Insulation-free magnet makes space for sustained nuclear fusion reactions
One of the many ways scientists are working to realize the potential of nuclear fusion as a virtually inexhaustible and clean source of energy is through new and improved magnets, which confine fields of plasma for the critical reactions to take place. A new example representing a "revolutionary change" in how these components are made could form a key piece of the puzzle, by facilitating the type of super-hot and sustained streams of plasma needed for fusion power to become a reality.
The magnet was developed by scientists at the Princeton Plasma Physics Laboratory (PPPL) with an eye to improving the performance of what are known as tokamak fusion reactors. These doughnut-shaped devices are designed to confine circular streams of plasma that fuse atoms together under extreme pressure and heat, releasing huge amounts of energy on an ongoing basis.
But one of the many difficulties in achieving these sustained streams of plasma is the threat they pose to the condition of the central electromagnet, a solenoid that generates electrical currents and the magnetic field. Energetic subatomic particles called neutrons emanate from the plasma and can erode the insulation coating the magnet's coils of wires, compromising their performance and longevity.
“If we are designing a power plant that will run continuously for hours or days, then we can’t use current magnets,” said Yuhu Zhai, a principal engineer at PPPL and lead author of a paper describing the research. “Those facilities will produce more high-energy particles than current experimental facilities do. The magnets in production today would not last long enough for future facilities like commercial fusion power plants.”
To develop their new type of magnet, the scientists crafted wires made of niobium and tin, which were heated in a special way to form a new type of superconductor. This new wiring material allows for electrical currents to flow at extremely low temperatures and with little resistance, which reduces the need for insulation. The result is wiring that is less prone to degradation, and which the researchers say offers other improvements in terms of performance.
“During our tests, our magnet produced about 83 percent of the maximum amount of electrical current the wires can carry, a very good amount,”Zhai said. “Scientists typically only use 70 percent of the superconducting wire electrical current capacity when designing and building high-power magnets. And large-scale magnets like those used in ITER, the international fusion facility being constructed in France, often use only 50 percent.”
The magnet is also said to be simpler and cheaper to fabricate than current solutions. And because it can operate at higher current densities, it could occupy less space inside the tokamak device while allowing for the generation of stronger magnetic fields.
“This is a revolutionary change in how you make electromagnets,” said Michael Zarnstorff, PPPL’s Chief Science Officer. “By creating a magnet with just metal and removing the need to use insulation, you get rid of a lot of costly steps and reduce the number of opportunities for the coil to malfunction. This is really important stuff.”
The research was published in the journal Superconductor Science and Technology.