Posted on: 28th January 2013
There is a predator on the loose in Madrid. It’s been discovered in the fusion laboratories of CIEMAT, preying on its favourite victim… Turbulence!
This predator is not a large feline, but a type of plasma behaviour known as zonal flows. Strangely enough the relationship between zonal flows and turbulence follows exactly the same pattern as that between the numbers of predators and their prey in the wild – a significant discovery for fusion scientists in the quest to control the turbulence that allows energy to escape from their experiments.
In the same way that a large number of prey – for example, deer – might support an increasing population of tigers, in certain conditions turbulence seems to trigger the growth of the zonal flow pattern. However, as they grow, the zonal flows inhibit the turbulence, which dies away, eventually undermining the zonal flows, in the same way as a large population of tigers would cause a decline in the deer population, leading to their own downfall. But as the tiger population drops, the deer begin to thrive again, and so the cycle begins anew. This oscillatory predator-prey relationship is exactly what has been measured in the TJ-II stellarator at CIEMAT, as shown in the figure.
The scientist that made the discovery is Dr Teresa Estrada, head of microwave and laser diagnostics at TJ-II. “It was a lucky discovery!” she said. “I was not looking for this, the timescale of the oscillations is milliseconds so I could easily have missed it. I saw [the oscillations] first in the turbulence, and so I looked at the flow and was excited to see them there too, with the characteristic ninety degree phase delay.”
Zonal flows are large areas of even flow around the minor radius of the plasma. They were initially predicted theoretically, and subsequent observations in other devices around the world suggested zonal flows did exist. However, the new measurements at TJ-II are the first to show the dynamical relationship between the zonal flows and the turbulence, thanks to a recently installed diagnostic system, Doppler Reflectometry. The technique, which injects microwave radiation into the plasma and measures the reflected signal, allows both the radial electric field and the density fluctuations to be measured simultaneously, and can see further into the core of the plasma than previous techniques.
The onset of the predator-prey oscillations occurs when as the plasma makes a transition into H-mode, a regime of high confinement of energy and particles that will be the basis of future fusion devices.The insight into how to create H-mode is what makes the discovery of zonal flows so significant, says Dr Estrada:
“If we can find ways to make the zonal flows stronger so they damp the turbulence, we possibly could identify one path to achieve H-mode at lower power!” she explains.
H-mode is planned to be the main operating mode of the high-powered fusion device ITER, which is under construction in the south of France. Controlling the heat and turbulence of a fusion plasma ten times larger than ever before produced is one of ITER’s main challenges. The information from TJ-II perhaps suggests that the ITER plasma could be thought of as an eco-system – and to be healthy it needs predators.
CIEMAT is the Spanish signatory to EFDA