From zero to 500000 W in a matter of hours

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 J. Ongena    04-04-2023     Leggi in PDF
Computer graphics of the plasma vessel and complex three-dimensionally shaped magnet coils of the Wendelstein 7-X fusion device. (Credit: Max-Planck Institute for Plasma Physics, Computer Graphics Department)

Great success for German-Belgian fusion research: In the world's largest stellarator Wendelstein 7-X at the Max Planck Institute for Plasma Physics (IPP) in Greifswald, an innovative heating system with radio waves has recently been used for the first time. Using a new type of antenna, a power of more than 500,000 watts could be radiated into the plasma for four seconds. This was obtained in the first commissioning hours of the heating system, an achievement that has never been realised before. The power delivered to the plasma is equivalent to that of 500 microwave ovens operating simultaneously at full power.

The ion-cyclotron resonance heating (ICRH) system for the Greifswald fusion experiment is built according to the design of the Laboratory for Plasma Physics at the Royal Military Academy in Brussels (LPP-ERM/KMS) and was constructed in an intensive collaboration with the Institute for Plasma Physics and the Central Institute for Engineering of the Research Centre in Jülich.

This is the culmination of the work of the last ten years. The new heating system contains several improvements over traditional designs, based on earlier research in the TEXTOR tokamak experiment at the Forschungszentrum Jülich. In addition, the antenna can be moved to the optimal position for power deposition in the plasma.

In the coming weeks, the power will be further increased to more than one million watts for ten seconds. The goal is to reach an ion temperature of at least 100 million degrees, which is required for the fusion of hydrogen isotopes into helium.

The stellarator is the lesser-known competitor of the tokamak, currently the most performing device in magnetic fusion research. In these machines the plasma is heated in a donut-shaped chamber and is confined using magnetic fields. A giant tokamak, ITER, is currently under construction in Cadarache, southern France.

In a stellarator, too, the plasma is magnetically confined. But that is where the comparison with a tokamak ends. Whereas the latter, like a donut, is very symmetrical in shape, a stellarator like the Wendelstein 7-X has a seemingly irregular shape. Fusion research in the European Union, coordinated by the EUROfusion Consortium invests in both lines of research. Whereas a tokamak is essentially a pulsed device, a stellarator can operate continuously. Three dimensionally shaped magnetic field coils are used to confine the plasma in a rather distorted plasma chamber. That Wendelstein 7-X is less well known than ITER is because modern stellarators required a large amount of theoretical work to optimize plasma confinement. For the design of the complex magnets, modern supercomputers are needed. Only then could this wonder of fusion engineering be realised.

Jef Ongena – Research Director at Plasma Physics Laboratory LPP-ERM/KMS in Brussels and project leader for the ICRH system at Wendelstein 7-X in Greifswald. Together with his colleague Yevgen Kazakov and two colleagues of the tokamak team in MIT, Boston, he received in 2018 the EPS/APS Landau-Spitzer Award for the development of the 3-ion scenario, a very flexible new method to heat plasmas using radiowaves.