonstration reactor that will not use the twisted magnet coil of W7-X. Instead, it will rely on a mosaic of about 1000 tiny square coils made of high- temperature superconductor. Operators can change the shape of the applied field by changing the magnetic field produced by each coil. Gates says that it puts complexity in the control system. The firm wants to develop a reactor that will use cheap, abundant deuterium to make radioisotopes. The firm will aim for a power- producing reactor if it succeeds.

Renaissance Fusion, based in France, has raised 16 million and plans to coat segments of the plasma vessel. Engineers will use a laser to etch a twisting pattern of magnets. They want to have a full prototype by the year 2027.

Type One Energy received funding from the U.S. Department of Energy to develop HTS cables. The cable would be wound into a coil with the help of a computer-controlled etching machine. David Anderson says advanced manufacturing technology opens the door for the stellarator.

The next phase of W7-X will accelerate the boom in stellarator efforts, according to Anderson. He says that with half-hour discharges you are essentially steady- state. This is a significant event.