Het binnenste van de Tokamak-fusiereactor van EPFL uit het Zwitserse Lausanne
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Things get extremely hot inside a nuclear fusion reactor, what with hydrogen plasma that can heat up to tens of millions of degrees Celsius. These high temperatures are necessary because the fusion reaction that needs to take place can only happen under such extreme conditions. Only then can vast amounts of clean energy be released.

One problem that must be dealt with during this process is keeping the plasma floating in the center of the ‘tokamak.’ Which is the most common fusion reactor that is shaped like a donut with magnetic coils around it.

The hot plasma should never touch the reactor walls because otherwise it will melt and moreover immediately cool down. The unfortunate thing is that a bit of hydrogen always manages to escape from the plasma. In order to prevent this, this escaped hydrogen must be cooled before it reaches the wall.

Researchers from the DIFFER and the EPFL Swiss Plasma Center have now developed an incredibly rigorous measurement and control method to cool these extremely hot particles escapingfrom the fusion plasma. According to the two centers, this is a milestone in fusion research. It was recently published in the scientific journal Nature Communications.

Swiss-Dutch collaboration

DIFFER stands for Dutch Institute for Fundamental Energy Research and is affiliated with the Eindhoven University of Technology (TU/e). Its main goal is to achieve as much sustainable and CO2-neutral energy as possible for everyone around the world. Nuclear fusion can play an important role in this.

Christian Theiler of EPFL explains that cooling the hydrogen particles that escape is possible in several ways. One of which is by injecting a gas. “You don’t want to cool it down too much, then your plasma will die off,” he says. Therefore, it is a matter of finding precisely the best possible point at which the reactor can still handle the load.

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The closed loop of measuring, calculating and control to ensure
that the wall is not destroyed.. Image: DIFFER/Julia van Leeuwen


The ability to control cooling properly is explicitly mentioned in the roadmap of the European nuclear fusion program (EUROfusion) as a much-needed step towards energy from nuclear fusion. “It’s a wonderful thing to be able to contribute to that,” says Matthijs van Berkel of DIFFER.

In Nature Communications, the scientists describe how to cool the particle stream that is escaping in a fast and controlled manner. They can do this by means of an innovative control technology system that constantly makes adjustments. The system was tested in the tokamak at EPFL in Lausanne in Switzerland.

No longer based on instinct

Hydrogen that escapes from the hot plasma is discharged through the reactor’s so-called divertor. This is a kind of vent where the heat from the plasma is captured. This vigorous cooling of the plasma near the vent is known in industry jargon as divertor detachment. It causes a drop in temperature and pressure near the divertor wall.

Nuclear physicists already have plenty of experience with this process, but still rely partly on their gut instinct. Which is not an exact science, but that is exactly what EPFL and DIFFER hope to change.

To accomplish this, the researchers use the MANTIS camera system on the TCV tokamak. MANTIS stands for Multispectral Advanced Narrowband Tokamak Imaging System. The system was developed at DIFFER together with EPFL and MIT.

800 times per second

The researchers adapted the system so that real-time camera images are converted into data, after which a computer model calculates the optimal cooling temperatures. This is all done with extreme precision: The state of the plasma is measured 800 times per second.

A new real-time imaging processing algorithm, which was also developed at DIFFER, then analyzes these MANTIS images. This algorithm determines how much cooling is needed, and then automatically controls the gas valves. Last but not least, the researchers made a model of the system by analyzing, again with the help of the camera, how the plasma responds to the gas intake. ” We use this model to determine the dynamic relationship between the actuation of the gas valve and the location of the heat flux,” Van Berkel explains.

Find more articles on nuclear fusion and DIFFER here.