China's Fusion Reactor Breaks Plasma Limit
- Jenner Nex
- Jan 14
- 3 min read
EAST Fusion Facility Exceeds Fusion Plasma Density Limit
New Plasma Regime: The Chinese fusion reactor EAST has overcome a crucial density limit for fusion plasma, achieving the so-called density-free regime for the first time. This allows the plasma to be compressed to a higher density without dangerous instabilities occurring near the reactor vessel walls. The EAST fusion facility thus paves the way for the extreme conditions necessary for nuclear fusion.
Nuclear fusion is considered a potential energy source of the future. Test reactors such as JET, Wendelstein 7-X, and the large-scale ITER reactor are testing various technologies for this purpose. However, to fuse enough atoms in the fusion plasma, extreme temperatures of more than 100 million degrees and a high plasma density are required. Only then does a chain reaction begin, in which a fusion reactor generates more energy than it requires to input.
Greenwald Limit and Plasma Instabilities
However, there is a problem: In tokamak-type fusion reactors, which confine their plasma with strong magnetic fields, instabilities occur above a certain density. This creates turbulence and eruptions at the outer edge of the plasma, which can disrupt nuclear fusion and even damage the reactor. "Achieving operation with plasma densities above this so-called Greenwald limit is therefore a challenge for magnetic confinement reactors," explain Ping Zhu of Huazhong University in China and his colleagues.
While some fusion reactors have already briefly exceeded this Greenwald limit, including the ASDEX test reactor in Garching, Bavaria, and two tokamak test reactors in the USA, this was achieved primarily through optimization of the magnetic confinement and the plasma density at the edge of the hot fusion fuel. However, it has not yet been possible to keep the plasma stable at increasing density and temperature, thus achieving a so-called density-free regime.
Beyond the Density Limit
Zhu and his colleagues have now succeeded in doing just that at the EAST fusion test reactor in Hefei, China. As early as 2025, the team set a record by keeping the fusion plasma in so-called high-confinement mode for 1,066 seconds. In this state, the plasma forms a particularly dense, compressed layer on its outer surface.
Now, the physicists have further optimized the interaction of the fusion plasma with the reactor wall. This enabled them to exceed the Greenwald limit and, for the first time, achieve a density-free regime. "We achieved an average electron density of 1.3 to 1.65 times the Greenwald limit. This is significantly higher than the normal operating mode of EAST," report Zhu and his team. Despite this, the fusion plasma remained stable, and there were no eruptions. These high electron densities were significantly higher than the normal operating mode of EAST.
Optimization of Gas Pressure and Cyclotron Heating
This was made possible by adjusting the pressure of the hydrogen gas and optimizing plasma heating using electron cyclotron resonance heating (ECRH). This significantly reduced the interactions between the reactor wall and the plasma, especially during the start-up phase. As a result, energy losses were reduced, impurity accumulation was minimized, and the plasma density could be increased without significant side effects.
"Our results point to a practical and scalable way to overcome the density limits in tokamaks and next-generation fusion devices," says Zhu. He and his team plan to test this methodology next during the high-confinement mode of their fusion reactor. (Science Advances, 2026; doi: 10.1126/sciadv.adz3040)
Source: Chinese Academy of Sciences
