A breakthrough once described as impossible b

image: Physicist Emily Paul and Matt Landreman with illustrative characters behind them.
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Credit: Arthur Lin for Paul’s photo, Faye Levine for Landreman’s photo; figures at the top left and right of the PRL paper; bottom computer-generated visualizations of a tokamak, left, and a stellarator, right by Paul and Landreman. Collage by Kiran Sudarsanan.

Scientists have achieved a remarkable breakthrough in the design of twisted stellarators, experimental magnetic facilities that could replicate on Earth the fusion energy that powers the sun and stars. This breakthrough shows how to more precisely shape the enveloping magnetic fields in stellarators to create an unprecedented ability to hold fusion fuel together.

“The key was to develop software that allows you to quickly try new design methods,” said Elizabeth Paul, Princeton University Presidential Postdoctoral Fellow at the U.S. Department of Health’s Princeton Plasma Physics Laboratory (PPPL). energy and co-author of a document that details the discovery in Physical examination letters. The results produced by Paul and lead author Matt Landreman of the University of Maryland could boost the ability of stellarators to harvest fusion to generate safe, carbon-free electrical power for humanity.

Stellar Rebirth

Stellarators, invented by Princeton astrophysicist and PPPL founder Lyman Spitzer in the 1950s, have long been upstaged by tokamaks in the global effort to produce controlled fusion power. But recent developments which include the impressive performance of the Wendelstein 7-X (W7-X) stellarator in Germany, the extensive 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 stellar coils has sparked renewed 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 composed of free electrons and atomic nuclei, or ions, which make up 99% of the visible universe. Stellarators could produce lab versions of the process without risking the damaging disruptions faced by more widely used tokamak fusion facilities.

However, the torsion magnetic fields in stellarators have been less effective at confining the trajectories of ions and electrons than the symmetric donut-shaped fields in tokamaks regularly do, causing a large and sustained loss of the extreme heat needed to collect the ions. release fusion energy. Additionally, the complex coils that produce the star fields are difficult to design and build.

The current breakthrough is producing what is called “quasisymmetry” in stellarators to nearly match the symmetrical field confinement capability of a tokamak. While scientists have long sought to produce near-symmetry in torsional stellarators, the new research is developing a trick to create it almost precisely. The trick uses new open-source software called SIMSOPT (Simons Optimization Suite) which is designed to optimize stellarators by slowly refining the simulated plasma boundary shape that delineates magnetic fields. “The ability to automate things and quickly try things out with this new software makes those setups possible,” Landreman said.

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

Revolutionary assumptions

Breakthrough made some simplifying assumptions that will require improvement. For simplicity, for example, the research considered a regime in which the pressure and electric current in the plasma were low. “We made some simplifying assumptions, but the research is an important step forward because we’ve shown that you can actually achieve precise quasi-symmetry that has long been thought impossible,” Paul said.

New stellarator coils and detailed engineering of the stellarator design must also be developed before discoveries can be made. The magnetic field could be provided in part by the permanent magnets PPPL is developing to streamline today’s twisted stellar coils. “The biggest missing pieces are magnets, pressure and current,” Landreman said.

Paul’s work on the PRL document is part of the accomplishments of the second year of his Princeton Presidential Fellowship. She previously won the 2021 Marshall N. Rosenbluth Outstanding Doctoral Thesis Award from the American Physical Society. for his thesis at the University of Maryland, on which Landreman was an adviser. She is now working with PPPL graduate student Richard Nies, who recently published a paper that applies the mathematical tools his thesis from Maryland developed to speed up the production of quasisymmetry.

PPPL physicist Amitava Bhattacharjee, professor of astrophysical sciences at Princeton, also oversees the ‘Hidden Symmetries and Fusion Energy’ project sponsored by the Simons Foundation in New York which funded the PRL paper. “Matt and Elizabeth’s work deftly uses the mathematical and computational tools developed in recent years on stellar optimization, and establishes beyond doubt that we can design near-symmetrical stellar magnetic fields with an unprecedented level of precision. It’s a triumph of computer design.

Stellarator’s work on the Simons project parallels PPPL research to develop the promising device the Lab invented some 70 years ago. Such a development would combine the best features of stellarators and tokamaks to design a disturbance-free facility with strong plasma confinement to reproduce a virtually unlimited source of fusion energy.

PPPL, at Princeton University’s Forrestal Campus in Plainsboro, NJ, is dedicated to creating new knowledge about the physics of plasmas – ultra-hot, charged gases – and developing practical solutions for creating energy of merger. The lab is operated by the University for the U.S. Department of Energy Office of Science, which is the largest supporter of basic physical science research in the United States and works to address some of the most pressing challenges From our era. For more information, visit energy.gov/science.

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