Straight coils for twisty reactors give new shape to nuclear fusion
Scientists are pursuing the proposition of clean and inexhaustible power from nuclear fusion through a variety of experimental reactor designs, and one of the more interesting examples is a roughly circular device known as a stellarator. While these have conventionally featured irregularly shaped magnetic coils, scientists have now developed simpler and straighter versions they say can offer some important benefits.
Stellarators have some similarities to another popular fusion reactor design called the tokamak, in that both are designed to confine streams of superheated plasma in looping chambers through a series of magnetic coils. In doing so, they create the heat and pressure needed for atomic nuclei to collide at incredible speeds to form helium and release massive amounts of energy.
But where tokamaks take on a neat and relatively simple doughnut shape, stellarators send their streams of plasma around in irregular circles that twist and turn, which actually offers greater stability. These streams are guided by equally irregular magnetic coils that are incredibly complex, so much so that some require supercomputers to design.
Things could be made simpler if sections of these coils were relatively straight, which would not only make stellarators far simpler to construct but also to maintain. This would, however, come at a cost to the reactor's performance.
“In principle, you can always make straighter coils, but the trade-off is that their magnetic fields might not confine the plasma as well as those produced by twistier coils,” said lead author of the new research Nicola Lonigro, a Ph.D. candidate at the University of York in Britain. “But our research showed that you could make a simpler coil with straighter sections that makes the same magnetic field shape and strength as conventional ones do.”
Lonigro and his colleagues came to this realization by experimenting with a mathematical technique called "spline representation," which they used to improve the computer code that generates designs for magnetic coils. This led to the design of magnets with straighter sections but still a strong and accurate enough magnetic field to confine the plasma. As a demonstration, the scientists designed straighter and simpler new magnets for an experimental stellarator at the University of Wisconsin-Madison.
“In the future, people will have to replace components within stellarators as they wear out, which requires large openings between the coils of the magnets,” said physicist Caoxiang Zhu, author of the paper from University of Science and Technology of China. “But it’s hard to have large openings in stellarators because the electromagnetic coils zig and zag and are really complex.”
While the breakthrough is impressive it also serves as a reminder of the countless complex problems physicists face in trying to make nuclear fusion a reality. These straighter coils are just one tiny piece of an incredibly complicated puzzle, but one that may some day prove important to help make limitless clean energy a reality.
“In the long term, this work is a contribution to the larger effort trying to make stellarators commercially viable,” Lonigro said.
The research was published in the journal Nuclear Fusion.