A disappearing feast: Mean flows remain slim after eating eddies

From American Physical Society:

IMAGE: Plasma physics experiment shows that the reduction in turbulence energy can’t be explained by the increase in the mean flow energy, ruling out the predator-prey model in magnetic confinement fusion… view more

Credit: Princeton Plasma Physics Laboratory

Magnetic confinement fusion holds the promise of almost limitless amounts of energy, available on demand and producing zero carbon dioxide. But in order to harness that energy, we must trap plasma — an ionized gas — hotter than the center of the sun inside a donut-shaped magnetic facility called a tokamak that measures just a few yards across. As you might guess, the confined plasma becomes turbulent, and that turbulence leaks energy out from the ultra-hot core to the room-temperature wall.

But a slight increase in heating power can reduce the turbulence near the edge of the tokamak and cause the energy to leak much less. This new state of high confinement, known technically as “H-mode” and discovered in Germany in 1982, opened a promising new avenue towards the production of fusion energy.

Yet there is still no conclusive explanation for the disappearing turbulence. One popular contender, the “predator-prey” model, posits that the turbulence spontaneously dumps all of its energy into a benign spinning of the plasma called “mean flow” that does not transport heat. According to this model, the spinning acts as a predator that feeds on eddies (prey) in the turbulence. If the predator is too successful, the population of eddies plummets and the mean flows (predators) grow accordingly. The predator-prey model suggests that the energy in the mean flows must increase by roughly the same amount that the energy in the turbulence drops. But does this really happen?

At the U.S. Department of Energy’s Princeton Plasma Physics Laboratory (PPPL), researchers have …

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