Scientists seeking to bring the fusion reaction that powers the sun and stars to Earth must keep the superhot plasma free from disruptions. Now researchers at the U.S. Department of Energy's (DOE) Princeton Plasma Physics Laboratory (PPPL) have discovered a process that can help to control the disruptions thought to be most dangerous.
Replicating fusion, which releases boundless energy by fusing atomic nuclei in the state of matter known as plasma, could produce clean and virtually limitless power for generating electricity for cities and industries everywhere. Capturing and controlling fusion energyistherefore a key scientific and engineering challenge for researchers across the globe.
The PPPL finding, reported in Physical Review Letters, focuses on so-called tearing modes—instabilities in the plasma that create magnetic islands, a key source of plasma disruptions. These islands, bubble-like structures that form in the plasma, can grow and trigger disruptive events that halt fusion reactions and damage doughnut-shaped facilities called "tokamaks" that house the reactions.
Researchers found in the 1980s that using radio-frequency (RF) waves to drive current in the plasma could stabilize tearing modes and reduce the risk of disruptions. However, the researchers failed to notice that small changes—or perturbations—in the temperature of the plasma could improve the stabilization process, once a key threshold in power is exceeded. The physical mechanism that PPPL has identified works like this:
• The temperature perturbations affect the strength of the current drive and the amount of RF power deposited in the islands.
• The perturbations and their impact on the deposition of power feedback against each other in a complex—or nonlinear—manner.
• When the feedback combines with the sensitivity of the current drive to temperature perturbations, the efficiency of the stabilization process increases.
• Furthermore, the improved stabilization is less to likely to be affected by misaligned current drives that fail to hit the center of the island.
The overall impact of this process creates what is technically called "RF current condensation," or concentration of RF power inside the island that keeps it from growing. "The power deposition is greatly increased," said Allan Reiman, a theoretical physicist at PPPL and lead author of the paper. "When the power deposition in the island exceeds a threshold level, there is a jump in the temperature that greatly strengthens the stabilizing effect. This allows the stabilization of larger islands than previously thought possible.