"Chirp, chirp, chirp." The familiar sound of birds is also what researchers call a wave in plasma that breaks from a single note into rapidly changing notes. This behavior can cause heat in the form of high energy particles -- or fast ions -- to leak from the core of plasma inside tokamaks -- doughnut-shaped facilities that house fusion reactions.
Physicists want to prevent these waves from chirping because they may cause too many fast ions to escape, cooling the plasma. As the plasma cools, the atomic nuclei in the tokamak are less likely to come together and release energy and the fusion reactions will sputter to a halt.
"Chirping modes can be very harmful because they can steal energy from the fast ions in an extended region of the plasma," said Vinícius Duarte, a graduate student from the University of São Paulo. Duarte is at the U.S. Department of Energy's (DOE) Princeton Plasma Physics Laboratory (PPPL) conducting research for his dissertation. Support for this work comes from the DOE Office of Science.
Chirping modes often have frequencies far above what the human ear can hear. The name -- "chirping" -- stems from the change in the waves' frequency over time. Typically, the modes start with a high frequency and drop down in frequency very rapidly. The chirping of modes has been studied for decades as physicists seek to understand and eliminate them.
In a recent theoretical study, Duarte discovered some conditions within plasma that can make the chirping of modes more likely. A paper he is preparing on this topic explains the phenomenon and may help to optimize the design of fusion energy plants in the future. Collaborating on the research were physicists at PPPL, General Atomics, the University of California-Irvine, and the University of Texas at Austin. Physicist Nikolai Gorelenkov, Duarte's PPPL advisor, introduced him to the software code that enabled this work, Prof. Herbert Berk of the University of Texas co-advised on the project and researchers from the DIII-D National Fusion Facility that General Atomics operates for the DOE provided the data for comparison with the theory.
The researchers began by noting that the chirping of modes seems to occur in some tokamaks more often than in others. They are rare in the DIII-D tokamak, for example, but were common in the National Spherical Torus Experiment (NSTX), PPPL's former flagship fusion device, which has recently been upgraded.
By running simulations on PPPL computers, Duarte and the team found that plasma turbulence -- or random fluctuation -- was a factor that helped explain the chirping of modes. Chirping can occur when there is a strong concentration of fast ions bunched together, while other particles are widely spaced.
The surprise is that substantial turbulence can break up concentrations of fast ions, and therefore help to extinguish the chirping of modes.
The simulations matched the data from experiments. In NSTX, the turbulence has little effect on fast ions and chirping modes are common, whereas DIII-D has relatively high interaction between turbulence and fast ions and chirping modes are rare. In DIII-D, chirping starts only when the interaction between the turbulence and fast-ions markedly decreases.
These findings could lead to fusion facilities that leak less heat than current machines and could improve the efficiency of ITER, the international tokamak under construction in France to demonstrate the feasibility of fusion power. "In ITER, where fast ions from fusion reactions are expected to sustain a burning plasma, the good confinement of these particles is a crucial issue," said Duarte.
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