China's EAST Fusion Reactor Reaches Plasma Density Above Previous Limit
Chinese researchers at the EAST tokamak achieved a theoretically predicted regime with plasma densities above the previous limit for the first time.
(Image: HFIPS)
Chinese scientists have reportedly achieved a theoretically predicted operating regime in the EAST (Experimental Advanced Superconducting Tokamak) fusion reactor, also known as the "artificial sun," for the first time, where the usual density limit no longer applies. They achieved a plasma density that is 30 to 65 percent above the so-called Greenwald limit – without the plasma becoming unstable. According to the team's account, this marks the first experimental proof of the "density-free regime," as the researchers report in the journal Science Advances.
Why Density is Crucial for Fusion
In nuclear fusion, light atomic nuclei – typically the hydrogen variants deuterium and tritium – are fused into heavier helium nuclei. This process releases enormous energy and also powers our sun. For the reaction to work in a reactor on Earth, the plasma must meet three conditions: extremely high temperatures of over 100 million degrees, a sufficiently long confinement time, and the highest possible density. The more particles come together in a confined space, the more frequently they collide and fuse – and the more energy is released. In fact, fusion power scales with the square of the fuel density.
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The Greenwald Limit as an Obstacle
In tokamaks – toroidal reactors that confine plasma with strong magnetic fields – there is an empirical upper limit to the achievable plasma density. This limit, named after the US physicist Martin Greenwald, describes the point at which the plasma becomes unstable and collapses. Such "disruptions" can damage the reactor and must be avoided. The usual operating range at EAST, according to the researchers, is between 0.8 and 1.0 of the Greenwald density.
While the Greenwald limit has been exceeded in previous experiments – for example, at the tokamaks ASDEX Upgrade and DIII-D, where densities up to about 1.5 times the limit were achieved using pellet injection. However, what is new about the EAST experiments is the method and the theoretical framework.
Comparison with Previous Experiments
In May 2024, a team at the US DIII-D tokamak exceeded the Greenwald limit by 20 percent – but only for 2.2 seconds. According to the published data, the Chinese experiments at EAST achieved plasma discharges lasting about 6 to 7 seconds – about three times longer. Furthermore, the achieved density, up to 65 percent above the Greenwald limit, significantly surpassed the value of the US experiments.
This combination of higher density and longer duration suggests that the method enables a more stable operating mode. This would be crucial for future fusion power plants: they would need to be able to maintain high densities for minutes or even hours.
Microwave Heating and Higher Gas Pressure as Keys
The team at EAST in Hefei combined two measures during the reactor's startup phase: Firstly, the researchers used so-called electron cyclotron resonance heating (ECRH) – which involves injecting microwaves that specifically heat the electrons in the plasma. Secondly, they increased the initial pressure of the deuterium fuel gas.
This combination led to a cooler edge region of the plasma at the reactor walls. This is crucial: at lower temperatures, fewer atoms are knocked out of the wall material – in this case, tungsten. These impurities would otherwise enter the plasma and cool it down by radiating energy. By optimizing the plasma-wall interaction from the outset, impurities and energy losses were reduced. With a cleaner plasma, the discharge remained stable, even though the density reached 1.3 to 1.65 times the Greenwald limit.
Theory of French Physicists Confirmed for the First Time
According to the authors, the experiments confirm a theory on "plasma-wall self-organization" (PWSO) developed in 2022 by physicists including Dominique Escande from the French research center CNRS and the University of Aix-Marseille. This theory distinguishes between two operating regimes: In the classical "density limit regime," the plasma temperature at the reactor walls is relatively high, leading to greater erosion of wall material – here, the known Greenwald limit applies. In the so-called "density-free regime," on the other hand, the wall temperature is so low that hardly any material is eroded, and the plasma remains clean. The term "density-free" does not mean that the plasma has no density, but rather that there is no longer a practical density limit – the theoretical upper limit shifts to extremely high values.
According to the theory, the use of high-temperature metals like tungsten as wall material is crucial for accessing the density-free regime: the heavy metal emits very few particles into the plasma at low temperatures, unlike, for example, carbon. The EAST experiments have now experimentally confirmed this concept for the first time, according to the researchers.
The EAST reactor had already set a record in January 2025: at that time, it was possible to stably maintain a plasma with over 100 million degrees Celsius for almost 18 minutes. The results on the density limit now published come from a different series of experiments and address a different aspect of fusion research: while the January record maximized confinement time, the current focus was on achieving the highest possible density – two parameters that have previously been mutually restrictive.
Significance for Future Reactors
The results could show a way to increase the performance of fusion reactors. In principle, the method could also be transferred to other facilities – including the international research reactor ITER, which is currently being built in southern France and in which China is also involved. In Germany, the Max Planck Institute for Plasma Physics operates two important experimental facilities: the tokamak ASDEX Upgrade in Garching and the stellarator Wendelstein 7-X in Greifswald.
As a next step, the Chinese researchers intend to test the method in the so-called H-mode – a particularly efficient operating mode in which the plasma is confined by an edge barrier and loses less energy. "We plan to apply the new method soon during high-confinement operation at EAST to achieve the density-free regime under high-performance plasma conditions," said Associate Prof. Yan. If successful, it would be another step on the long road to commercial nuclear fusion.
(vza)
