Nuclear fusion: breakthrough with diamond balls from Freiburg – knowledge

The Company Diamond Materials you have to find it first. It is tucked away in an industrial area in the north of Freiburg, on the premises of a Japanese semiconductor manufacturer. When you finally get there and crane your neck in the conference room of the 30-employee company, you can spot the Black Forest in the distance. But Christoph Wild has no eyes for the Black Forest at the moment, he has a tiny ball with him, about two millimeters in size. He grabs it with tweezers. “It’s fascinating, the proportions,” says the managing director of the company, which has only existed since 2004 as a spin-off from the Fraunhofer Institute for Applied Solid State Physics.

He means the 192 lasers at the National Ignition Facility (NIF) in California – the most powerful in the world – and all the accessories made up of laser medium blocks, amplifiers and waveguides, which cover an area of ​​around 50,000 square meters. And all this just to blast a single tiny diamond sphere, like the one Wild is holding in his tweezers, with laser light and ignite a nuclear fusion reaction inside.

Inconspicuous, but technically perfect: Christoph Wild holds a diamond ball.

(Photo: Andreas Jäger)

In December at the NIF it was possible for the first time to extract more fusion energy from such an experiment than was put in with the laser. this was celebrated worldwide as a breakthrough. The dream of clean and practically unlimited fusion energy, despite all the delays and problems, suddenly seemed a little closer. The bullet for this came from Wild’s company in Freiburgby Diamond Materials.

The diamond balls are among the roundest objects that exist in the world

The enormous energy consumption of the lasers was not taken into account in the balance calculation of the Californian researchers, a real net energy gain was not even remotely achieved. Nevertheless, the experiment is considered a great success. For a long time, laser-based demonstration reactors, such as those at the NIF, fell short of expectations nuclear fusion want to ignite in a gas with a temperature of around one hundred million degrees Celsius in a magnetic cage. This includes, for example, the major international fusion project Iter in southern France.

In contrast to such reactors, what is known as inertial fusion is used in California. The fuel in the sphere, usually a mixture of the hydrogen isotopes deuterium and tritium, is heated up and subjected to enormous pressure at the same time, so that the positively charged atomic nuclei overcome their mutual repulsion and fuse to form a new, energetically more favorable nucleus becomes free.

Before the laser shot, the fuel is trapped in a container – the diamond ball. It may sound like expensive jewelry, but nothing sparkles here. Only a dark, metallic sheen emanates from the balls, reminiscent of ballpoint pens. However, the inconspicuous appearance is deceptive. diamondalso synthetic, as it is made in Freiburg, is an “extreme material with extreme applications,” says Wild.

At the NIF, the bullet was not fired directly, instead the lasers aimed at the inner walls of a cage that encloses it. The laser beams are thus converted into X-rays, which radiate evenly from the inner walls of the cage cavity onto the sphere in the center. The spherical shell – the diamond – vaporizes and implodes. Directed inward, the fuel is squeezed together under tremendous pressure and heated at the same time, the fusion reaction can ignite.

In order for everything to work like this, however, every detail must be perfectly balanced. “When the ball implodes, it is compressed to a ten-thousandth,” says Christoph Wild. Thanks to a special grinding process, the diamond balls from Baden are among the roundest objects that exist. How to do that? company secret. But what the physicist reveals: The process is not dissimilar to rolling lumps of dough between the palms of your hands.

The diamond is not mined, but extracted from a gas mixture

Perfection is necessary so that no deformations occur during the implosion. If the sphere is even slightly potato-shaped, the fuel will be compressed unevenly and not sufficiently compacted. Then the fusion burning cannot continue by itself from the center of the sphere outwards, so that in the end there is an energy gain.

Nevertheless, it sounds a bit exaggerated to burn diamonds of all things just like that. Aren’t the stones way too expensive? “You need a light element,” says Wild. Simulations and tests have shown that carbon is ideal. Diamond is specially arranged carbon that is normally formed in the earth’s crust under high pressure. Its crystal structure makes it the hardest natural substance there is. In addition, diamond conducts heat particularly well. The absorption properties can also be tailored by introducing foreign atoms so that the X-rays are absorbed particularly well.

However, no natural diamonds from a mine are processed in Freiburg. The company’s various products, including spheres or discs called “wafers” for other uses, are man-made CVD diamonds. “CVD” stands for chemical vapor deposition, which can be translated as “chemical vapor deposition”. CVD diamond is not pressed from graphite as it is in nature, but is obtained from a mixture of hydrogen gas and methane, with the methane supplying the required carbon atoms. The reaction takes place in an egg-shaped container. egg shaped? “Ellipsoid of revolution!” corrects Wild.

Energy: The do-it-yourself look is deceptive: the artificial diamonds are produced in such reactors.

The do-it-yourself look is deceptive: the artificial diamonds are produced in such reactors.

(Photo: Andreas Jäger)

It takes two months to produce a batch of around 20 diamond balls. Gradually, layer by layer, each diamond grows on a silicon core, the substrate. The silicon framework is later etched out again through a tiny, two-micrometer hole, and only then is the diamond sphere hollow on the inside. During laser fusion, the fuel is filled into the ball through the same hole.

There is still a long way to go to a laser-driven fusion power plant

However, diamond balls are unlikely to be used in a possible fusion power plant. If you want to scale up laser fusion, says Christoph Wild, even artificial diamonds are far too expensive. Hollow plastic spheres could be an alternative if they collapsed symmetrically like diamonds. They don’t do that yet.

Apart from that, there are other hurdles in the way of a laser-driven fusion power plant, such as the fact that the hydrogen isotope tritium for the fuel would have to be produced at great expense; a magnetic confinement reactor could breed it itself, at least in theory. In addition, the high-power lasers at the NIF currently have to cool down for hours before they can fire again. However, a power plant could only be operated at a rate of several shots per second.

Another German company wants to solve all these problems: Marvel Fusion. The Munich start-up is planning a laser-driven fusion power plant. In contrast to most competing systems, Marvel Fusion does not want to fuse nuclei of the hydrogen isotopes deuterium and tritium, but rather an ordinary hydrogen nucleus, i.e. a proton, with a boron nucleus, resulting in three helium nuclei. This offers – at least theoretically – the advantage that no neutrons are released, which over time turn the fusion reactor walls into radioactive emitters. However, an even greater mutual repulsion of the nuclei must be overcome. The concept envisages bringing the fuel mixture into the laser chamber in the form of pellets, ten pieces per second – without any encapsulation.

Either way, a fusion power plant remains a long way off. On the way there, however, a small Freiburg company, if you will, with its product, was the center of attention for a brief moment.

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