At the experimental level, researchers have succeeded in breaking through two significant obstacles in the field of nuclear fusion, and if they can reproduce their results on a large scale, it could lead to commercial and efficient fusion reactors, writes New Scientist.
For the optimal energy production of the nuclear fusion reaction, two more things were needed: to increase the plasma density and to contain the denser plasma. Researchers have now achieved this in an experimental tokamak reactor, but we are still years away from a commercial reactor operating.
Tokamaks are the most common type of fusion reactor: a device that can store high-temperature plasma, hotter than the surface of the Sun, in a donut-shaped magnetic field created by an electromagnet.
Until now, scientists believed that there were limits to increasing the plasma density: if they went above the so-called Greenwald limit, the plasma would escape from the magnetic field and damage the reactor. Increasing density, on the other hand, is a very important factor in performance. It has been shown that the power of tokamak reactors increases in proportion to the square of the density of the fuel.
According to the new results, the Greenwald border can also be crossed safely. In San Diego, the research team led by Siye Ding operated the tokamak reactor at the DIII-D National Fusion Facility for 2.2 seconds at an average density exceeding the Greenwald limit by 20 percent, so that the system remained stable. They were not the first to cross the Greenwald limit, but the previous experiment lasted less time and was less stable.
DIII’s tokamak chamber – Photo: Wikipedia
Gianluca Sarri, a researcher at Queen’s University Belfast, said that reactors are increasingly able to show stable operation and consistently reach the optimum point required for this. According to him, if the results of the small reactor can be extrapolated to a larger machine, they can achieve a significant increase in performance.
Sarri said that in the DIII-D experiment, they did not use their new approach, but they mixed the existing ones very well. The team used a higher density in the core of the doughnut-shaped plasma to increase power, while allowing it to fall below the Greenwald limit at the near edges of the storage chamber, thus preventing the plasma from escaping. In addition, deuterium gas was injected into the plasma to dampen the reactions in certain locations.
The outer radius of the DIII-D plasma chamber is only 1.6 meters, and it is not yet known whether the same method would work for the next generation of tokamaks under construction in France, ITER, which will have a radius of 6.2 meters and is expected as early as 2025 will produce plasma.
“These plasmas are very complicated. A small change in circumstances results in a big change in behavior. Experimentally, it was more of a “trial-and-luck” approach, where we tried many different configurations and basically saw which one worked best. It’s all about forcing the plasma to do something that’s completely against its nature, which it doesn’t really want to do,” Sarri said.
According to Ding, the experiment bodes well for the future of fusion energy. “Many reactor designs require high containment and high density at the same time. Experimentally, this is the first time this has been realized. The next step is expensive, and research is currently moving in many directions. I hope this study will help coordinate efforts worldwide.”
According to Sarri, another step has been taken towards a practical fusion power plant, but no one should expect to see a commercial reactor in the next five or ten years.
Fusion energy, which for a long time was only a topic of science fiction, has now become a simple technical problem, so according to experts, the question today is not whether fusion energy is feasible, but how it will be cheap enough.
