Gyrotrons are microwave oscillators generating a temperature of up to 150 million degrees Celsius in the reactor, similar to a very big microwave. This high temperature makes the tritium fuel reach the plasma state required for fusion. To guide microwave radiation from the gyrotrons into the plasma and in order to maintain a vacuum and keep the radioactive tritium inside the reactor, a team around Dr. Dirk Strauss and Professor Theo Scherer of KIT’s Institute for Applied Materials (IAM) designs appropriate window units.
The Diamond disks guide radiation of more than one megawatt power into the ITER research reactor, numerous diamond windows have been designed by IAM and produced in cooperation with industry partners. Meanwhile, scientists are also working on window units for ITER’s successor called DEMO, in which power will be produced from 2050 onwards. As a consequence of planned multi-frequency operation of the microwave heating system in DEMO, however, new types of gyrotrons will be required.
The disks are made of synthetic diamond by chemical vapor deposition (CVD), a special coating technique. The CVD diamonds grow on a silicon surface in a small vacuum reactor filled with a gas mixture. By means of microwave irradiation, this mixture is turned into a plasma, similar to what happens in a fusion reactor, but with much smaller energy consumption. The plasma consists of atomic hydrogen that prevents undesired graphite formation and a small amount of methane that supplies carbon for the diamond. Diamond only grows at few micrometers per hour so it takes weeks to reach the 2-millimeter thickness that is needed. The diamond disks will cost millions of dollars for the DEMO reactor.
Currently diamond disks have a polycrystalline structure but future disks could use monocrystalline diamond.
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