Princeton Plasma Physics scientists to test new theory for a puzzling problem

PPPL1.JPGFrom left, physicists Luis Delgado-Aparicio and David Gates at the Princeton Plasma Physics Laboratory in Plainsboro.They have discovered a possible solution to a mystery that has long baffled researchers working to harness fusion.

Researchers at the Princeton Plasma Physics Lab have offered a theory explaining a decades-old puzzle in fusion research: plasma reactions that break down before reaching the optimum conditions needed to generate power efficiently.

The researchers believe bubble-like islands in the plasma are the culprit, and if confirmed by experiments, their explanation could overcome one of the major barriers to generating clean energy from nuclear fusion.

Nuclear fusion occurs when atomic nuclei within plasma — heated, electrically charged gases — join together. Forcing atomic nuclei to fuse requires a combination of high temperatures, increasing the speed at which particles move, and high density, which forces particles closer together. The combination increases the rate at which particles collide, generating a burst of energy that scientists believe can be harnessed as a new, cleaner source of electricity.

But when physicists increase the density of plasma in a type of reactor used in plasma experiments called a tokamak, the plasma breaks down well below the optimal density for fusion reactions.

This density limit has been a mystery for decades, said David Gates, a research physicist at PPPL who co-authored the proposed solution, published in the journal Physical Review Letters last week with Luis Delgado-Aparicio, a postdoctoral fellow at PPPL.

Gates and Delgado-Aparicio’s insight was connecting the mysterious behavior at the density limit to another phenomenon that hadn’t been fully explained — the islands that grow as plasma reaches the density limit, Gates said.

During fusion experiments, scientists increase the temperature and density of the plasma in the tokamak through ohmic heating, the same process that heats a toaster. An electric current flowing through the plasma generates energy and creates a magnetic field that keeps the plasma together.

But islands in the plasma collect impurities from the walls of the reactor, which Gates and Delgado-Aparico said cool the plasma and shield it from energy that should heat it. As the plasma approaches the density limit, the islands grow until the electric current collapses and the plasma flies apart.

French physicist Paul-Henri Rebut, in the mid-1980s, was the first to describe plasma islands. Gates learned about them a decade later while working with Wolfgang Suttrop, a German physicist whose paper speculating that islands might be connected to the density limit sparked Gates’ and Delgado-Aparicio’s idea.

But Gates said he never actually worked on the problem until he learned about Delgado-Aparicio’s research explaining corkscrew-shaped phenomena called snakes that were very similar to the plasma islands described by Rebut and Suttrop.

Gates said he introduced Delgado-Aparicio to the prior research on islands and the density limit, and eight months later, Delgado-Aparicio wrote a paper in which he developed an equation relating the growth of islands to the density limit.

They quickly realized that if islands were the explanation for the density limit, that equation held the answer, Gates said. When the two met to work out the solution, it was even easier than they realized, and they found the solution to the decades-old problem in just 15 minutes, Gates said.

“It was really a very simple idea,” Gates said. “It was just a matter of putting all the pieces together.”

Princeton University Dean for Research A.J. Stewart Smith wasn’t surprised that Gates, “one of the most creative scientists at the lab,” was one of the people to put those pieces together. “This is a major development,” Smith said.

Their next step is testing the theory experimentally, Gates said. Although PPPL’s reactor is currently undergoing renovations, Gates and Delgado-Aparicio have submitted research proposals to other labs, including the Massachusetts Institute of Technology, where Delgado-Aparicio is a visiting scientist.

If the results verify their theory, knowing that islands cause the density limit suggests ways of overcoming it, Gates said. An experiment they’ve proposed would test whether cooling and shrinking the islands with targeted blasts of radiation can keep them from disrupting the plasma. If that lets researchers use higher densities, they could generate fusion at lower temperatures.

“If you can raise the density, the tokamak becomes a much more viable fusion device,” Gates said. “It gives you much more freedom in designing it.”

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