China's Experimental Advanced Superconducting Tokamak (EAST) reported the first experimental verification of what plasma operating regime in January 2026?

In January 2026, China's Experimental Advanced Superconducting Tokamak (EAST), often called the "artificial sun," successfully achieved the first experimental verification of a "density-free" plasma operating regime. This groundbreaking achievement addresses a long-standing challenge in magnetic confinement fusion research: the empirical upper limit on plasma density, known as the Greenwald density limit, which has historically constrained the performance of tokamak devices. By breaking through this limit, EAST demonstrated that fusion plasmas can remain stable at densities previously thought impossible, opening a promising pathway toward sustained fusion reactions.

Achieving high plasma density is crucial for nuclear fusion, the process that powers the sun and stars, to generate significant energy. Fusion power output scales with the square of the fuel density, meaning higher densities lead to more efficient energy production. However, as plasma density increases in tokamaks, instabilities often arise, degrading confinement and potentially leading to disruptive shutdowns. The "density-free" regime, first predicted by a plasma-wall self-organization (PWSO) theory, proposes that a delicate balance between the plasma and the reactor's metallic walls can allow for stable operation at much higher densities, effectively removing the traditional density cap.

The EAST experiments achieved this by carefully controlling the initial fuel gas pressure and employing electron cyclotron resonance heating (ECRH) during the plasma start-up phase. This approach effectively optimized plasma-wall interactions, significantly reducing impurity accumulation and energy losses, which are common culprits behind density-driven instabilities. By suppressing these interactions and guiding the plasma into this new regime, EAST maintained stable operation at densities up to 1.65 times the Greenwald limit. This breakthrough, a collaborative effort published in Science Advances, offers vital physical evidence for high-density tokamak operation and represents a significant step toward realizing fusion ignition in future devices, potentially leading to a clean and sustainable energy source.