Researchers at the US Department of Energy’s (DOE) Princeton Plasma Physics Laboratory (PPPL) have found a way to build powerful magnets that are smaller than before, facilitating the design and construction of machines that could help the world harness the sun’s power to create electricity without produce greenhouse gases that contribute to climate change.
The researchers found a way to build high-temperature superconducting magnets that are made of materials that conduct electricity with little or no resistance at temperatures hotter than before. Such powerful magnets would fit more easily into the tight space inside spherical tokamaks, which are shaped more like a cored apple than the donut-like shape of conventional tokamaks, and are being explored as a possible design for future fusion power plants.
Because the magnets could be placed separately from other machines in the spherical tokamak’s central cavity to hold the hot plasma to fuel fusion reactions, researchers could repair them without having to take anything else apart. “To do this, you need a magnet with a stronger magnetic field and a smaller size than current magnets,” said Yuhu Zhai, chief engineer at PPPL and lead author of a paper reporting the findings in IEEE Transactions on Applied Superconductivity. “The only way you do that is with superconducting wires, and that’s what we’ve done.”
Fusion, the force that powers the sun and stars, combines light elements in the form of plasma – the hot, charged state of matter made up of free electrons and atomic nuclei – which generates enormous amounts of energy. Scientists are trying to replicate fusion on Earth for a virtually inexhaustible supply of safe and clean power to generate electricity.
High temperature superconducting magnets have several advantages over copper magnets. They can be turned on for longer periods than copper magnets can because they don’t heat up as quickly, making them better suited for use in future fusion power plants that will need to run for months at a time. Superconducting wires are also powerful, able to transmit the same amount of electrical current as a copper wire many times wider while producing a stronger magnetic field.
The magnets could also help researchers continue to shrink the size of tokamaks, improve performance and reduce construction costs. “Tokamaks are sensitive to the conditions in their central regions, including the size of the central magnet, or solenoid, the shielding and the vacuum vessel,” said Jon Menard, PPPL’s associate director of research. “A lot depends on the center. So if you can shrink things in the center, you can shrink the whole machine and reduce costs while, in theory, improving performance.”
These new magnets take advantage of a technique refined by Zhai and researchers at Advanced Conductor Technologies, the University of Colorado, Boulder, and the National High Magnetic Field Laboratory in Tallahassee, Florida. The technology means that the wires do not need conventional epoxy and fiberglass insulation to ensure current flow. While simplifying construction, the technology also lowers costs. “The cost of winding the coils is much lower because we don’t have to go through the expensive and error-prone epoxy vacuum impregnation process,” Zhai said. “Instead, you directly wind the conductor to the coil form.”
Additionally, “high-temperature superconducting magnets can help spherical tokamak design because the higher current density and smaller windings allow more space for support structure that helps the device withstand the high magnetic fields, improving operating conditions,” said Thomas Brown, a PPPL engineer who contributed to the research. “The smaller, more powerful magnets also give the machine designer more opportunities to design a spherical tokamak with geometry that can improve overall tokamak performance. We’re not quite there yet, but we’re closer, and maybe close enough.”
This research was supported by the US Department of Energy (Small Business Innovation Research and Laboratory Directed Research and Development).
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