Breakthrough in the magnetic fields of stellarators: closer fusion

Scientists have reached an important milestone in the conceptual design of stellarators, experimental structures that could reproduce the fusion energy that powers the sun and stars on Earth. The breakthrough shows how to more accurately model the magnetic fields that are encased in stellarators to create an unprecedented ability to hold fusion fuel together.

“The key thing was to develop software that would allow you to quickly try out new design methods,” said Elizabeth Paul, a Princeton University presidential postdoctoral fellow at the Department of Science’s Princeton Plasma Physics Laboratory (PPPL). US Energy and co-author of a paper detailing the discovery in Physical Review Letters. The findings produced by Paul and lead author Matt Landreman of the University of Maryland could increase the ability of stellarators to use fusion to generate electricity that is safe and carbon-free for humanity.

Stellarators, invented by Princeton astrophysicist and PPPL founder Lyman Spitzer in 1950, have long since taken second place to tokamaks in the worldwide effort to produce controlled fusion energy. But recent developments which include the impressive performance of the Wendelstein 7-X (W7-X) stellarator in Germany, the large results of the Large Helical Device (LHD) in Japan, the promising results of the Helically Symmetric Experiment (HSX) in Madison, wisconsin, and the proposed use of simple permanent magnets to replace complex stellarator coils, have created a resurgence of interest in winding machines.

Fusion creates vast energy throughout the universe by combining light elements in the form of plasma, the hot, charged state of matter made up of free electrons and atomic nuclei, or ions, which make up 99% of the visible universe. Stellarators could produce laboratory versions of the process without the risk of harmful disruptions that more widely used tokamak smelting plants face.

However, the tortuous magnetic fields in the stellarators were less effective at confining the ion and electron pathways than the donut-shaped symmetrical fields in the tokamaks, causing a large and prolonged loss of the extreme heat needed to bring the ions together to release energy of merger. Furthermore, the complex coils that produce star fields are difficult to design and build.

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The current turn produces what is called “quasisymmetry” in the stellarators to almost equal the limiting capacity of the symmetrical fields of a tokamak. While scientists have long sought to produce quasisymmetry in stellarator twisting, new research develops a trick to create it almost precisely. The trick uses new open source software called SIMSOPT (Simons Optimization Suite) designed to optimize stellarators by slowly refining the simulated shape of the plasma boundary that marks the magnetic fields. “The ability to automate things and try things out quickly with this new software makes these configurations possible,” said Landreman.

Scientists could also apply the findings to studying astrophysical problems, he explained. In Germany, a team is developing a quasi-symmetric stellarator to confine and study antimatter particles such as those found in space. “It’s exactly the same challenge as the merger,” Landreman explained. “You just have to make sure the particles stay confined.”

The turning point will require an improvement. For simplicity, for example, the research considered a regime in which the pressure and electric current in the plasma were small. “We have made some simplifying assumptions, but the research is a step forward because we have shown that it is possible to obtain precise quasisymmetry that was thought not possible for a long time,” said Paul.

Further development is also needed before the results can be realized, the new stellarator coils and detailed engineering of the stellarator design. The magnetic field could be provided in part by the permanent magnets that PPPL is developing to streamline today’s twisted stellarator coils. “The biggest missing pieces are the magnets, the pressure and the current,” Landreman explained.

Overseeing Paul’s work at Princeton is PPPL physicist Amitava Bhattacharjee, a professor of astrophysical sciences at Princeton who also oversees the Simons Foundation of New York-sponsored “Hidden Symmetries and Fusion Energy” project that funded the PRL paper. Stellarator’s work on the Simons project runs parallel to the PPPL research to develop the promising device that the Laboratory invented some 70 years ago. Such a development would combine the best features of stellarator and tokamak to design a seamless structure with strong plasma confinement to reproduce a virtually unlimited source of fusion energy.