Various kinds of turbulence can stir up this hot soup of particles and dissipate some of the intense heat, and a major problem has been to understand and predict exactly how this turbulence works, and thus how to overcome it.
A long-standing discrepancy between predictions and observed results in test reactors has been called “The great unsolved problem” in understanding the turbulence that leads to a loss of heat in fusion reactors.
In a result so surprising that the researchers themselves found it hard to believe their own results at first, it turns out that interactions between turbulence at the tiniest scale, that of electrons, and turbulence at a scale 60 times larger, that of ions, can account for the mysterious mismatch between theory and experimental results.
For the first time, researchers show two types of turbulence within plasma that cause significant heat loss.
The expectation by physicists for more than a decade had been that turbulence associated with ions was so much larger than turbulence caused by electrons – nearly two orders of magnitude smaller – that the latter would be completely smeared out by the much larger eddies.
The two scales of turbulence do indeed coexist, the researchers found, and they interact with each other so strongly that it’s impossible to understand their effects without including both kinds in any simulations.
Far from being eliminated by the larger-scale turbulence, the tiny eddies produced by electrons continue to be clearly visible in the results, stretched out into long ribbons that wind around the donut-shaped vacuum chamber that characterizes a tokamak fusion reactor.
Previously, scientists had thought that simply simulating turbulence separately at the two different size scales and adding the results together would give a close enough approximation, but they kept finding discrepancies between those predictions and the actual results seen in test reactors.
In addition to the theoretical simulations, MIT graduate student Ruiz Ruiz, lead author of the second paper, has analyzed a series of experiments at the Princeton Plasma Physics Laboratory, which provided direct evidence of electron-scale turbulence that supports the new simulations.
The results offer clear, independent evidence that the electron-scale turbulence really does play an important role, and they show that this is a general phenomenon, not one specific to a particular reactor design.
That’s because Howard’s simulations were based on MIT’s Alcator C-Mod tokamak reactor, whereas Ruiz Ruiz’s results were from a different type of reactor called the National Spherical Torus Experiment, which has a significantly different configuration.
Understanding the details of these different mechanisms of turbulence has been “An outstanding challenge” in the field of fusion research, White says, and these new findings could greatly improve the understanding of what’s really going on inside the 10 tokamak research reactors that exist around the world, as well as in future experimental reactors under construction or planning.
“The evidence from both of these papers, that electron energy transport in tokamaks has a significant contribution from both ion and electron-scale turbulence and that multiscale simulations are needed to predict the transport, is profoundly important,” says Gary Staebler, a researcher at General Atomics who was not involved in this work.