American physicists reached a milestone on their way to using nuclear fusion: they used lasers to ignite a fusion reaction that was largely self-sufficient.
Christian J. Mayer
In the town of Livermore near San Francisco, physicists want to realize what many consider a dream: bringing the sun’s energy source to Earth. In the depths of the star, atomic nuclei of hydrogen fuse to form helium atomic nuclei, releasing the large binding energy of the new nucleus and making the sun shine. If fusion power plants could be built on Earth, humanity would potentially be energy safe, with virtually no emissions.
Scientists led by Alex Zelstra of Lawrence Livermore National Laboratory (LLNL) are now as close to this goal as you are. It was reported in the scientific journal “Nature”. With a laser pulse they ignited a nuclear fusion that continued to fuel itself. Physicists say nuclear fusion fuel is “burning”. Then much less heat must be provided from outside to maintain fusion. Only then can you extract more energy from nuclear fusion than you put into it – it becomes conceivable to build power plants.
Atomic nuclei are attracted to each other almost naked
This work is an “important milestone,” comments Marcus Roth of the Technical University of Darmstadt, who has conducted research in the field for 25 years. However, the current experience is only one building block in fusion research. More will follow before the technology is used to generate power.
Because nuclear fusion takes a lot. The pressure in the center of the Sun is 200 billion times the pressure at the Earth’s surface, and the temperature is 15 million degrees. In these extreme conditions, hydrogen atoms lose their electrons and form what is called a plasma. In it, the atomic nuclei revolve around the bare, so to speak. If two of them collide fast enough, they can fuse into a helium nucleus.
At a small point, conditions such as those in the interior of the Sun prevail for milliseconds.
There are two ways to recreate this on Earth. If you dispense with the high pressure, you would have to heat the plasma much more than the sun to ignite the fusion. Since no material can withstand this, physicists try to keep the plasma suspended in a magnetic field. Research reactors such as ITER at Cadarache in southern France and Wendelstein 7-X at Greifswald in Germany are working on this highly complex task.
Researchers at LLNL’s National Ignition Facility do it differently. In addition to high temperature, they also generate extremely high pressures in the plasma. To do this, they fill a capsule the size of a pinhead with hydrogen and use a powerful laser to suddenly heat it up, causing the capsule shell to explode. According to the principle of recoil, the hydrogen inside the ball explodes so violently that it is strongly compressed into the center of the ball. For milliseconds, conditions are in a small spot like those in the interior of the Sun, so plasma forms and nuclear fusion occurs.
This principle, called inertial fusion, has been tried for a long time. But so far, plasma cannot be made to burn. The team around Alex Zylstra has now achieved this breakthrough: the fusion heated the plasma ten times as much as the laser.
However, it is still not possible to produce energy in this way. There is still a lot of energy losses in the overall system for that. For example, it is not possible to focus all the laser energy on the capsule. Another part of the energy is consumed by the explosion of the projectile. Overall, the fusion energy generated was only ten percent of the total energy provided by the laser. In a subsequent experiment, the researchers were able to increase this percentage to seventy percent. So you are very close to the point where the fusion energy exceeds that of the laser.
With more efficient lasers, the target gets closer
A German company made a decisive contribution to the success. Heidelberg diamond materials supplied the diamond spheres in which hydrogen fills. “The requirements for capsules are very high,” says Christoph Wild, one of the managing directors, who is very pleased with the research’s success. The material must absorb laser radiation as completely as possible and the capsules must be polished to a precision of a few nanometers (billionths of a meter). In a future power plant, a stream of these beads could flow through the irradiated region by the laser, so that fusion energy would be produced continuously. This will heat the water and the steam will power the generators.
Fusion expert Markus Roth of the Technical University of Darmstadt praised the work from California, in which he was not involved: “a great experiment to understand physics,” says the physicist. The scientific results are very important for the further development of inertial fusion. Roth might benefit from this himself: he’s a co-founder of German-American startup Focused Energy, which wants to develop an inertial fusion power plant. In relatively small companies, he sees the task of translating search results into practice.
Because they can adapt new technologies more easily, Roth says, such as efficient lasers. The laser system life of the National Ignition Facility is 13 years. It requires 200 times the power of the laser light itself. Such a disparity is out of the question for a fusion power plant. “But now there are more efficient lasers,” Roth says. Development is going fast. If things continue like this, Roth estimates, nuclear fusion could provide energy within 10 to 15 years.