Scientists at the Institute of Plasma Physics under the Chinese Academy of Sciences (ASIPP) in Hefei, Anhui Province said experiments at the Experimental Advanced Superconducting Tokamak (EAST) had achieved a plasma density that was previously thought impossible. Using a new process called plasma-wall self organisation (PWSO), the researchers were able to keep the plasma stable at unprecedented density levels. They said that, by pushing plasma density well past long-standing empirical limits, fusion ignition can be achieved with far higher energy outputs. “The findings suggest a practical and scalable pathway for extending density limits in tokamaks and next-generation burning plasma fusion devices,” said Professor Ping Zhu from Huazhong University of Science and Technology, who co-led the research.
Professor Zhu’s team now plan to apply this new method at the EAST reactor to confirm that it will work under high-performance plasma conditions. The latest breakthrough was reported in Science Advances (Vol. 12 Issue 1, January 2026) in a study titled Accessing the density-free regime with ECRH-assisted ohmic start-up on EAST.
This work was a collaborative effort by ASIPP, Huazhong University of Science & Technology, and Aix-Marseille University, supported by the National Magnetic Confinement Fusion Project. Since it began operation in 2006, EAST has been an open test platform for Chinese and international scientists to conduct fusion-related experiments and research.
The researchers explained that high plasma density operation is crucial for a tokamak to achieve energy breakeven and burning plasma. However, there is often an empirical upper limit of electron density in tokamak operation, known as the Greenwald density limit, above which tokamaks generally disrupt. “Achieving high-density operation above the density limit has been a long-standing challenge in magnetic confinement fusion research.” Recent experimental results on EAST achieved line-averaged electron density in the range of 1.3 to 1.65, which was significantly above the typical EAST operational range of 0.8 to 1.0.
These experiments were shown to operate in the density-free regime first predicted by a recent plasma-wall self-organisation (PWSO) theory. “These results suggest a promising scheme for substantially increasing the density limit in tokamaks, a critical advancement toward achieving burning plasma,” the researchers said.
The PWSO theory describes a physical mechanism in magnetic confinement fusion devices where the plasma and the reactor wall interact in a self-organizing manner, primarily affecting the plasma’s density limit and impurity levels. The theory predicts a radiative density limit where the amount of radiation becomes unstable as density increases.
The theory also predicts two possible operating regimes: the normal regime, which has a hard density limit; and a “density-free” regime. This is potentially achievable with specific start-up conditions such as electron cyclotron resonance heating (ECRH) assistance and metallic wall materials, where impurities collapse and the density limit is effectively pushed higher.
In particular, the power-dependent scalings of density limit derived from this theory have been supported by experimental results in multiple tokamak devices. In addition, a relatively higher density limit is reached in stellarator when the start-up is performed by using higher ECRH power. Both PWSO theory and stellarator results were an incentive to perform experiments in Joint Texas Experiment Tokamak (J-TEXT) at Huazhong University of Science and Technology which directly validated key aspects of PWSO theory. Unlike EAST, which has metallic walls (plasma-facing components) J-TEXT has carbon walls.
The carbon walls facilitate research into the interaction between the hot plasma and the surrounding material surfaces. “These experiments demonstrate that increasing ECRH power or prefilled gas pressure enhances the achievable density limit by decreasing impurity radiation and increasing target region plasma temperatures but by remaining in the density limit basin of PWSO theory.”
Building on the success of these experiments, the EAST tokamak provided a unique platform to extend the validation of the PWSO model. “With the tungsten plasma-facing components, the EAST tokamak allows the exploration of PWSO-induced density limits under conditions distinct from those of J-TEXT. Whereas with carbon walls, the chemical sputtering is important, and for tungsten walls, the physical sputtering dominates, which might enable reaching the density-free basin predicted by the PWSO theory.”
Chemical sputtering is where the plasma interacts with the carbon to create impurities. Physical sputtering in a tokamak is where high-energy plasma particles collide with the reactor’s inner wall materials, ejecting wall atoms (impurities) into the plasma.
In the latest EAST experiments, by controlling the physical conditions of the target plate, tungsten impurity-dominated physical sputtering was reduced, allowing the plasma to break through the density limit into the density-free region.
“These results demonstrate the potential of a practical scheme for substantially increasing the density limit in tokamaks, which is also germane to the stellarator start-up,” the researchers said. “The breaking of Greenwald density limit and the successful access to the density-free regime as demonstrated in this work opens a promising path advancing toward achieving the fusion ignition condition.”
