University of Texas-led Team Solves a Big Problem for Fusion Energy

Abundant, low-cost, clean energy — the envisioned result if scientists and engineers can successfully produce a reliable method of generating and sustaining fusion energy — took one step closer to reality, as a team of researchers from The University of Texas at Austin, Los Alamos National Laboratory and Type One Energy Group solved a longstanding problem in the field. The work was supported by the U.S. Department of Energy.

One of the big challenges holding fusion energy back has been the ability to contain high-energy particles inside fusion reactors. When high-energy alpha particles leak from a reactor, that prevents the plasma from getting hot and dense enough to sustain the fusion reaction. To prevent them from leaking, engineers design elaborate magnetic confinement systems, but there are often holes in the magnetic field, and a tremendous amount of computational time is required to predict their locations and eliminate them.

In their paper published in Physical Review Letters, the research team describes having discovered a shortcut that can help engineers design leak-proof magnetic confinement systems 10 times as fast as the gold standard method, without sacrificing accuracy. While several other big challenges remain for all magnetic fusion designs, this advance addresses the biggest challenge that’s specific to a type of fusion reactor first proposed in the 1950s, called a stellarator.

“What’s most exciting is that we’re solving something that’s been an open problem for almost 70 years,” said Josh Burby, assistant professor of physics at UT and first author of the paper. “It’s a paradigm shift in how we design these reactors.”

A stellarator uses external coils carrying electric currents that generate magnetic fields to confine a plasma and high-energy particles. This confinement system is often described as a “magnetic bottle.”

There is a way to identify where the holes are in the magnetic bottle using Newton’s laws of motion, which is very precise but takes an enormous amount of computational time. Worse still, to design a stellarator, scientists might need to simulate hundreds or thousands of slightly different designs, tweaking the layout of the magnetic coils and iterating to eliminate the holes — a process that would require a prohibitive amount of computation on top of that.

So, to save time and money, scientists and engineers routinely use a simpler method for approximating where the holes are, using an approach called perturbation theory. But that method is much less accurate, which has slowed the development of stellarators. The new method relies on symmetry theory, a different way of understanding the system.

“There is currently no practical way to find a theoretical answer to the alpha-particle confinement question without our results,” Burby said. “Direct application of Newton’s laws is too expensive. Perturbation methods commit gross errors. Ours is the first theory that circumvents these pitfalls.”

This new method also can help with a similar but different problem in another popular magnetic fusion reactor design called a tokamak. In that design, there’s a problem with runaway electrons — high-energy electrons that can punch a hole in the surrounding walls. This new method can help identify holes in the magnetic field where these electrons might leak.

Burby’s co-authors from UT are postdoctoral researcher Max Ruth and graduate student Ivan Maldonado. Other authors are Dan Messenger, a postdoctoral fellow at Los Alamos, and Leopoldo Carbajal, a computational scientist and data scientist at Type One Energy Group, a company planning to build stellarators for power generation.