Scientists at a nuclear fusion reactor in China recently surmounted an important obstacle in operating reactor vessels at high density. They pushed plasma density 65% beyond a special threshold, entering a stable state that overcomes a long-standing barrier to achieving burning plasma, the stage where a fusion reaction becomes self-sustaining.
Fusion power mimics what happens inside the sun. Hydrogen atoms slam together so hard that they fuse into helium, releasing copious amounts of energy. But this reaction only works when the atoms are packed into a small space at an extreme temperature, typically more than 100,000,000º C.
Greenwald limit
In the reactors designed to sustain these reactions, scientists measure success using the triple product: the density multiplied by the temperature multiplied by the confinement time. All three numbers need to be very high for engineers to reach ignition, a state in which a fusion reaction sustains itself. Density is the number of fuel particles that can be squeezed into the reactor. More density means more collisions and more fusion.
But there’s a catch. For decades, tokamaks, the donut-shaped magnetic vessels designed to hold the superhot plasma, ran into the Greenwald density limit. Beyond this limit, the plasma collapses in a disruption that could damage the reactor. The Greenwald formula links this limit to the plasma current and the reactor’s size.
The EAST fusion reactor in Hefei in China has typically operated between 80% and 100% of this limit.But in a January 1 paper in Science Advances, the EAST team reported it had achieved stable plasmas at densities 1.3x to 1.65x of the limit.
Temperature at divertor
The team achieved this by combining two techniques. First, they used electron cyclotron resonance heating (ECRH) during start-up. In ECRH, microwave beams are shot into the plasma, heating electrons to millions of degrees. This happens before ramping up the plasma current, a large electric current that flows through the plasma to heat it and help create the magnetic cage. Second, the team started with more deuterium gas in the chamber, then fed hydrogen fuel as the plasma heated up.
For the experiments, EAST’s tungsten surfaces were also coated with a thin layer of lithium to condition them and reduce impurities.
The overall combination changed how the plasma interacted with the reactor walls.When the plasma touches the walls, tungsten atoms from the walls are released into the plasma. Tungsten is an impurity that radiates a lot of heat away, potentially causing the plasma to collapse.
This creates a vicious cycle. Hot plasma strikes the walls, releases impurities, the impurities radiate heat, the plasma gets hotter in the spots that try to compensate, those hot spots hit the walls harder, releasing more impurities. Eventually the system may spiral into disruption.
In 2021, physicist Dominique Escande and colleagues at the Aix-Marseille University in France developed the plasma-wall self-organisation (PWSO) theory to predict this behaviour mathematically. The theory says two stable states exist: a density-limit regime near the Greenwald limit and a density-free regime where the density limit shoots past the limit.
The difference between the two states is the temperature at the divertor, the part of the reactor where plasma meets the walls. A cooler divertor means more gentle collisions between particles and the wall, fewer impurities, and thus cleaner plasma, which can pack hydrogen atoms more densely.
Less sputtering
The EAST team ran two sets of experiments. In the first, they held the ECRH power at 600 kW and varied the gas pressure. In the second, they fixed the gas pressure and varied the ECRH power.
This way, the team found that more gas in the chamber led to a cooler divertor and less tungsten contamination.
Varying the ECRH power had a smaller effect; in the paper, researchers wrote that that could be because they used relatively low gas pressure in tests.
Schematic illustration of the EAST tokamak operation during ECRH-assisted start-up.
| Photo Credit:
Yan Ning
One unexpected finding emerged from repeated ECRH shots with identical settings. The later experiments reached higher densities than the first ones, even with the same power and gas inputs. The team found this was because the wall’s conditions improved over time as multiple high-density plasmas ‘conditioned’ its tungsten surface, making it less prone to sputtering.
The experiments were able to achieve densities of up to 5.6 × 1019 particles per cubic metre, around 65% higher than the EAST reactor’s normal 3.4 × 1019. The plasma temperature near the divertor target also dropped by roughly a third, from about 1.1 million to 0.7-0.8 million degrees C. The plasma also had fewer heavy atoms mixed in.
Beyond the limit
The measurements matched the predictions of the PWSO theory remarkably well. The team tested both a simplified zero-dimensional model and a more complex one-dimensional model to simulate the way temperature and density of impurities changed in the plasma. Both models placed EAST’s results squarely in the density-free regime, the stable state in which the plasma density exceeds the Greenwald limit.
Previous experiments in the J-TEXT tokamak in Wuhan stayed in the density-limit regime. The difference could be the J-TEXT reactor’s carbon walls. While tungsten sputters by bombarding the plasma, carbon undergoes additional chemical reactions and releases more impurities.
The new advance doesn’t ‘solve’ fusion energy. The EAST tests ran at relatively low power and plasma current, and lasted several seconds rather than the hours needed for a power plant.
The density-free regime also isn’t truly unlimited. At extreme density, different types of turbulence and instabilities can arise independent of the divertor. As density increases, engineers will also need more power to keep the plasma hot enough to fuse. But those constraints arise far beyond the Greenwald limit.
In future experiments, per the paper, engineers could further lower the divertor temperature by increasing both the ECRH power and the gas pressure, possibly reaching full detachment, a condition in which the plasma barely touches the walls. Detached plasmas could operate at densities several times the Greenwald limit.
Matters for ITER
“The findings suggest a practical and scalable pathway for extending density limits in tokamaks and next-generation burning plasma fusion devices,” Zhu Ping, of the Huazhong University of Science and Technology in Wuhan and a co-lead of the new study, said in a release.
Fusion researchers often focus on temperature and confinement time while treating density as constrained by the Greenwald limit. The new experiments challenge that assumption. The potential implication is that if a reactor can be run at twice the fuel density, it might achieve the conditions for ignition at lower temperatures or with shorter confinement times.
This matters for ITER, the large international fusion experiment under construction in France, and in which India has invested.
“Density limit is one of the critical issues in tokamak plasma for a fusion power plant. Plasma-wall self-organisation theory proposed in the … paper should be validated to overcome the density limit in ITER,” Ryoji Hiwatari of the Japan National Institutes for Quantum and Radiological Science and Technology wrote on X.com.
mukunth.v@thehindu.co.in
Published – January 13, 2026 06:00 am IST
