Researchers have accomplished an extraordinary breakthrough in the conceptual design of twisty stellarators - experimental magnetic facilities that could replicate on Earth the fusion energy that fuels the sun and stars.
Physicist Emily Paul and Matt Landreman with figures behind them. (Credits: Arthur Lin for Paul photo, Faye Levine for Landreman photo; top left and right figures from PRL paper; bottom computer-generated visualizations of a tokamak, left, and a stellarator, right by Paul and Landreman. (Image Credit: Collage by Kiran Sudarsanan.)
The innovation demonstrates how to more accurately control the enclosing magnetic fields in stellarators to form an unparalleled ability to keep the fusion fuel together.
“The key thing was developing a piece of software that allows you to rapidly try out new design methods,” said Elizabeth Paul, a Princeton University Presidential Postdoctoral Fellow at the U.S. Department of Energy’s Princeton Plasma Physics Laboratory (PPPL). She is the co-author of the study, and details of the study can be found in the journal Physical Review Letters.
The results generated by Paul and lead author Matt Landreman of the University of Maryland could improve the capability of stellarators to harvest fusion to make carbon-free and safe electrical power for humans.
Stellarator Renaissance
Stellarators, invented by Princeton astrophysicist and PPPL founder Lyman Spitzer in the 1950s, have not been as widely noted as tokamaks in the international effort to create controlled fusion energy.
But new developments that include the remarkable performance of the Wendelstein 7-X (W7-X) stellarator in Germany, the wide-ranging results from the Large Helical Device (LHD) in Japan, the favorable results from the Helically Symmetric Experiment (HSX) in Madison, Wisconsin, and the recommended use of simple permanent magnets to substitute complex stellarator coils have generated a renaissance of interest in the twisty machines.
Fusion generates massive energy all over the universe by integrating light elements in the form of plasma - the hot, charged state of matter made up of free electrons and atomic nuclei - or ions that account for 99% of the visible universe. Stellarators could create laboratory versions of the process without the damaging disruptions that the more commonly used tokamak fusion facilities deal with.
Nevertheless, the twisting magnetic fields in stellarators have been less effective at limiting the paths of the ions and electrons than the symmetrical, doughnut-shaped fields in tokamaks regularly do, causing a large and continuous loss of the extreme heat needed to bring the ions together to discharge fusion energy. Furthermore, the complex coils that yield the stellarator fields are hard to design and build.
The current innovation creates what is known as “quasisymmetry” in stellarators to almost match the confining ability of the symmetrical fields of a tokamak. While researchers have been eager to create quasisymmetry in twisting stellarators for a long time, the new study develops a trick to form it quite precisely.
The trick employs new open-source software called SIMSOPT (Simons Optimization Suite) that is programmed to enhance stellarators by gradually refining the simulated shape of the boundary of the plasma that demarcates the magnetic fields.
The ability to automate things and rapidly try things out with this new software makes these configurations possible.
Matt Landreman, Researcher and Study Lead Author, University of Maryland
Researchers could also apply the results to the study of astrophysical complications, he said.
In Germany, a team is building a quasisymmetric stellarator to confine and explore antimatter particles such as those located in space.
It’s exactly the same challenge as with fusion. You just need to make sure that the particles stay confined.
Matt Landreman, Researcher and Study Lead Author, University of Maryland
Breakthrough Assumptions
The breakthrough made some straightforward assumptions that will necessitate improvement. For simplicity, for example, the research looked at a regime in which the electric current and pressure in the plasma were small.
“We’ve made some simplifying assumptions but the research is a significant step going forward because we’ve shown that you can actually get precise quasisymmetry that for a long time was thought not to be possible,” Paul said.
Also demanding further development before the findings can be attained are new stellarator coils and comprehensive engineering of the stellarator design. The magnetic field could be supplied in part by the permanent magnets that PPPL is building to simplify today’s twisted stellarator coils.
The biggest missing pieces are the magnets and the pressure and current.
Matt Landreman, Researcher and Study Lead Author, University of Maryland
Paul’s study on the PRL article is among the achievements during the second year of her Princeton Presidential fellowship. She has also previously won the American Physical Society’s highly competitive 2021 Marshall N. Rosenbluth Outstanding Doctoral Thesis Award for her dissertation at the University of Maryland. Landreman was the advisor on her dissertation.
She is currently working with PPPL graduate student Richard Nies, who recently published an article that applies the mathematical tools that her Maryland thesis created to quicken the creation of quasisymmetry.
Supervising Paul’s Princeton work is PPPL physicist Amitava Bhattacharjee, a Princeton professor of astrophysical sciences who also supervises the “Hidden Symmetries and Fusion Energy” project supported by the Simons Foundation in New York that sponsored the PRL article.
“Matt’s and Elizabeth’s work makes adroit use of the mathematical and computational tools developed in recent years on stellarator optimization, and establishes beyond doubt that we can design quasisymmetric stellarator magnetic fields with an unprecedented level of accuracy. It is a triumph of computational design.”
Stellarator work on the Simons project is following the PPPL research to create the promising device the Laboratory invented nearly 70 years ago. Such development would unite the top features of stellarators and tokamaks to engineer a disruption-free facility with powerful plasma confinement to replicate an almost unlimited source of fusion energy.
Journal Reference:
Landreman, M. & Paul, E., (2022) Magnetic Fields with Precise Quasisymmetry for Plasma Confinement. Physical Review Letters. doi.org/10.1103/PhysRevLett.128.035001.