China fusion reactor breaks theoretical density limit

NUCLEAR physicists have exceeded a long-standing theoretical density limit at an experimental fusion reactor in China, suggesting that future power plants could generate more energy than previously thought.

Until recently, models predicted that the plasma fuelling fusion reactions would become unstable above a certain density known as the Greenwald limit. However, researchers at China’s Experimental Advanced Superconducting Tokamak (EAST), also known as the “artificial sun”, found that the plasma remained stable at densities up to 1.65 times the limit, provided interactions with the reactor walls were carefully controlled. The results were published in Science Advances this month.

Fusion power scales roughly with the square of density. If the EAST findings can be replicated elsewhere, future fusion plants could generate significantly more power than predicted by earlier models.

The EAST experiment validates a 2017 theoretical study by physicists in France that first challenged the Greenwald limit. The EAST team, which included more than 18 researchers from Huazhong University of Science and Technology, the Chinese Academy of Sciences and Aix-Marseille Université in France, used electron cyclotron resonance heating and ohmic startup to achieve “plasma-wall self organisation”. This allowed them to access a “density-free regime” without triggering plasma instability.

Chris Eaglen, vice-chair of IChemE’s nuclear technology special interest group, who was not involved in the study, said the findings could influence future reactor design.

“The Greenwald limit was treated as a hard operational ceiling for decades,” he said. “Fusion engineers have sized machines, fuelled plasmas and set safety margins around it.”

He added that the results from EAST show that “the limit is not a fundamental law, but a consequence of how plasmas are formed and interact with walls”, which “changes the design assumption”.

“It means that reactors may not need to be as large or as conservative in density assumptions,” Eaglen said. However, he cautioned that the breakthrough “improves confidence in future reactor designs rather than accelerating timelines”, stressing that the improved understanding of density is “not a shortcut to power-producing fusion”.

Fusion is still a long way from commercial reality. Key challenges include achieving net energy gain and sustaining stable plasma over long periods. Net energy was first demonstrated in 2022 at the Lawrence Livermore Laboratory in California, while the record for sustained plasma is currently 22 minutes, recorded last year at France’s WEST reactor.