Background of the Research
Fusion power generation is expected to be a sustainable energy source that emits no carbon dioxide during power generation. To achieve this, research is underway worldwide to confine high-temperature plasma*1 using magnetic fields. To initiate a fusion reaction, high-energy particles or waves must be injected from the outside to heat the plasma to extremely high temperatures.Once the fusion reaction begins, “self-heating”*2 occurs, in which high-energy particles generated by the reaction further heat the plasma, allowing the high-temperature state to be maintained even if external heating is reduced. This self-heating process is complex, and large-scale experimental facilities such as ITER are necessary to demonstrate it in its entirety. Demonstration of self-heating is expected around 2040.On the other hand, one of the key processes underpinning self-heating—the phenomenon in which “waves generated by plasma instabilities heat the plasma itself”—can be verified even with current experimental facilities. In this study, experiments using the Large Helical Device (LHD)*3 demonstrated the process in which “plasma heats itself using waves it generates.” This achievement represents an important step toward understanding the mechanisms of self-heating necessary for fusion power generation .
Research Findings
Until now, it was believed that when plasma becomes unstable and waves are generated, the plasma loses energy and its temperature drops. Consequently, research has focused on how to suppress plasma instability. The research group led by Professor Katsumi Iida shifted their perspective from this conventional thinking and reasoned, “Rather than suppressing instability, isn’t it more important to observe unstable plasma in detail and clarify its characteristics?”Consequently, they deliberately generated unstable plasma and examined its state in detail. As a result, they uncovered a phenomenon in which the plasma heats itself through the waves it generates. It had already been known that when waves occur, a temporary “distortion” arises in the motion of the particles comprising the plasma (distribution function)*4. However, it had not been clear how the plasma’s temperature changed afterward.The research group therefore conducted a detailed analysis of the motion of the plasma particles (distribution function). As a result, they discovered that after the “distortion” in the distribution function caused by the waves had returned to normal, the spread of the particles’ motion had increased. This indicates that the plasma temperature had risen.In other words, this study succeeded in capturing the following sequence of events: “Plasma becomes unstable → Waves are generated → Distortions occur in the distribution function → The spread of the distribution function increases (the plasma is heated).”This study demonstrates that waves generated by instabilities not only cause the plasma to lose energy but, conversely, also heat the plasma. This discovery provides a new perspective for understanding the mechanism by which fusion plasma maintains its high temperature.
Significance of the Findings and Future Directions
Currently, there is no device capable of demonstrating the entire “self-heating” process in nuclear fusion.“Self-heating” is an essential element for realizing fusion power generation, and demonstrating even a single part of this process is of great significance. The phenomenon observed in this experiment—in which “waves generated by plasma instabilities heat the plasma itself”—is one of the key processes underpinning self-heating. The experimental observation of this phenomenon is a major achievement that advances our understanding of the mechanisms of self-heating in fusion plasma.Furthermore, this phenomenon—in which “waves cause the plasma to heat itself”—is a universal physical process that occurs not only in fusion plasma but also in the plasma of Earth’s magnetosphere. The findings of this study bridge the distinct research fields of fusion and space plasma, and are expected to contribute to the future development of interdisciplinary research.
[Glossary]
*1 Plasma
The fourth state of matter, following solid, liquid, and gas. A state in which, due to high temperatures, atoms are dissociated into electrons and ions, allowing the electrons and ions to move freely.
*2 Self-heating
Self-heating is a phenomenon in which the temperature of a substance rises due to energy or reactions generated within the substance itself. In a fusion reactor, the phenomenon in which energy generated by fusion reactions heats the plasma and maintains its high-temperature state is called “self-heating.”
*3 Large Helical Device (LHD)
The main experimental facility at the National Institute for Fusion Science, the LHD is the world’s largest superconducting helical plasma generation device that utilizes a heliotron magnetic field based on Japan’s original concept. LHD stands for Large Helical Device. Experiments began in March 1998. Its magnetic field structure differs from that of tokamak-type plasma generation devices, enabling unique research that takes advantage of these characteristics.
*4 Reference Information
Press Release: World’s First Observation of the Process by Which Waves Transport Heat in Plasma
https://www.nifs.ac.jp/news/researches/220929.html
Communications Physics
Experimental study
Direct observation of phase-space dynamics during energetic-particle-driven MHD bursts
10-Nov-2026