Scientists working with China's fully superconducting Experimental Advanced Superconducting Tokamak (EAST) have successfully reached a long-theorized "density-free regime" in fusion plasma experiments. In this state, the plasma remains stable even when its density rises far beyond traditional limits. The results, published in Science Advances on January 1, shed new light on how one of fusion energy's most stubborn physical barriers might finally be overcome on the road to ignition.

The research was co-led by Prof. Ping Zhu of Huazhong University of Science and Technology and Associate Prof. Ning Yan of the Hefei Institutes of Physical Science at the Chinese Academy of Sciences. By developing a new high-density operating approach for EAST, the team showed that plasma density can be pushed well past long-standing empirical limits without triggering the disruptive instabilities that usually end experiments. This finding challenges decades of assumptions about how tokamak plasmas behave at high density.

Why Density Limits Have Held Fusion Back

Nuclear fusion is widely seen as a potential source of clean and sustainable energy. In deuterium-tritium fusion, the fuel must be heated to about 13 keV (150 million kelvin) to reach optimal conditions. At such temperatures, the amount of fusion power produced increases with the square of the plasma density. Despite this advantage, tokamak experiments have long been constrained by an upper density limit. When that limit is exceeded, the plasma often becomes unstable, disrupting confinement and threatening the operation of the device. These instabilities have been a major obstacle to improving fusion performance.

A newer theoretical framework known as plasma-wall self organization (PWSO) offers a different explanation for why density limits arise. The concept was first proposed by D.F. Escande et al. from the French National Center for Scientific Research and Aix-Marseille University. According to PWSO theory, a density-free regime can emerge when the interaction between the plasma and the reactor's metallic walls reaches a carefully balanced state. In this regime, physical sputtering plays a dominant role in shaping plasma behavior.

The EAST experiments provided the first experimental confirmation of this theoretical idea. Researchers carefully controlled the initial fuel gas pressure and applied electron cyclotron resonance heating during the startup phase of each discharge. This strategy allowed plasma-wall interactions to be optimized from the very beginning. As a result, impurity buildup and energy losses were greatly reduced, allowing the plasma density to increase steadily by the end of startup. Under these conditions, EAST successfully entered the PWSO-predicted density-free regime, where stable operation was maintained even at densities far exceeding empirical limits.

Implications for Fusion Ignition