Researchers from the National Institute for Fusion Science, led by Associate Professor Tatsuya Kobayashi, Assistant Professor Mikiro Yoshinuma, and Professor Katsumi Ida, used medical-grade tomography technology to measure the plasma phase distribution at high speed and with high precision in a recent study that was published in the journal Proceedings of the National Academy of Sciences.
3D spatial tomography estimates the 3D structure of a subject from images taken from multiple directions. On the other hand, newly developed phase-space tomography estimates the plasma phase-space distribution by combining data obtained from highly resolved measurements in (A) velocity versus space, (B) velocity versus time, and (C) time versus space, respectively. Image Credit: National Institute for Fusion Science
Fusion energy is being researched and developed as a potential new source of electricity that could help achieve a carbon-neutral society. The Large Helical Device (LHD) at the National Institute for Fusion Science is being used to study magnetically confined plasma. Plasma differs from other gases primarily due to its low density.
Collisions between constituent particles are rare in magnetically confined plasma, which has a density only a millionth that of the atmosphere. This leads to a distortion of the velocity distribution function, which is the histogram of particle motion.
Understanding the background physics is desired because distortions in the velocity distribution function can result in unexpected plasma dynamics, such as abrupt changes in plasma temperature and the generation of currents.
A common technique for determining the plasma velocity distribution function is spectroscopy, which measures the light that plasma emits. Due to the limited amount of light, measuring the time variation of the velocity distribution function requires sacrificing spatial resolution.
However, to predict and control the plasma and to build a fusion power reactor, it is necessary to know the change in the plasma's phase-space distribution, resolved in velocity and space coordinates.
Along with the already-existing “high-resolution spectrometer” and “high-speed spectrometer,” they also installed a new “high-speed luminescence intensity monitor” and operated the three instruments in unison.
The original plasma phase-space distribution was reconstructed by integrating the acquired data and performing tomographic analysis. This made it possible to measure the plasma phase-space distribution at a high speed of 10,000 Hz (10,000 times per second) for the first time in history. Compared with the previous two hundred hertz, this is a fifty-fold improvement.
Energy exchange between plasma particles and beam particles via waves in the LHD experiment was observed using phase-space tomography, which showed changes in the plasma phase-space distribution. It is well known that waves accelerate and impart energy to particles traveling at near-wave velocities (wave-particle interaction).
This is comparable to how surfers accelerate by moving in tandem with the waves. Achieving highly efficient fusion energy requires using waves to heat plasma. Waves have been found to interact with the plasma and move mostly in a toroidal direction.
New cases of simultaneous rightward and leftward waves in phase-space tomography have been found. Those waves accelerate more particles, which is believed to result in more effective plasma heating.
This study has shown that integrating data and running multiple diagnostic systems simultaneously can yield measurement performance that is superior to that of each instrument alone.
Future studies on fusion energy experiments are anticipated to employ this measurement method to determine how to control plasmas based on data from the plasma phase-space distribution. In addition to magnetic confinement plasmas, collisionless plasmas are frequently observed in the sun, auroras, and other celestial bodies.
Consequently, comprehensive measurements of the plasma phase-space distribution are also needed for these various systems. Future applications of phase-space tomography are anticipated in various domains.
Journal Reference:
Kobayashi, T., et al. (2024) Detection of bifurcation in phase-space perturbative structures across transient wave–particle interaction in laboratory plasmas. Proceedings of the National Academy of Sciences. doi.org/10.1073/pnas.2408112121.