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 Caroline Darbos and Mark Henderson are counting on diamonds for ITER's Electron-Cyclotron Heating.  ITER's Electron-Cyclotron Heating system will require 80 diamond windows. © FZK Karlsruhe This together with other properties, such as their heat-dissipating capacity, makes diamonds very interesting for fusion devices. In ITER, the machine's Electron-Cyclotron Heating (ECH) system will use the largest synthetic diamonds ever produced. The ECH heating system is sometimes presented as a giant microwave cooking system. Radiofrequency waves are generated by an array of devices called "gyrotrons" located some 100 metres away from the tokamak in order to avoid perturbations from its magnetic field. A "wave guide" leads the radio waves to the chamber wall which they must pass in order to transmit their energy 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. "We cannot use ordinary glass for this", explains Caroline Darbos, engineer in the ECR team, and Mark Henderson, the physicist responsible for the system. "It would melt under the heat load. About 1 MW of power has to go through these windows. And glass is not a good thermal conductor nor is it sufficiently transparent to the 170 GHz frequency we're using." Optically, thermally and structurally, diamonds are an ideal material. "They're just a bit costly", says Henderson. ITER's heating system will need some 80 diamond windows — 56 for the vacuum chamber, one for each of the 24 gyrotrons. Each window is 1.1 millimetres thick and ranges between 67 and 106 millimetres in diameter. The price for one is 30,000 to 100,000 €. Manufacturing synthetic diamonds is a slow and quite secret industrial process based on carbon plasma — heated by an ECH system very similar to the one used for ITER — crystal growth and precision polishing. It takes about two weeks to "grow" a disk of 1.1mm thickness. The diamond windows for ITER are currently being tested in a joint collaboration between Forschungszentrum Karlsruhe (FZK) in Germany and Japan Atomic Energy Agency (JAEA). Manufacturing should start in 2011 for a delivery in 2015. And, sorry, despite strong demand, Caroline and Mark cannot provide free samples. << return to Newsline #67 |
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