This powerful electromagnet could help make fusion energy a reality

The elusive goal of fusion energy may be a lot closer to reality thanks to a new type of superconducting magnet. Or so claim researchers with Commonwealth Fusion Systems (CFS), who unveiled the 2 meter long, 1 meter wide, D-shaped electromagnet at an online press conference today and said it had produced a magnetic field roughly 500,000 times Earth’s natural field, twice as strong as any similar superconducting magnet. Researchers at the Massachusetts-based company say their magnet technology should enable them to build a relatively small prototype fusion power plant by 2025—although they acknowledge they must still overcome multiple other technological challenges.

Spun out of the Massachusetts Institute of Technology (MIT) in 2018, CFS has wagered its existence on developing magnets made of exotic high-temperature superconducting materials that can produce fields twice as strong as conventional superconducting magnets. So the successful test of the magnet, achieved on 5 September, marks a triumph for the company. “We went from 3 years ago, when we didn’t even know whether such a magnet could exist, to having it today,” says Bob Mumgaard, a plasma physicist and CFS’s co-founder and CEO.

A fusion reactor, or tokamak, aims to capture the energy released when nuclei of deuterium and tritium, the heavy isotopes of hydrogen, fuse to produce helium and energetic neutrons. To do that, a tokamak relies on intense magnetic fields to trap and squeeze a super-hot ionized gas, or plasma, within a doughnut-shaped vacuum chamber. However, researchers have yet to build a tokamak that yields more energy than it consumes, and they have long believed such a reactor needs to be large to reach that breakeven point. For example, the international ITER tokamak, which is under construction in France and aims surpass breakeven, has a vacuum chamber 11 meters tall and 19 meters wide.

With high-field magnets, however, CFS researchers say tokamaks can be dramatically smaller—and therefore cheaper and easier to build. CFS researchers set out to make the required magnets by winding coils composed of high-temperature superconductors called rare-Earth barium copper oxides, rather than ordinary superconducting metals like niobium tin. When cooled to near absolute zero, a superconductor carries electrical current without resistance, as long as the current and the magnetic field do not grow too large. High-temperature superconductors—so-named because they superconduct at comparatively balmy temperatures, some above liquid nitrogen’s temperature of above 77 Kelvin— can withstand higher magnetic fields than conventional superconductors.

The challenge was mainly to fashion a magnet that can bear the enormous mechanical stresses generated as the magnetic field itself pushes back on the current-carrying coils, says Brian LaBombard, a plasma physicist and engineer at MIT who worked on the magnet. “You can think of it almost like pressurizing a balloon,” he says. Ordinary superconductors can be fashioned into rugged wire that can be wound into a coil, but high-temperature superconductor comes in a relatively fragile tape. So to develop its magnet, CFS researchers came up with a design in which thin layers of tape are sandwiched between stronger layers of metal. “You need to basically have as much metal as you can,” LaBombard says. “And the design we have here is pushing that to the limit.”

In the recent test, the new magnet produced a field of 20 Tesla for about 5 hours, although CFS researchers say they could have sustained the field indefinitely. With the magnet in hand, the company say it’s ready to shoot for its next goal: developing a prototype reactor called SPARC that—just like ITER—will aim to show that a tokamak can generate more energy than it consumes. In SPARC, researcher will use 18 coils like the 20 Tesla prototype to surround a toroidal vacuum chamber. “This magnet allowed us to develop the manufacturing processes and equipment and the supply chain at a scale that is relevant for commercial fusion,” says Joy Dunn, a manufacturing engineer with CFS.

A magnet alone does not a tokamak make, however. Last year, a report from the National Academies of Science, Engineering and Medicine found that to make a prototype fusion power plant a reality by 2040, the field must still overcome numerous other technological challenges. Among those needs are materials that can face up to heat and the neutron bombardment from the plasma, and better schemes for venting the hot helium exhaust from the vacuum chamber. Mumgaard agrees that those problems must still be solved. But he argues they would all become significantly easier to address in a high-field, compact tokamak.

More generally, the new magnet may signal a sea-change for how all fusion developers envision future reactors, no matter the peculiarities of their designs, says Dennis Whyte, a plasma physicist and engineer at MIT: “This is, in my view, the tide that lifts all boats.”

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