A new method in which to stop a common instability from halting fusion reactors has been discovered this week. The instability causing the problems is known as a global Alfven eigenmode (GAE) and is a wave-like disturbance that can cause the reactors to breakdown. A second neutral beam injector was used to achieve suppression in the end which took just a small amount of highly energetic particles to shut down the GAE’s. These experiments were performed on the National Spherical Torus Experiment-Upgrade (NSTX-U), located at the Department of Energy’s Princeton Plasma Physics Laboratory (PPPL).
These instabilities that occur have been described as being similar to that of a dragon that’s swallowing its own tail. Once GAE’s become triggered by ions inside the gas they can rise up and drive out the particles, cooling the plasma and bringing the fusion reactors to a standstill. Beams from the second injector suppressed the arousal as they flowed through the plasma in roughly the same direction as the magnetic field containing the hot gas. These beams have been aptly named “outboard” beams in order to distinguish them from those that the original NSTX-U injector produces. Once the outboard beam had been injected, the GAE’s were suppressed in milliseconds. The density of the ions was increased as a result which altered their distribution in the plasma. This then caused the gradient of the ion density to reduce and stop the GAE’s from forming. These are fantastic results in terms of fusion development.
Jonathan Menard is head of research on NSTX-U and he said, “Normally when you inject energetic particles, you drive up instabilities. The fact that the second neutral beam was able to turn them off by varying the fast-ion distribution with a small with a small amount of particles provides our research with flexibility and is a welcome discovery.” Eric Fredrickson, the lead author of the paper, states, “This research demonstrates suppression of GAE’s with just a small population of energetic particles. It gives confidence that by using this code, reasonable predictions of GAE stability can be made for ITER.”
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