The results, reported in Science Advances on January 1, address a long-standing physical constraint on the path toward nuclear fusion ignition by showing that plasma density limits in tokamaks can be relaxed without triggering disruptive instabilities.
The study was co-led by Prof. ZHU Ping of Huazhong University of Science and Technology and Associate Prof. YAN Ning of the Hefei Institutes of Physical Science, Chinese Academy of Sciences, who implemented a new high-density operating scheme on EAST to extend the achievable density range.
For deuterium-tritium fusion reactions, plasmas must be heated to an optimal temperature of around 13 keV, corresponding to about 150 million kelvin, and under these conditions thermonuclear power output scales with the square of the fuel density.
In conventional tokamak operation, however, plasma density has long been restricted by an empirical upper limit, beyond which instabilities typically arise, degrade confinement, and jeopardize the safe operation of the device, limiting efforts to raise fusion performance.
A recently developed plasma - wall self organization (PWSO) theory offers a new perspective on this disruptive density limit, having been proposed by D.F. Escande and colleagues at the French National Center for Scientific Research and Aix-Marseille University to describe how plasmas and metallic walls can co-evolve under conditions dominated by physical sputtering.
According to PWSO, a density-free regime becomes accessible if the plasma and wall reach a specific balance in which sputtering and impurity dynamics allow higher densities without driving disruptive events, effectively removing the traditional density cap.
The EAST experiments verify this physical concept for the first time by combining control of the initial fuel gas pressure with electron cyclotron resonance heating during the start-up phase of the discharge.
This approach enables optimization of plasma - wall interactions from the earliest moments of the discharge, so that impurity influx and associated energy losses are reduced as the plasma current and temperature rise.
With impurity accumulation and plasma - wall interactions suppressed, the plasma density can be increased throughout start-up, eventually reaching values at the end of this phase that would typically be associated with instability and disruption in standard operation.
Under these conditions, the team reports that EAST accessed the density-free regime predicted by PWSO theory, with the plasma remaining stable even when operating at densities that far exceeded empirical limits documented in earlier tokamak studies.
These experimental results provide new physical insight into how the density limit in tokamak operation can be overcome and are viewed as an important step toward realizing fusion ignition in future burning plasma devices.
"The findings suggest a practical and scalable pathway for extending density limits in tokamaks and next-generation burning plasma fusion devices," said Prof. ZHU.
Associate Prof. YAN added that the research team plans to apply the new method during high-confinement operation on EAST in the near future in an attempt to access the density-free regime under high-performance plasma conditions.
Research Report:Accessing the density-free regime with ECRH-assisted ohmic start-up on EAST
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Hefei Institutes of Physical Science
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