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Physicists from PPPL, the U.S. Department of Energy’s Princeton Plasma Physics Laboratory, have discovered a possible solution to a mystery that has long baffled researchers working to harness fusion. If confirmed by experiment, the finding could help scientists eliminate a major impediment to the development of fusion as a clean and abundant source of energy for producing electric power.

Image from the Wikimedia Commons



Nuclear fusion is the process by which two or more atomic nuclei join together, or “fuse”, to form a single heavier nucleus. This is usually accompanied by the release of large quantities of energy. Fusion is the process that powers active stars, the hydrogen bomb and some experimental devices examining fusion power for electrical generation.

From left: physicists Luis Delgado-Aparicio and David Gates

From left: physicists Luis Delgado-Aparicio and David Gates.

Photo credit: Elle Starkman, PPPL Office of Communications



An in-depth analysis by scientists from PPPL zeroed in on tiny, bubble-like islands that appear in the hot, charged gases—or plasmas—during experiments. These minute islands collect impurities that cool the plasma. And it is these islands, the scientists report in the April 20 issue of Physical Review Letters, that are at the root of a long-standing problem known as the “density limit” that can prevent fusion reactors from operating at maximum efficiency.
Fusion occurs when plasmas become hot and dense enough for the atomic nuclei contained within the hot gas to combine and release energy. But when the plasmas in experimental reactors called tokamaks reach the mysterious density limit, they can spiral apart into a flash of light.
“The big mystery is why adding more heating power to the plasma doesn’t get you to higher density,” said David A. Gates, a principal research physicist at PPPL and co-author of the proposed solution with Luis Delgado-Aparicio, a post-doctoral fellow at PPPL and a visiting scientist at MIT’s Plasma Science Fusion Center. “This is critical because density is the key parameter in reaching fusion and people have been puzzling about this for 30 or 40 years.”

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