Tokamak experiments exceed plasma density limit, offering new approach to fusion ignition
Schematic illustration of the EAST tokamak operation during ECRH-assisted Ohmic start-up. Credit: 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, 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.
Schematic comparison of EAST experimental results with plasma–wall self-organization theory prediction. Credit: Yan Ning
The physical concept of 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 are significantly reduced. The plasma is eventually pushed to a high enough density at the end of startup. 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 Prof. 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.
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