Chinese nuclear fusion reactor surpasses Greenwald plasma density limit

Scientists operating the EAST tokamak report plasma densities 30%–65% above the long‑accepted Greenwald limit, using high‑power microwaves and increased neutral‑gas injection to reduce impurities; the results were published in Science Advances on 1 January.

Researchers report that China's Experimental Advanced Superconducting Tokamak (EAST) has operated plasmas at densities beyond a widely accepted ceiling for tokamak devices. That ceiling, known as the Greenwald limit, has long constrained tokamak operation. The team published its findings in Science Advances on 1 January. Their method combined higher‑efficiency heating and increased neutral‑gas injection to limit impurities and help the plasma remain stable. Key findings: - EAST, a tokamak in Hefei, reported plasma densities 30% to 65% higher than those normally reached by the device, surpassing the Greenwald limit. - The results were published on 1 January in Science Advances and are described as experimental demonstrations rather than final proofs of sustained fusion energy production. - Researchers used high‑power microwaves to heat the initial fuel more efficiently, which reduced the number of metal atoms eroded from the inner wall and lowered plasma impurities. - The team also injected larger amounts of neutral gas to provide additional fuel for high densities and to cool the plasma edge, further limiting impurity production. - The paper builds on a 2021 proposal by Dominique Escande and colleagues that certain wall–plasma conditions could be mutually stabilising, allowing densities above the Greenwald estimate. - The article and invited experts note possible application of the approach to other tokamaks and to ITER, and the EAST team intends to propose repeating the experiment at ITER in France. Summary: Experts described the work as showing a route to higher‑density tokamak operation without relying solely on stronger confinement magnets. Researchers reported that the next technical steps are to combine high‑density operation with advanced stability‑control techniques and to test the approach on other devices; the EAST team plans to seek a repeat at ITER. Broader achievement of the conditions needed for a self‑sustaining, burning plasma remains to be demonstrated.