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Fusion energy is emerging as a critical component in achieving a carbon-neutral society by providing a sustainable source of electric power. At the National Institute for Fusion Science, researchers are advancing this effort through studies on magnetically confined plasma using the Large Helical Device (LHD). Unlike other gases, plasma exhibits low density, approximately one-millionth that of at 
by Riko Seibo
Tokyo, Japan (SPX) Dec 04, 2024
Fusion energy is emerging as a critical component in achieving a carbon-neutral society by providing a sustainable source of electric power. At the National Institute for Fusion Science, researchers are advancing this effort through studies on magnetically confined plasma using the Large Helical Device (LHD). Unlike other gases, plasma exhibits low density, approximately one-millionth that of atmospheric air, resulting in rare collisions between its particles. This unique state of matter necessitates specialized understanding, as distortions in its velocity distribution function can trigger sudden temperature shifts and current generation within the plasma.
Traditionally, spectroscopy has been used to study the velocity distribution of plasma particles. However, limitations in the total light emitted by plasma have forced researchers to sacrifice spatial resolution to measure time variations effectively. Achieving a deeper understanding of plasma phase-space - resolved across velocity and spatial dimensions - is vital for predicting and controlling plasma behaviors, particularly in developing efficient fusion reactors.
A research team led by Associate Professor Tatsuya Kobayashi, Assistant Professor Mikiro Yoshinuma, and Professor Katsumi Ida has pioneered a method to precisely measure plasma phase distribution at unprecedented speeds. By integrating tomography techniques, commonly employed in medical imaging, with advanced spectrometers, the team achieved this breakthrough.
The researchers combined a newly installed "high-speed luminescence intensity monitor" with an existing "high-resolution spectrometer" and "high-speed spectrometer." Through coordinated operation of these instruments, they reconstructed the plasma phase-space distribution via tomographic analysis. The result was a measurement speed of 10,000 Hz (10,000 times per second) - a 50-fold increase over the previous benchmark of 200 Hz.
This advanced phase-space tomography was utilized in LHD experiments to observe energy exchanges between plasma and beam particles via waves, a process akin to surfers gaining speed by synchronizing with ocean waves. These wave-particle interactions, essential for efficient plasma heating, revealed surprising findings: waves traveling in opposite directions can occur simultaneously, accelerating more particles and potentially enhancing plasma heating efficiency.
The success of this method underscores the power of integrating diverse diagnostic tools to surpass individual instrument limitations. The implications are significant, not only for fusion research but also for understanding plasma in other contexts, such as astronomical phenomena, solar activity, and auroras. The team's work sets the stage for applying phase-space tomography across a range of scientific domains.
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National Institute for Fusion Science
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