Scientists discover a process that stabilizes fusion plasmas by Staff Writers Plainsboro NJ (SPX) Jan 09, 2019
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 energy is therefore a key scientific and engineering challenge for researchers across the globe.
Creating magnetic islands 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."
Beneficial to ITER Reiman worked with Professor Nat Fisch, associate director for academic affairs at PPPL and coauthor of the report. Fisch had demonstrated in a landmark 1970s paper that RF waves could be used to drive currents to confine tokamak plasmas through a process now called "RF current drive." Fisch points out how "it was Reiman's groundbreaking paper in 1983 that predicted that these RF currents could also stabilize tearing modes. The use of RF current drive for stabilization of tearing modes was perhaps even more crucial to the tokamak program than using these currents to confine the plasma," Fisch said. "Hence," he said, "Reiman's 1983 paper essentially launched experimental campaigns on tokamaks worldwide to stabilize tearing modes." Moreover, he added, "Significantly, in addition to predicting the stabilization of tearing modes by RF, the 1983 paper also pointed out the importance of the temperature perturbation in magnetic islands."
Underappreciated feature The theoreticians began their recent work with a simple model and advanced to more complex ones to address the key issues. They now plan to produce a more detailed picture with still-more sophisticated models. They are also working to suggest experimental campaigns that will expose these new effects. Support for this research comes from the DOE Office of Science.
Developing new materials for the fusion reactor Tokyo, Japan (SPX) Dec 12, 2018 In the future fusion reactor, plasma is confined by using the magnetic field inside the doughnut-shaped vacuum vessel. The blanket is in a location where it almost touches the plasma, and as if to enfold the plasma the blanket is placed on the inner surface of the vacuum chamber. The blanket, by absorbing the high-speed particles generated by the fusion reaction inside the plasma, releases heat and a higher temperature is achieved. Then, cooling materials are passed through piping in the blanket, ... read more
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