"The perfect diamond sphere requires a great deal of expertise and a complex process“

Dr. Christoph Wild and Tobias Fehrenbach from Diamond Materials produced the perfect diamond sphere for the successful nuclear fusion experiment at the Lawrence Livermore National Laboratory. In this interview, they talk about their beginnings in diamond research, the challenges of producing diamond spheres and their collaboration with LLNL.

How did your enthusiasm for diamond begin?

Wild — During my diploma thesis at Fraunhofer IAF, I had already worked on amorphous diamond-like carbon layers. At the same time, the first reports on the deposition of crystalline diamond layers appeared. We were very excited by this and started to carry out initial experiments. I still remember the moment when we saw the first diamond layers under the microscope and my colleagues could not believe it when I told them: “These are diamond crystals.” In the 1990s, there was a real diamond hype worldwide. The extraordinary properties of diamond were well known and the deposition of diamond in layer form on disks made it clear that many new applications would emerge.

Fehrenbach — When I came to diamond, its research was already well established. I studied geosciences and specialized in crystallography. For me, it is very exciting that this kind of technical application is possible with a material that has its origins in nature. Having already worked a lot with silicon at Fraunhofer ISE, I was very interested in working with diamond. There is still a lot of research potential there.

Mr. Wild, what was your vision when you founded Diamond Materials?

Wild — At Fraunhofer IAF, we worked intensively on the further development of diamond technology. We successfully realized new reactor concepts, such as the ellipsoid reactor, and developed various processing methods. This enabled us to sell our first diamond products at the beginning of the 2000s. We founded Diamond Materials to manufacture and market diamond products and thus meet customer demands in the long term. As a scientist, it is of course particularly exciting to put the results of the own research into practice. This year we are celebrating our 20th anniversary!

In the beginning, did you realize that diamond could play a role in nuclear fusion research? 

Wild — No, not at all. But meanwhile, we even have two points of contact with nuclear fusion: firstly, of course, through the diamond spheres as targets in inertial confinement fusion and, secondly, we manufacture diamond windows for coupling the microwave into the plasma reactor in the ITER project in southern France, which is working on the concept of magnetically confined fusion plasma.

How did the collaboration with the Lawrence Livermore National Laboratory come about?  

Wild — I met Jürgen Biener at a diamond conference in Japan in 1994. He then went to LLNL and called me about ten years later to ask if we could make hollow spheres out of diamond. Then, LLNL was looking for an ideal target material for carrying out nuclear fusion experiments. The material had to have the lowest possible atomic number and a high density. At that time, people mainly worked with beryllium and plastic; the diamond spheres were more of a backup option. However, it turned out that significantly better results could be achieved with diamond. So we very quickly developed the basis for production and received the Fraunhofer Prize for this work in 2006. 

Of all things, why is diamond suitable for this nuclear fusion experiment? 

Fehrenbach — In the experiment, the laser light is converted into X-rays, which cause the sphere to implode. Therefore, a high-density material with a low atomic number is required that is also transparent to the radiation. Diamond also has the advantage of being able to incorporate foreign atoms. The inner layers of the diamond spheres are doped with tungsten — this allows the X-ray absorption to be adjusted in a targeted manner

Diamond wafers are grown layer by layer on top of each other. How do you produce hollow spheres?

Wild — For the production of diamond hollow spheres, silicon spheres are coated with several diamond layers. The individual layers differ, for example, in their doping. Finally, a tiny hole of just a few micrometers is lasered into the coating and the silicon core is etched out. The targets for the experiments at LLNL have to be perfect diamond hollow spheres, which is of course a challenge. 

And how do you achieve the perfect diamond hollow sphere?

Fehrenbach —The most important thing is an exactly homogeneous layer thickness distribution. As the layers are deposited from above, the spheres in the reactor have to be moved a lot. But everything rolling can have small defects and every irregularity, no matter how small, has an effect on the shape of the sphere and therefore on the entire experiment. That is why we have gathered a great deal of know-how and developed complex processes to produce diamond spheres with layers that are as homogeneous as possible. Our partners at LLNL give us very specific guidelines for each shift. We get to 90% quite quickly, but the last bit to perfection is still a lot of work. 

How did you perceive the success of the fusion experiment?

Wild — Of course, we were quite euphoric. The breakthrough had been hinted at a few months earlier, but this is a huge achievement: the scientists at LLNL worked for 30 years on the successful demonstration and finally achieved their goal in December 2022. And the use of the diamond sphere as the target was a key factor in the experiment. This effort is not economical for practical energy production, but these experiments are helping to gain knowledge and generate ideas that will form the basis for future nuclear fusion technology.

Diamond as the basis for nuclear fusion, for quantum technologies — in which other areas will diamond be indispensable?

Wild — Diamond is used wherever its outstanding properties can be utilized — for example, where high performance is required, where other materials would melt or where its extreme hardness is needed. These are often very specialized applications where the benefits outweigh the costs, and new and sometimes surprising applications are constantly being added, for example in medicine. Currently, we are working together with Fraunhofer IAF on the development of diamond substrates for quantum technology applications in the GrodiaQ project and both sides benefit greatly from the exchange.