China Fusion Reactor Breaks Longstanding Density Barrier

Scientists operating China’s Experimental Advanced Superconducting Tokamak have achieved a major breakthrough in nuclear fusion research by sustaining plasma densities far beyond a long-established theoretical limit. The advance marks a significant step towards achieving burning plasma, a critical stage where fusion reactions become self-sustaining and viable for future power generation.

Breaking the Greenwald Density Limit

Fusion reactors rely on extremely hot, dense plasma confined by magnetic fields. For decades, tokamak reactors faced the Greenwald density limit, beyond which plasma becomes unstable and collapses. Researchers at the EAST facility in Hefei reported stable plasma densities reaching 1.3 to 1.65 times this limit, surpassing previous operational ceilings. The results were published on January 1 in Science Advances, demonstrating a stable, high-density operating regime once thought unattainable.

Innovative Heating and Wall Conditioning

The breakthrough was achieved through a combination of electron cyclotron resonance heating during plasma start-up and modified fuel injection strategies. Scientists began with higher deuterium gas levels before introducing hydrogen as temperatures rose. Tungsten reactor walls were coated with lithium to reduce impurity release. These steps altered plasma–wall interactions, lowering impurity contamination and enabling denser plasma confinement without triggering disruptions.

Plasma-Wall Self-Organisation Theory Confirmed

The experiments closely matched predictions from plasma-wall self-organisation theory, developed in 2021, which proposes two stable plasma states: one limited by density and another allowing density to rise well beyond the Greenwald threshold. EAST’s results fell clearly into the density-free regime, aided by a cooler divertor region where plasma meets the reactor walls, reducing sputtering and heat loss.

Imporatnt Facts for Exams

• The Greenwald limit defines the maximum stable plasma density in tokamaks.
• Tokamaks use magnetic confinement to sustain high-temperature plasma.
• Electron cyclotron resonance heating uses microwaves to heat plasma electrons.
• EAST is a key experimental fusion reactor operated by China.

Implications for ITER and Fusion Energy

Although the experiments were short and at relatively low power, the findings challenge the long-held assumption that plasma density is tightly constrained. Higher fuel density could allow fusion reactors to reach ignition at lower temperatures or shorter confinement times. This has important implications for ITER, the world’s largest fusion experiment under construction in France with India as a partner, where overcoming density limits is considered vital for future fusion power plants.