Their analysis focused on tiny, bubble-like islands that appear in the hot plasmas during experiments. These minute islands collect impurities that cool the plasma. And it is these islands, the scientists report in 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 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. This is critical because density is the key parameter in reaching fusion and people have been puzzling about this for 30 or 40 years.

—David A. Gates, a principal research physicist at PPPL

“,” said and co-author of the proposed solution with . “

These islands actually inflict double damage, Gates and colleague Luis Delgado-Aparicio, a post-doctoral fellow at PPPL and a visiting scientist at MIT’s Plasma Science Fusion Center, said. Besides cooling the plasma, the islands act as shields that block out added power. The balance tips when more power escapes from the islands than researchers can pump into the plasma through ohmic heating. When the islands grow large enough, the electric current that helps to heat and confine the plasma collapses, allowing the plasma to fly apart.

Gates and Delgado-Aparicio now hope to test their theory with experiments on a tokamak called Alcator C-Mod at MIT, and on the DIII-D tokamak at General Atomics in San Diego. Among other things, they intend to see if injecting power directly into the islands will lead to higher density. If so, that could help future tokamaks reach the extreme density and 100-million-degree temperatures that fusion requires.

The scientists’ theory represents a fresh approach to the density limit, which also is known as the “Greenwald limit” after MIT physicist Martin Greenwald, who has derived an equation that describes it. Greenwald has another potential explanation of the source of the limit. He thinks it may occur when turbulence creates fluctuations that cool the edge of the plasma and squeeze too much current into too little space in the core of the plasma, causing the current to become unstable and crash.

Conquering the limit could provide essential improvements for future tokamaks that will need to produce self-sustaining fusion reactions, or “burning plasmas,” to generate electric power. Such machines include proposed successors to ITER, a $20-billion experimental reactor that is being built in Cadarache, France, by the European Union, the United States and five other countries.

PPPL, in Plainsboro, N.J., is devoted both to creating new knowledge about the physics of plasmas—ultra-hot, charged gases—and to developing practical solutions for the creation of fusion energy. Princeton Plasma Physics Laboratory is managed by Princeton University for the U.S. Department of Energy’s Office of Science.

Resources

• D. A. Gates and L. Delgado-Aparicio (2012) Origin of Tokamak Density Limit Scalings. Phys. Rev. Lett. 108, 165004 doi: 10.1103/PhysRevLett.108.165004