Science

China's EAST tokamak fusion reactor has achieved a breakthrough by reaching a "density-free regime" where plasma remains stable at densities far above traditional limits[^1][^2].

Led by Professor Ping Zhu and Associate Professor Ning Yan, researchers used electron cyclotron resonance heating (ECRH) and controlled initial fuel gas pressure during plasma startup to optimize plasma-wall interactions. This allowed them to achieve line-averaged electron densities of 1.3 to 1.65 times the Greenwald density limit, significantly higher than EAST's typical operational range of 0.8 to 1.0 times the limit[^4].

The experiments provided the first experimental validation of plasma-wall self-organization (PWSO) theory, which predicts two density regimes: a conventional density-limit regime and a density-free regime where much higher plasma densities become possible[^4].

Key findings:

• Higher plasma density was achieved by reducing impurity radiation and improving plasma-wall interactions
• The tungsten walls of EAST were crucial, as physical sputtering dominates over chemical sputtering
• Results suggest a practical path to substantially increase density limits in tokamaks
• Higher density operation is essential for achieving fusion energy breakeven and burning plasma

"The findings suggest a practical and scalable pathway for extending density limits in tokamaks and next-generation burning plasma fusion devices," said Professor Zhu[^5].

[^1]: Science Advances - Accessing the density-free regime with ECRH-assisted ohmic start-up on EAST [^2]: Instagram - Hashem Al-Ghaili post on EAST breakthrough [^4]: Science Advances - Accessing the density-free regime with ECRH-assisted ohmic start-up on EAST [^5]: Chinese Academy of Sciences - EAST Tokamak Experiments Exceed Plasma Density Limit