An icy airbag to protect the future ITER nuclear fusion reactor

During the works, research continues. In Saint-Paul-lez-Durance (Bouches-du-Rhône), one of the biggest research sites, ITER is moving forward to hope to carry out nuclear fusion experiments in five years. 1,400 kilometers away, on a much smaller equivalent, the JET installed in England, a group of researchers has just removed a thorn from the foot of the future giant, which could have penalized its operation. This team brings together members of ITER, the European research consortium Eurofusion and the American company General Atomics. She described her remedy, April 30, in Physical Review Letters (PRL). “We checked that we could have a kind of airbag to protect the machine”, sums up Cédric Reux, from the CEA’s Institute for Research into Magnetic Confinement Fusion, in Cadarache.

The accident in question is not dangerous and would not cause the machine to explode. It is nevertheless a question of correcting a defect which accelerates the deterioration of the walls of the heart of the device. If it has been known for a long time, it was not until then considered to be problematic on smaller experiments. But ITER’s core will be about ten times the size of its predecessors and will contain thirty times more energy.

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The reactor will merge heavy hydrogen nuclei to release energy. To this end, it is necessary to heat strongly, to 150 million degrees, and to concentrate the reactive material for quite a long time (about ten minutes). This first involves the transformation of fuels into plasma, a low density material electrically charged in positive nuclei and negative electrons, then by trapping this plasma using intense magnetic fields. But this scale model of a star’s heart is fragile. It can be destabilized and disappear, in what specialists call a “disruption”, which leads to strong releases of energy on the walls. More precisely, streams of electrons are ejected at the speed of light and damage the enclosure on the surface, or even over more than a centimeter in thickness.

Slow down the destructive stunt

It is almost by chance that in 2018 an American team found a way to prevent the destructive effect of these electrons in the event of disruption, during the injection of deuterium ice cubes (the name of the heavy hydrogen in plasma ). Three years later, the effect was verified and above all explained on the JET by the authors of the article by PRL. During disruption, various electromagnetic effects accelerate the electrons which hit the walls. Adding deuterium ice cubes “spreads” these fluxes over a larger area, thus reducing damage. It also prevents the creation of new electron flows, stopping the destructive cascade. “We were very happy to see this effect. It was immediately noticeable because, on the control screens, we did not see the expected sparks or hot spots ”, appreciates Cédric Reux. More precise measurements confirmed that where energy flows of some 10 megajoules per square meter (MJ / m2) were expected, with the “airbags”, they fell to less than 0.5 MJ / m2.

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An icy airbag to protect the future ITER nuclear fusion reactor