China's 'artificial sun' reactor shatters major fusion limit — a step closer to near-limitless clean energy

China's nuclear fusion reactor, dubbed the "artificial sun," has breached a major fusion limit by firing plasma beyond its usual operational range, advancing humanity's slow progress towards near-limitless clean energy.

The Experimental Advanced Superconducting Tokamak (EAST) kept plasma — the high-energy fourth state of matter — stable at extreme densities, which was previously seen as a major obstacle in the development of nuclear fusion, according to a statement released by the Chinese Academy of Sciences.

"The findings suggest a practical and scalable pathway for extending density limits in tokamaks and next-generation burning plasma fusion devices," study co-lead author Ping Zhu, a professor in the School of Electrical and Electronic Engineering at the University of Science and Technology in China, said in the statement.

Nuclear fusion offers the potential for near-limitless clean energy. In other words, energy without much nuclear waste or climate-warming greenhouse gas emissions released by burning fossil fuels. The new findings, published Jan.1 in the journal Science Advances, could bring our species one step closer to unlocking this energy source, which some researchers claim we could harness within decades.

However, nuclear fusion technology has been in development for more than 70 years, and it's still very much an experimental science, with reactors typically consuming more energy than they can produce. Meanwhile, climate scientists are calling for deep cuts to greenhouse gas emissions now as the impacts of climate change are already being felt around the world. Nuclear fusion is therefore unlikely to represent a practical solution to the current climate crisis — but could power our world in the future.

Fusion reactors are designed to fuse two light atoms into a single heavy atom via heat and pressure. By doing so, they generate energy in a similar way to the sun. However, the sun has a lot more pressure than Earth's reactors, so scientists compensate by corralling hot plasma at temperatures far hotter than the sun.

China's EAST is a magnetic confinement reactor, or tokamak, designed to keep plasma continuously burning for prolonged periods. The reactor heats plasma and traps it inside a donut-shaped chamber using powerful magnetic fields. Tokamak reactors have yet to achieve fusion ignition, which is the point at which the fusion process becomes self-sustaining, but the EAST reactor has been increasing the amount of time it can maintain a steady, highly confined loop of plasma.

One hurdle for fusion researchers is a density limit called the Greenwald Limit, beyond which plasma typically becomes unstable. This limit is a problem because, while higher plasma densities enable more atoms to whack into one another, thereby lowering the energy cost of ignition, instability also kills the fusion reaction.

To overcome the Greenwald limit, scientists at EAST carefully managed the plasma's interaction with the reactor's walls by controlling two key parameters upon starting the reactor: the initial fuel gas pressure and the electron cyclotron resonance heating, or the frequency at which electrons in the plasma absorbed microwaves. This kept the plasma stable at extreme densities of 1.3 to 1.65 times beyond the Greenwald Limit — much higher than the tokamak's usual operational range of 0.8 to 1, according to the study.

This isn't the first time the Greenwald Limit has been breached. For example, the U.S. Department of Energy's DIII-D National Fusion Facility tokamak in San Diego broke through the limit in 2022, and in 2024, researchers at the University of Wisconsin–Madison in Wisconsin announced that they had maintained a stable tokamak plasma at about 10 times the Greenwald Limit using an experimental device.

However, the breach at EAST enabled the researchers to heat the plasma to a previously theorized state called the "density-free regime" for the first time, where the plasma remained stable as the density increased. The research is based on a theory called plasma-wall self organization (PWSO), which proposes that a density-free regime could be possible when the interaction between the plasma and the reactor's walls is in a carefully balanced state, according to the statement.

Progress made at EAST and in the U.S. will inform the development of new reactors. China and the U.S. are both part of the International Thermonuclear Experimental Reactor (ITER) program, which is a collaboration between dozens of countries to build the world's largest tokamak in France.

ITER will be another experimental reactor designed to create sustained fusion for research purposes, but could pave the way for fusion power plants. The ITER reactor is expected to begin producing full-scale fusion reactions in 2039.