We are talking about a milestone in the civilian use of nuclear fusion: In the USA, researchers have succeeded for the first time in using a fusion process to generate more energy than they put into it. The method is laser fusion. But what is that? And is it good for a power plant?
For the first time, scientists in the USA have succeeded in obtaining more energy from controlled nuclear fusion than was required for ignition. After around 70 years of research, an important milestone in research has been reached, which raises hopes that fusion energy can be used for civil purposes on earth. This opens up the prospect of an almost unlimited and CO2-free energy source. The breakthrough came at the National Ignition Facility (NIF) at Lawrence Livermore National Laboratory in California using laser fusion. But what is it really?
In a nuclear fusion, atomic nuclei fuse together – hence the name. This process releases an incredible amount of energy. This process also makes the sun shine – inside it, hydrogen atoms fuse together to form helium. Light and heat from the sun are the basis of life on earth.
Generating nuclear fusion in technical facilities on earth is an extremely tempting prospect. One gram of hydrogen contains as much energy as eleven tons of coal. But the undertaking is extremely difficult: researchers have been trying to fuse atomic nuclei together since the 1950s. The two most suitable types of hydrogen are deuterium and tritium. However, extreme conditions are required in fusion reactors to fuse the fuel: it has to be very hot and dense at the same time.
There are different approaches to technical nuclear fusion: The most common are magnetic confinement and inertial fusion, which also includes laser fusion. In magnetic confinement, hydrogen plasma is confined in a magnetic field and heated to such an extent that the atomic nuclei fuse with one another. The planned ITER experimental reactor in France is based on the principle, as is the Wendelstein 7-X experimental reactor in Greifswald. This type of fusion is also the best researched so far.
Inertial Fusion works a little differently. Here, the hydrogen is extremely compressed and heated for only a very short time. The atomic nuclei fuse and the inertia of the mass keeps the plasma from immediately flying apart again – hence the name.
It’s the same principle as with hydrogen bombs. But in civil projects, the amount of fuel per ignition is much lower: It is usually small, about the size of a pinhead, called pellets. Tiny nuclear bombs, if you will. They are made of beryllium or plastic. They contain a mixture of deuterium and tritium. During laser fusion, the fuel in these pellets is compressed and heated with powerful laser beams. The atomic nuclei fuse and inertia helps them.
There are two different types of laser fusion: direct and indirect. The indirect variant was used for the breakthrough at the NIF. The laser beams are not aimed directly at the pellet, but at the edge of a small cavity in which it is located. This creates X-rays, which evenly fill the cavity. The energy of the X-rays arriving from all sides causes the pellet to implode and the fuel is compressed and heated to such an extent that the atomic nuclei fuse – a mini hydrogen bomb is ignited.
A huge system is used for this at the NIF in California – it is the largest laser in the world: 192 laser beams are directed at the target, causing a temperature of 100 million degrees and a pressure of 100 billion earth atmospheres to develop in the pellet.
But this indirect laser fusion is very inefficient. At the NIF, too, 500 megajoules of energy had to be fed in to generate the laser beams in order to finally direct around 2 megajoules onto the target. However, 2.5 megajoules were released in the process – the first excess energy in fusion research. If you calculate the total energy consumption of the system, however, only 0.5 percent of the energy input could be recovered.
According to experts, direct laser fusion is therefore considered more promising. As the name suggests, the pellet is directly irradiated with the fuel from lasers and thus caused to implode. However, this method is much more prone to instabilities. Years of research are expected to avoid this. It is not certain whether it will succeed.
According to Sibylle Günter, Scientific Director at the Max Planck Institute for Plasma Physics in Garching, there is another “efficiency problem” with laser fusion. It is much more efficient to compress cold matter than hot. “Here, too, the community agrees that on the way to a power plant, you first have to compress the relatively cold matter to the required density and then find a way to increase the temperature in the center of the pellet,” says Günter. There are initial ideas for this, but that is difficult.
Another problem with a laser fusion power plant: it previously took several days to position a pellet for an ignition attempt. “A power plant would have to do this more than ten times a second. That’s another challenge,” says Günter. Further challenges are the production of the fuel tritium, a type of hydrogen that is very rare in nature, but which can be obtained artificially from lithium.
