Diamonds for fusion

Its unbreakable, transparent nature and capacity to dissipate heat five times higher than that of copper, makes diamonds, the hardest material on earth, particularly useful for ITER’s tokamak Electron Cyclotron (EC) heating system. The EC system, will heat the plasma up to 150 million° C by transferring the energy from electromagnetic waves into the electrons of the plasma which is necessary for the fusion reaction to occur, by injecting an unprecedented 1 to 2 MW of power up to 56 beam-lines (tubes that guide the power). This very high power has to be transmitted safely and efficiently across the 80 mm diameter diamond disk, for durations of up to 3 000 seconds.

The power supplies of the EC system convert the electricity from the grid to provide the correct current and voltage to the source of the EC waves (gyrotrons) that in turn generate the electromagnetic waves. The gyrotrons are located some 100 metres away from the tokamak in order to avoid perturbations from its magnetic field. The beam-line leads the electro-magnetic waves from the gyrotron to the ITER vacuum vessel through an antenna which transmits and directs the waves to the electrons inside the plasma chamber. Since both the gyrotrons and the chamber have to remain vacuum tight, and since the radiofrequency waves propagate in a manner similar to light, the only way to get them out of the gyrotrons and into the chamber is through a window — a diamond window – and F4E is responsible for procuring 60 such windows.

The core of the window is the diamond disc, and in collaboration with ITER IO and Karlsruhe Institute of Technology (KIT), F4E has entrusted the manufacturing of two prototypes diamond disks to a German SME named Diamond Material which is based in Freiburg. Manufacturing of the diamond disc, the 1.1 mm thick part of the window where the electro-magnetic waves will travel, is currently under way and is the result of some 10 years of development at KIT and the Fraunhofer Institute (IAF) where the owners of Diamond Material previously worked.

“The diamond disc will be integrated into a mechanical structure which needs to be extra robust and mechanically stable in order to withstand the harsh ITER tokamak environment and ensure that the diamond window does not fail. Vacuum-tightness is also of upmost importance since the window will confine the tritium gas used for fuelling the ITER plasma and thus the disc is key element for the confinement”, explains Gabriella Saibene, F4E Project Team Manager for Antennas and Plasma Engineering.

How exactly is a diamond window produced? The production of the disc is a slow industrial process: chemical vapour deposition (CVD) is the method by which diamond can be ‘grown’ from a hydrocarbon gas mixture. The CVD growth involves substrate preparation, feeding varying amounts of gases into a chamber and energising them. By controlling the pressure and temperature of about 800°C in the growth chamber, the hydrocarbon gas is ionized into chemically active radicals using microwave power. During the growth, the gas in the chamber is ionised, (a plasma is formed), the molecules are broken off and they deposit very slowly on the substrate (about 1 micron per hour) and incorporated in the diamond thus making it “grow”. After weeks of this process, the diamond disc has grown to its required thickness, so that it can be polished, measured and then mounted into a metallic structure to make a ‘window’.

“Thanks to the very successful collaboration between F4E, ITER IO and KIT and Diamond Material, we have been able to progress exceptionally well”, enthuses Gabriella Saibene. “The next step will to carry out optical and mechanical testing, as well as check how the disc will resist fractures (fracture toughness) and how well it brazes (joins with metal) as it needs to be attached to a copper mechanical structure. Conclusion of the testing and the conclusion of the entire two diamond window prototypes is expected in less than one year’s time”, she adds.