SCI-FI-TEK 70 YRS IN THE MAKING
EAST Tokamak experiments exceed plasma density limit, offering new approach to fusion ignition
Chinese Academy of Sciences Headquarters
image:
Schematic illustration of the EAST tokamak operation during ECRH-assisted Ohmic start-up
view moreCredit: YAN Ning
Researchers working on China's fully superconducting Experimental Advanced Superconducting Tokamak (EAST) have experimentally accessed a theorized "density-free regime" for fusion plasmas, achieving stable operation at densities well beyond conventional limits. The results, reported in Science Advances on January 1, provide new insights into overcoming one of the most persistent physical obstacles on the path toward nuclear fusion ignition.
The study was co-led by Prof. ZHU Ping from Huazhong University of Science and Technology and Associate Prof. YAN Ning from the Hefei Institutes of Physical Science of the Chinese Academy of Sciences. By realizing a novel high-density operating scheme on EAST, the team demonstrated that plasma density, long constrained by empirical limits in tokamak operation, can be substantially extended without triggering disruptive instabilities.
Nuclear fusion is widely regarded as a promising source of clean and sustainable energy. For deuterium-tritium fusion reactions, plasmas must be heated to an optimal temperature of around 13 keV (150 million kelvin). Under these conditions, thermonuclear power scales with the square of fuel density. However, in conventional tokamak operation, plasma density has long been restricted by an empirical upper limit. Exceeding this limit often leads to instabilities that disrupt plasma confinement and endanger tokamak operation, posing a major challenge to improving fusion performance.
The recent development of the plasma–wall self organization (PWSO) theory provides a novel perspective on understanding the disruptive density limit. PWSO was originally proposed by D.F. Escande et al. from the French National Center for Scientific Research and Aix-Marseille University. The theory predicts that a new density-free regime could be accessed by achieving a delicate balance between the plasma and the metallic walls of the device, which are dominated by physical sputtering.
The physical concept on the density-free regime has been verified for the first time on EAST in this work. The EAST experiments combine control of the initial fuel gas pressure with electron cyclotron resonance heating during the startup phase, allowing effective optimization of plasma–wall interactions from the very beginning of the discharge. Through this approach, plasma–wall interactions, impurity accumulation and energy losses were significantly reduced, plasma is eventually pushed into a high enough density at the end of start-up. The researchers successfully accessed the PWSO theoretical density-free regime, in which the plasma can remain stable even when operating at densities that far exceeded empirical limits.
These experimental achievements provide new physical insights into breaking through the long-standing density limit in tokamak operation in pursuit of fusion ignition.
"The findings suggest a practical and scalable pathway for extending density limits in tokamaks and next-generation burning plasma fusion devices," said Prof. ZHU.
Associate Pro. YAN added that the research team plans to apply the new method during high-confinement operation on EAST in the near future in an attempt to access the density-free regime under high-performance plasma conditions.
Journal
Science Advances
Method of Research
Experimental study
Subject of Research
Not applicable
Article Title
Accessing the density-free regime with ECRH-assisted ohmic start-up on EAST
Article Publication Date
1-Jan-2026
Study advances understanding of uncertainty propagation in tokamak equilibria
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The uncertainty quantification of the separatrix (a); the sensitivity analysis of the upper (b) and lower (c) half of the separatrix Z-coordinate; the sensitivity analysis of the left-side (d) and right-side (e) half of the separatrix R-coordinate.
view moreCredit: LIU Haiqing
A research team led by Prof. LIU Haiqing at the Hefei Institutes of Physical Science of the Chinese Academy of Sciences, has carried out a comprehensive analysis of uncertainty propagation in free-boundary plasma equilibrium reconstruction.
Their findings, published in Nuclear Fusion, offer new insights into how input measurement errors influence the accuracy of tokamak equilibrium calculations.
Plasma equilibrium reconstruction is essential for tokamak operation, providing the basis for plasma control, stability evaluation, and the interpretation of diagnostic data. However, uncertainties in experimental inputs can significantly affect the reliability of these calculations.
In this study, researchers systematically map how such uncertainties translate into variations in key equilibrium parameters. They find reliable reconstruction requires core diagnostic inputs—such as magnetic probe measurements and the toroidal field—to remain within a controlled accuracy range. Improved precision in midplane and X-point position data can further enhance reconstruction quality.
The analysis shows that different parts of the plasma respond differently to input uncertainties. The core q-profile is most affected by the toroidal field and initial plasma current, while the edge q-profile is highly sensitive to the X-point and outer midplane positions. Plasma shape accuracy is similarly influenced by uncertainties in these boundary-related measurements.
The team also found that the toroidal magnetic field exhibits strong sensitivity near the midplane, whereas uncertainties in other regions remain comparatively low. Global parameters such as beta and plasma volume are mainly affected by midplane position and current-related inputs. The magnetic axis position is influenced by uncertainties in X-point and strike-point coordinates.
"Our study provides a useful reference for refining diagnostic setups and improving the robustness of plasma control strategies," added Prof. LIU.
Journal
Nuclear Fusion
Article Title
The uncertainty quantification of the free boundary G–S plasma equilibrium calculation on Experimental Advanced Superconducting Tokamak (EAST)
