(Hefei, Jan 2) A research team from the Institute of Plasma Physics, Hefei Institutes of Physical Science, Chinese Academy of Sciences, announced on Friday that experiments on EAST—China’s fully superconducting tokamak often dubbed the “artificial sun”—have confirmed the existence of a “density-unconstrained regime” in tokamaks and identified a practical pathway to operate at higher plasma density. The findings provide important physics support for high-density operation in magnetic confinement fusion devices.
According to Xinhua, the results were published in Science Advances.
A tokamak is a donut-shaped device that uses magnetic fields to confine ultra-hot plasma—effectively creating a “magnetic track” that keeps the plasma from touching the wall, a prerequisite for controlled fusion.
Plasma density is a key performance parameter because it directly affects fusion reaction rates. For decades, experiments have shown a density limit: once the plasma approaches this threshold, it can become unstable, disrupt, and escape magnetic confinement, releasing large energy loads onto the vessel walls and threatening safe operation.
The fusion community has long suspected that the processes triggering the density limit occur in the plasma–wall boundary region, but the detailed mechanism remained unclear.
In this work, the team developed a self-organization theoretical model of plasma–wall interactions and found that radiation-driven instability caused by edge impurities plays a decisive role in triggering the density limit, clarifying the underlying physics.
Leveraging EAST’s all-metal wall environment, researchers combined electron cyclotron resonance heating with coordinated pre-gas puff startup strategies to reduce impurity sputtering at the edge and delay the onset of the density limit and disruptions. By adjusting divertor target conditions, they further suppressed tungsten-impurity-dominated sputtering, enabling the plasma to surpass the density limit and transition into a new density-unconstrained regime. The experimental observations closely matched theoretical predictions, marking the first confirmation of this regime.
The project was carried out jointly by multiple institutions in China and abroad, including Huazhong University of Science and Technology and Aix-Marseille University in France, with support from China’s national magnetic confinement fusion program.
