Scientists see solution to critical barrier to fusion

Baku, May 1 (AzerTAc). Physicists 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.
An in-depth analysis by scientists from the U.S. Department of Energy`s Princeton Plasma Physics Laboratory (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. 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." The scientists hit upon their theory in what Gates called "a 10-minute `Aha!` moment." Working out equations on a whiteboard in Gates` office, the physicists focused on the islands and the impurities that drive away energy. The impurities stem from particles that the plasma kicks up from the tokamak wall. "When you hit this magical density limit, the islands grow and coalesce and the plasma ends up in a disruption," says Delgado-Aparacio. These islands actually inflict double damage, the scientists 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 a process called ohmic heating -- the same process that heats a toaster when electricity passes through it. When the islands grow large enough, the electric current that helps to heat and confine the plasma collapses, allowing the plasma to fly apart.
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.

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