The picture shows a polycrystalline CVD diamond disc for use in nuclear fusion reactors and gyrotrons. Image source: Karlsruhe Institute of Technology

Because nuclear power can provide people with an environmentally friendly energy source, scientists around the world are working hard to achieve this goal. In this highly sophisticated field of research, there is a little-known material, diamond, which is actually an indispensable material in nuclear fusion technology. Recently, researchers at the Karlsruhe Institute of Technology (KIT) have developed a diamond disc for heating raw materials into a plasma state in a nuclear fusion reaction. Working with a company called Diamond Materials, they have been able to produce a diamond disc with a diameter of 180 mm.

During the nuclear fusion reaction, hydrogen atoms and helium atoms fuse and release huge energy. If it is applied to a nuclear power plant, it will one day contribute to a sustainable and safe energy supply for humans. At the Karlsruhe Institute of Technology, researchers developed so-called gyrotrons for the ITER reactor, such as Wendelstein 7X and ASDEX Upgrade. The gyrotron is a microwave oscillator that produces up to 150 million degrees Celsius in the reactor, similar to a huge microwave oven. This high temperature allows the helium fuel to reach the plasma state required for melting. In order to direct the microwave radiation from the gyrotron into the plasma and keep the radioactive cesium in the reactor under vacuum, Dr. DITK Strauss and Professor Theo Scherer of the Institute of Applied Materials Research (IAM) of Karlsruhe Institute collaborated to design A suitable window unit. In this extreme environment, there is only one material that can do the heavy lifting, and that is diamond, Dr. Dirk Strauss said. Since it has not been found that other materials can exist under such extreme microwave radiation, even if it can be stably present, it does not necessarily have excellent permeability and low loss like diamond.

In order to introduce more than one megawatt of electrical radiation into the ITER reactor, the Karlsruhe Institute of Applied Materials has designed a number of diamond discs in collaboration with industrial partners. At the same time, scientists are still developing window units for ITER's second-generation DEMO, which is expected to be available in 2050. However, due to the need for microwave multi-frequency heating in DEMO reactors, a new type of gyrotron is also urgently needed. Currently responsible for this work is Professor John Jelonnek of the Institute of Pulse Power and Microwave Technology at the Karlsruhe Institute of Technology. And these new gyrotrons will require window units with larger diamond discs, and fortunately, the corresponding prototypes have been made. “Our diamond discs are 180 mm in diameter and 2 mm thick,” says Professor Theo Scherer. “This makes it the largest synthetic diamond structure ever.”

The diamond disc is made by a special coating technique, chemical vapor deposition (CVD). That is, the diamond is deposited on the surface of the silicon in the vacuum reactor in a gaseous environment for growth. By microwave radiation, the diamond becomes a plasma state, similar to the fusion reaction occurring in the reactor, but consumes much less energy. The plasma contains many hydrogen atoms, which can effectively inhibit the formation of graphite. “This is a very time consuming and very complicated process,” said Professor Dirk Strauss. “Because diamond discs only grow a few microns per hour, the final product becomes quite expensive.”

However, the technology for applying diamond materials in nuclear reaction technology has not been exhausted. So far, diamond discs with polycrystalline structures have been designed in IAM reactors. “At the moment, we are also working on the development of single crystal diamond discs,” Theo Scherer said. “This may further reduce losses during microwave transmission.”

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