“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, which often has frequencies far above what the human ear can hear, can cause heat in the form of high energy particles — or fast ions — to leak from the core of plasma inside the doughnut-shaped tokamaks 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. “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 who conducted research for his dissertation at PPPL.
Duarte and collaborators at General Atomics and the universities of California-Irvine and Texas at Austin recently discovered conditions within plasma that make chirping more likely. Research found that chirping was common in the National Spherical Torus Experiment (NSTX), PPPL’s former flagship fusion facility that has been upgraded, but was rare in the DIII-D tokamak that General Atomics operates for the DOE in San Diego.
Simulations on PPPL computers showed that substantial turbulence can break up concentrations of fast ions and help to extinguish the chirping. Turbulence has little effect on fast ions in NSTX and chirping modes are common, whereas DIII-D has relatively high interaction between turbulence and fast ions and chirping modes are rare.
These findings could lead to the development of 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.