In experiments using the Experimental Advanced Superconducting Tokamak (EAST), also referred to as the “artificial sun,” researchers from the Institute of Plasma Physics at the Hefei Institutes of Physical Science under the Chinese Academy of Sciences, Huazhong University of Science and Technology, and Aix-Marseille University in France, among other institutions, have discovered a way to exceed the plasma density limit. This provides a vital physical foundation for high-density operation in magnetic confinement fusion devices. The results were published in the journal Science Advances on January 2nd, 2026.
A toroidal device called a tokamak uses magnetic confinement to accomplish controlled nuclear fusion. Like a helical “magnetic racetrack,” it efficiently confines high-temperature plasma to encourage fusion processes. The rate of fusion reactions is directly impacted by plasma density, a critical factor influencing tokamak performance.
Historically, researchers have accepted an upper limit to plasma density. When this limit is reached, the plasma becomes unstable, escapes magnetic confinement, and expends significant energy on the device's inner walls, risking operational safety.
While long-term global fusion research has shown that the physical processes that cause the density limit occur in the plasma-wall boundary area, the fundamental causes remain unknown.
In this study, the Chinese researchers created a theoretical model of self-organized plasma-wall interactions. Using this model, they discovered the important role of radiation instability caused by border contaminants in triggering the density limit, clarifying the underlying process.
Based on this theoretical discovery, researchers experimentally directed the plasma to surpass the density limit, effectively guiding it into a new “density-free zone.”
These findings provide the first experimental validation of such a zone in tokamaks. According to the researchers, this new study not only gives significant insights into understanding the density limit, but it also lays the groundwork for high-density functioning in tokamaks.
Journal Reference:
Liu, J. et.al. (2025) Accessing the density-free regime with ECRH-assisted ohmic start-up on EAST. Science Advances. Doi: 10.1126/sciadv.adz3040. https://www.science.org/doi/10.1126/sciadv.adz3040