Permanent magnets could help to stabilize and simplify stellarator fusion reactor design.
Stellarators look like huge metal donuts fully greebled with hardware sprinkles.
Permanent magnets would replace expensive to operate and complicated electromagnets.
A researcher in New Jersey says he had a nuclear fusion breakthrough while helping his son with a science project. He realized he could explore which kinds of permanent magnets could be powerful and stable enough to be part of a fusion reactor concept called a stellarator.
Michael Zarnstorff, from the Max Planck-Princeton Research Center for Plasma Physics, worked with three other researchers on the permanent magnet design.
“It is shown that the magnetic-field coils of a stellarator can, at least in principle, be substantially simplified by the use of permanent magnets,” the scientists’ new paper begins. A permanent magnet is one whose magnetic charge comes from the chemical makeup of the material itself, like the interplay between individual electrons. This is different from something like an electromagnet—whether that’s a copper wire electromagnet or the magnetic field generated by the Earth. If behaviors or assemblies generate magnetic force, that’s a temporary magnet.
These magnets can’t generate the required plasma flux inside a stellarator, but researchers say they can shape the course of the plasma and help to rein it in.
Like a tokamak, a stellarator—such as the one at the University of Wisconsin-Madison shown above—is a donut-shaped (toroidal) plasma stream that generates power by fusing light particles into heavier ones. These generators must be brought up to temperatures like those of the sun and other, well, naturally occurring fusion plasma generators.
But the stability of these generators is really up to chance, based on a carefully managed magnetic field. Extremely hot moving plasma ends up corroding the materials containing it, and these small changes can drop the temperature and throw the fusion reactor out of the power generating zone. This is where permanent magnets could change the landscape of plasma reactor design.
Permanent magnets can’t replace the entire magnetic field by themselves, but they can supplement and add structure, the researchers say. Stellarators and tokamaks are both torus shaped, but stellarators require twisting the plasma so it swirls through the donut in a spiraling form.
“Such magnets cannot produce toroidal magnetic flux, but they can create poloidal flux and rotational transform of the magnetic field, and thus help to simplify stellarator design,” the researchers say. In other words, the permanent magnets can be used to make more effective twists, which has been a snarl in the design process for stellarator prototypes.
“In the original design, the magnetic field was created by 20 nonplanar, modular coils of 5 different types,” the researchers say. “Leaving permanent magnets to do most of the plasma shaping, a new optimization was now carried out where only 8 identical, planar, circular toroidal-field coils proved necessary.”
Permanent magnets are fixed solids that don’t require special equipment, and using them to replace complex electromagnetic coils isn’t just simpler to build or design around—it’s simpler to maintain and keep at high temperatures.
Scientists have wondered for thousands of years if magnets could be used to make perpetual motion machines, and if nuclear fusion ever comes to the wider commercial marketplace, they’ll really seem to generate extraordinary, long-term power from very little.