China’s nuclear fusion reactor pushes plasma beyond critical limit: what happens next?

Researchers working on an “artificial sun” in China report breaking a long-accepted threshold that has limited the operation of nuclear fusion reactors for decades.

The Experimental Advanced Superconducting Tokamak (EAST) reactor in China is a nuclear fusion research reactor in Hefei. Researchers hope to one day produce clean, virtually unlimited energy by replicating the fusion processes that power the sun.

In fusion reactors, lightweight atoms are compressed under intense pressure and heat to form heavier atoms. This process releases energy, but must be carefully optimized so that the reactor produces more energy than it consumes.

One promising reactor design, the tokamak, confines the plasma inside a donut-shaped chamber using magnetic fields. The plasma is then heated. To sustain fusion reactions, the plasma must reach an extremely high density, which means many molecules must be packed into a small volume.

But researchers believed that plasma could not exceed a certain density without becoming unstable. This upper limit – known as the Greenwald limit – has been a major obstacle to fusion research, especially for tokamak-type devices.

In a paper published on January 1 in sciences predecessor¹Scientists working on China’s EAST device report pushing plasma densities beyond that limit, achieving densities 30% to 65% higher than those normally achieved by EAST.

“These results are very promising and should be explored in other tokamak devices,” says Jeronimo Olaya, a fusion plasma physicist at the French Commission for Alternative Energies and Atomic Energy in Saint-Paul-les-Dorance.

Less impurities

In 2021, study co-author Dominique Escand, a plasma physicist at the University of Aix-Marseille in France, and his colleagues first proposed² The Greenwald limit can be exceeded by adjusting conditions such that the plasma and the inner wall of the reactor are in a stable and mutually enhanced state.

The EAST team used high-powered microwaves to raise the temperature of the primary fuel used to generate the plasma in a way that is more efficient than traditional methods. This reduced the number of metal atoms that fell from the tokamak’s inner walls and mixed with the plasma. Less impurities means less unwanted radiation, which helps the plasma remain stable even as its density increases.

The researchers also injected a large amount of neutral gas into the chamber. This provided more fuel for the plasma to reach higher densities later in the experiment, while simultaneously cooling the area near the walls and reducing impurity production.