April 13, 2024

Laboratory builds its first stellarator in 50 years and opens doors to research into new plasma physics

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A photo of MUSE, the first starship built at PPPL in 50 years and the first to use permanent magnets. Credit: Michael Livingston / PPPL Communications Department

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A photo of MUSE, the first starship built at PPPL in 50 years and the first to use permanent magnets. Credit: Michael Livingston / PPPL Communications Department

For the first time, scientists have built a fusion experiment using permanent magnets, a technique that could show a simple way to build future devices at a lower cost and allow researchers to test new concepts for future fusion power plants.

Researchers at the U.S. Department of Energy’s (DOE) Princeton Plasma Physics Laboratory (PPPL) have combined decades of experience in engineering, computing, and theoretical physics to design a new type of stellarator, a winding machine that confines plasma, the fourth electrically charged state of matter. , to harness the fusion process that powers the sun and stars and potentially generate clean electricity.

“Using permanent magnets is a completely new way to design stellars,” said Tony Qian, a graduate student in the Princeton Program in Plasma Physics, based at PPPL. Qian was the main author of articles published in Journal of Plasma Physics It is Nuclear fusion which detail the theory and engineering behind the device, known as MUSE. “This technique allows us to quickly test new plasma confinement ideas and build new devices with ease.”

Stellarators typically rely on complicated electromagnets that have complex shapes and create their magnetic fields through the flow of electricity. These electromagnets must be built with precision and with very little margin for error, increasing their cost.

However, permanent magnets, like the magnets that attach art to refrigerator doors, don’t need electrical currents to create their fields. They can also be ordered from industrial suppliers and then embedded in a 3D-printed housing around the device’s vacuum canister, which contains the plasma.

“MUSE is largely built from commercially available parts,” said Michael Zarnstorff, senior research physicist at PPPL and principal investigator on the project. “By working with 3D printing companies and magnet suppliers, we can research and buy the precision we need, rather than manufacturing it ourselves.”

The original realization that permanent magnets could be the basis for a new variety of more affordable stellar magnets came to Zarnstorff in 2014. “I realized that even if they were situated alongside other magnets, rare earth permanent magnets could generate and maintain the Magnetic fields need to confine the plasma so that fusion reactions can occur,” Zarnstorff said, “and that’s the property that makes this technique work.”


Left: Some of the permanent magnets that make the innovative MUSE concept possible. Right: A close-up of MUSE’s 3D-printed shell. Credit: Xu Chu/PPPL and Michael Livingston/PPPL Communications Department

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Left: Some of the permanent magnets that make the innovative MUSE concept possible. Right: A close-up of MUSE’s 3D-printed shell. Credit: Xu Chu/PPPL and Michael Livingston/PPPL Communications Department

Solving a long-standing engineering problem

Invented more than 70 years ago by PPPL founder Lyman Spitzer, stellarators are just one concept for fusion facilities. Another is donut- or apple-shaped tokamak, like PPPL’s ​​National Spherical Torus Experiment-Upgrade, which confines plasma using relatively simple magnets. For decades, this has been the preferred design by scientists around the world because of the way the devices confine plasma.

However, tokamaks also rely on magnetic fields created by electrical currents passing through the plasma, which creates instabilities that interfere with fusion reactions. However, Stellarators can operate without such currents and can therefore function for indefinite periods of time. But their complicated magnets, which are difficult to design and build, for years meant that stellarators were not economical or practical options for fusion power plants.

That’s why MUSE’s success in demonstrating that stellarators can operate using simple magnets is so important. “Typical star magnets are very difficult to machine because you have to do it very precisely,” said Amelia Chambliss, a graduate student in the Department of Applied Physics and Applied Mathematics at Columbia University who helped design MUSE during a internship at the DOE Graduate Science Laboratory at PPPL a few years ago. “So the idea that we can use lots of discrete magnets to do work is very interesting. It’s a much easier engineering problem.”

Noticing a theoretical property

In addition to being an engineering breakthrough, MUSE also exhibits a theoretical property known as quasi-symmetry to a greater degree than any other stellar to date. It is also the first completed device anywhere in the world that has been specifically designed to have a type of quasi-symmetry known as quasi-symmetry.

Conceived by PPPL physicist Allen Boozer in the early 1980s, quasi-symmetry means that although the shape of the magnetic field inside the stellarator may not be the same around the physical shape of the stellarator, the strength of the magnetic field is uniform around it. of the device, leading to good confinement of the plasma and a greater probability of fusion reactions occurring. “In fact, MUSE’s quasi-symmetry optimization is at least 100 times better than any existing stellarator,” said Zarnstorff.

“The fact that we were able to design and build this Stellarator is a true achievement,” said Qian.

In the future, the PPPL team plans to carry out a series of experiments to determine the exact nature of MUSE’s quasi-symmetry and thus discover the extent to which the device prevents hot particles from moving from the plasma core to the edge, hindering fusion reactions. . Methods will include more precise mapping of magnetic fields and measuring how the spinning plasma slows down, which depends on the quasi-symmetry of the device.

MUSE demonstrates the kind of innovation possible in a US national laboratory. “For me, the most important thing about MUSE is that it represents a creative way to solve a difficult problem,” said Chambliss. “It uses many innovative and open-minded approaches to solve stellar long-standing problems. As long as the community continues to think in this flexible way, we will be in good shape.”

More information:
TM Qian et al, Design and construction of the MUSE permanent magnet stellarator, Journal of Plasma Physics (2023). DOI: 10.1017/S0022377823000880

T. Qian et al, Simplest optimized Stellarators using permanent magnets, Nuclear fusion (2022). DOI: 10.1088/1741-4326/ac6c99

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