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In a potential new milestone for fusion energy research, researchers in China report achieving a state once only theorized for fusion plasmas, enabling stable operation under conditions that significantly exceed normal limits.

The achievement was made during experiments with China’s Experimental Advanced Superconducting Tokamak (EAST), which reportedly produced fusion plasmas in a “density-free regime,” overcoming a longstanding hurdle to nuclear fusion ignition.

The team’s findings were featured in a new study in Science Advances, offering a fresh perspective on tackling one of the most significant impediments to practical fusion energy.

The Plasma Density Limit

Amid growing concerns about access to clean, sustainable energy, nuclear fusion has long been seen as one of the most promising avenues for future energy sources.

Despite its promise, harnessing nuclear fusion is easier said than done, since it involves fusion reactions between deuterium and tritium that require heating plasmas to around 150 million kelvins—temperatures that still only represent a fraction of the intense conditions that occur naturally on the surface of the Sun.

Nonetheless, achieving such temperatures in conventional tokamaks—devices physicists use to conduct controlled fusion experiments with hot plasmas—is challenging because of the currently known upper limit on attainable plasma density. In essence, energy levels above this boundary typically lead to instabilities that not only affect plasma confinement but also cause disruptions that can damage tokamaks.

A Fusion Ignition Breakthrough

The recent work reported in Science Advances is significant because the EAST experiments now demonstrate that the plasma density limit, which has long constrained the operational capabilities of tokamaks, may finally have been overcome.

The research, co-led by Prof. Zhu Ping from Huazhong University of Science and Technology and Associate Prof. Yan Ning of the Hefei Institutes of Physical Science of the Chinese Academy of Sciences, details the achievement of high-density plasmas at EAST, potentially extending stable operating periods without causing plasma disruption.

At the heart of the Chinese team’s work is a novel concept known as plasma-wall self-organization (PWSO) theory, which offers a unique approach to overcoming the plasma density limit. This theoretical approach, first developed by French physicist Dominique Franck Escande and colleagues with the French National Center for Scientific Research and Aix-Marseille University, holds that the key to overcoming plasma density issues involves attaining harmonious conditions between the plasma within the tokomak and its metallic walls, where physical forces increasingly impact the plasmas and their confinement as temperatures increase.

Verification of PWSO Theory

Although PWSO theory was initially introduced in 2021, it has yet to see verification in practice until now. According to the Chinese research team, the recent EAST experiments have successfully demonstrated the concept by combining careful control of fuel pressure with increased temperature during the initial startup phase of tokamak operation. During this time, electron cyclotron resonance heating is initiated, and with optimal control between fuel pressure and heating, the resulting plasma-wall interactions become more manageable from the outset.

The EAST researchers report that employing this process helps reduce potentially harmful interactions between the heated plasmas and the tokamak wall, limit impurity accumulation during confinement, and reduce overall energy loss.

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