14th September 2018
Fusion breakthrough: improved control of plasma
Researchers have predicted the entire set of beneficial 3-D distortions for controlling edge localised modes (ELMs) in tokamak plasma.
A long-time puzzle in the effort to capture the power of fusion is how to lessen or eliminate a common instability that occurs in the plasma called edge localised modes (ELMs). Just as the Sun releases enormous bursts of energy in the form of solar flares, so flare-like bursts of ELMs can slam into the walls of doughnut-shaped tokamaks that house fusion reactions, potentially damaging the walls of the reactor.
To control these bursts, scientists disturb the plasma with small magnetic ripples, called resonant magnetic perturbations (RMPs) that distort the smooth, doughnut shape of the plasma – releasing excess pressure that lessens or prevents ELMs from occurring. The hard part is producing just the right amount of this 3-D distortion to eliminate the ELMs without triggering other instabilities and releasing too much energy that, in the worst case, can lead to a major disruption that terminates the plasma.
Making the task exceptionally difficult is the fact that a virtually limitless number of magnetic distortions can be applied to the plasma, causing finding precisely the right kind of distortion to be an extraordinary challenge. But no longer.
Physicists at the U.S. Department of Energy's (DOE) Princeton Plasma Physics Laboratory (PPPL), in collaboration with a team from the National Fusion Research Institute (NFRI) in South Korea, have successfully predicted the entire set of beneficial 3-D distortions for controlling ELMs without creating additional problems. Researchers validated these predictions on the Korean Superconducting Tokamak Advanced Research (KSTAR) facility – one of the world's most advanced superconducting tokamaks – located in Daejeon, South Korea.
KSTAR was ideal for testing the predictions, because of its advanced magnet controls for generating precise distortions in the near-perfect, doughnut-shaped symmetry of the plasma. Identifying the most beneficial distortions, which amount to less than one percent of all the possible distortions that could be produced inside KSTAR, would have been virtually impossible without the predictive model developed by the research team.
The result was a precedent-setting achievement: "We show for the first time the full 3-D field operating window in a tokamak to suppress ELMs without stirring up core instabilities or excessively degrading confinement," said Jong-Kyu Park, from Princeton University. His paper – written with 14 co-authors from the United States and South Korea – is published in Nature Physics. "For a long time, we thought it would be too computationally difficult to identify all beneficial symmetry-breaking fields, but our work now demonstrates a simple procedure to identify the set of all such configurations."
The team reduced the complexity of the calculations when they realised that the number of ways the plasma can distort is actually far fewer than the range of possible 3-D fields that can be applied to the plasma. By working backwards, from distortions to 3-D fields, the authors calculated the most effective fields for eliminating ELMs. The KSTAR experiments confirmed the predictions with remarkable accuracy.
The findings on KSTAR provide new confidence in the ability to predict optimal 3-D fields for ITER, the international tokamak under construction in France, which plans to employ special magnets to produce 3D distortions to control ELMs. Such control will be vital for ITER, whose goal is to produce 10 times more energy than it will take to heat the plasma.
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