Twisty device explores an alternative path to fusion | Science


Is the search for fusion energy, long dominated by doughnut-shaped devices called tokamaks, about to change shape? As ITER, the world’s largest – and tens of billions of dollars’ most expensive – tokamak draws to a close in the hills of southern France, a much smaller test bed with a more sinuous geometry will begin to reach full power in Germany.

If the 16-meter-wide device, called a stellarator, can match or surpass similarly sized tokamaks, it could cause fusion scientists to rethink the future of their field. Stellarators have several key advantages, including a natural ability to keep the super-hot boiling gases they contain stable enough to fuse nuclei and release energy. Even more crucial for a future fusion power plant, they can theoretically just run and go, whereas tokamaks must periodically shut down to reset their magnetic coils.

Within seconds, Germany’s billion-euro machine, dubbed Wendelstein 7-X (W7-X), is already achieving “tokamak-like performance”, says plasma physicist David Gates, who has proven himself adept at prevent particles and heat from escaping. superheated gas. If W7-X can pull off long draws, “it will definitely be in the lead,” he says. “That’s where Stellarators shine.” Theorist Josefine Proll from the Eindhoven University of Technology is equally enthusiastic: “All of a sudden the stellarators are back in the game.” The encouraging outlook is inspiring a group of startups, including one for which Gates is now leaving the Princeton Plasma Physics Laboratory, to develop their own Stellarators.

W7-X has been operating since 2015 at the Max Planck Institute for Plasma Physics (IPP) in Greifswald, Germany, but only at relatively low power levels and for short periods of time. Over the past 3 years, the creators of W7-X have stripped it down and replaced all interior walls and fixtures with water-cooled versions, paving the way for much longer and hotter runs. At a W7-X board meeting last week, the team reported that the redesigned plasma tank had no leaks and was ready to go. It is expected to restart later this month, with a view to showing whether it can really bring plasma to conditions that, in a future device, would trigger fusion.

The twisted inner surface of the Wendelstein 7-X is now water-cooled, allowing for longer strokes.IPP/JAN HOSAN

Stellarators and tokamaks create magnetic cages for gas at over 100 million degrees Celsius, so hot it would melt any metal container. Heating is provided by microwaves or high-energy particle beams. The extravagant temperatures produce a plasma – a bubbling mixture of nuclei and separate electrons – and slam the nuclei together with such force that they fuse together, releasing energy. A fusion power plant would be fueled by a mixture of the most readily reacting isotopes of hydrogen, deuterium and tritium. Research machines like W7-X that don’t try to generate power avoid radioactive tritium and stick to safer, more abundant hydrogen or deuterium.

To create their plasma-confining magnetic fields, tokamaks and stellarators use electromagnetic coils looped around the ship and through the central hole. But such a field is stronger near the hole than the outer edge, causing the plasma to drift towards the reactor wall.

Tokamaks tame drift by circulating plasma around the ring. This flow generates another magnetic field, twisting the ionized gas like a candy cane and stabilizing it. Stellarators use oddly shaped magnetic coils instead of scattering plasma to produce the twist. The tokamak scheme has long proven the most effective at holding plasma in place, but once plasma physicists had sufficiently powerful supercomputers, they could modify the complex geometries of the stellarator magnets to improve confinement, a process called optimization.

W7-X is the first optimized large stellarator and contains 50 oddly twisted superconducting coils, each weighing 6 tons. Its construction, which began in the mid-1990s, was tortuous, completed 10 years late and cost almost double the 550 million euros initially budgeted.

Despite the expectation, the researchers were not disappointed. “The machine worked immediately,” says Thomas Klinger, director of W7-X. “It’s a very easy-going machine. [It] just did what we told him to do. This contrasts with tokamaks, which are prone to “instabilities” – the plasma swells or oscillates in unpredictable ways – or more violent “disturbances”, often related to interrupted plasma flow. Because stellarators don’t rely on plasma current, it “suppresses a whole branch” of instabilities, says IPP theorist Sophia Henneberg.

In early stellarators, the geometry of the magnetic field caused some slower-moving particles to follow banana-shaped orbits until they collided with other particles and were blown out of the plasma, releasing energy. W7-X’s ability to remove this effect means that its “optimization worked as expected,” says Gates.

Once that Achilles’ heel is removed, the W7-X primarily loses heat through other forms of turbulence – small eddies that push particles towards the wall. Simulation of turbulence requires considerable computing power, and theorists have only recently mastered it. The next W7-X campaign should validate the simulations and test the means of combating turbulence.

The construction of the Wendelstein 7-X was long and complex (video made in 2015).

The campaign should also showcase a stellarator’s ability to operate continuously, unlike the pulsing operation of a tokamak. W7-X has run for periods of 100 seconds before – long by tokamak standards – but at relatively low power. Not only were its components uncooled, but the device’s microwave and particle heating systems could only provide 11.5 megawatts of power. The upgrade will increase the heating power by 60%. Running W7-X at high temperature, high plasma density, and for long periods of time will be the true test of the stellarators’ potential to produce fusion power. An initial goal, Klinger says, is to get the temperature of the ions up to 50 million degrees Celsius for 100 seconds. That would put W7-X “among the best machines in the world,” he says. Then the team will push it longer, up to 30 minutes. “We will go step by step, exploring uncharted territory,” he says.

W7-X’s achievements have prompted venture capitalists to back several startups developing commercial power-generating stellarators. First priority for startups: find an easier way to make the magnets.

Princeton Stellarators, founded this year by Gates and his colleagues, got $3 million and aims to build a demonstration reactor that will forego the twisted magnetic coils of W7-X. Instead, it will rely on a mosaic of around 1,000 tiny square coils made of high-temperature superconductor (HTS) on the outer surface of the plasma vessel. By varying the magnetic field produced by each coil, operators will be able to modify the shape of the applied field at will. “It takes the complexity out of the coils and puts them in the control system,” says Gates. The company initially hopes to develop a reactor that will fuse only cheap and abundant deuterium, to generate not electricity, but neutrons for making radioisotopes. If successful, the company will then aim for a power-generating reactor.

Renaissance Fusion, based in Grenoble, France, has raised 16 million euros and plans to coat segments of the plasma vessel in a multi-layer HTS, forming a uniform coating. Then, using a laser, engineers will burn tracks into the superconductor to etch a twisting pattern of magnetic coils. They aim to complete a meter-long test segment over the next 2 years and a full prototype by 2027.

A third company, Type One Energy in Madison, Wisconsin, received funding from the US Department of Energy to develop HTS cables with enough bend to be used in stellarator magnets. The company carved pieces of metal with computer-controlled engraving machines, carving twist channels into which the cable is wound to turn it into a coil. “Advanced manufacturing technology opens the door to the Stellarator,” says co-founder David Anderson of the University of Wisconsin-Madison.

Anderson says the next phase of W7-X’s operation will accelerate the boom in stellarator efforts. “With half-hour shots, you’re basically in a state of equilibrium,” he says. “This is a big deal.”


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