NIFS finds success in rapid measurement of plasma phase space distribution

A research group including Associate Professor Tatsuya Kobayashi, Assistant Professor Mikiro Yoshinuma, and Professor Katsumi Ida of the National Institute for Fusion Science applied tomography, a tool used in fields such as medicine, to plasma diagnostics. They succeeded for the first time in the world in rapidly measuring changes in the plasma phase space distribution, a previously unexplored space. The results are published in PNAS.

Phase space is a space defined by the coordinates of the real space in which we live and coordinates corresponding to the speeds of numerous plasma particles. Because a distortion in the phase space distribution for high-temperature plasmas can considerably impact plasma performance, an experimental observation of this distortion has been desired.

In this research, Kobayashi and his colleagues introduced a new high-speed luminescence intensity measuring instrument in addition to a conventional high-resolution spectrometer and a high-speed spectrometer. They operated the three instruments in coordination. The original plasma distribution was reconstructed by integrating the obtained data and performing tomographic analysis. Thus, for the first time in the world, phase space resolved into the velocity coordinates and spatial coordinates were measured at a high speed of 10,000 Hz. Using tomography, the researchers achieved a 50-fold improvement over the previously employed speed of 200 Hz.

The developed measurement technique was used to observe energy exchange between plasma and beam particles via waves in a Large Helical Device (LHD) experiment, revealing changes in the distortion of the phase space distribution.

In plasma, particles moving at speeds close to those of the generated waves are accelerated by energy from waves (wave−particle interaction). This phenomenon is similar to the acceleration of a surfer by moving in unison with waves. Plasma heating via waves is essential in achieving highly efficient fusion energy. It has been believed thus far that waves primarily travel in the direction of doughnut-shaped plasma rings and interact with plasma.

Using phase space tomography, the simultaneous occurrence of clockwise and counterclockwise waves was discovered, in addition to the previously discovered clockwise (in the direction opposite to the direction of magnetic field lines) traveling waves. Simultaneous waves accelerate more particles, which is expected to cause more efficient plasma heating.

This measurement technique is expected to be utilized in future research in nuclear fusion experimental devices and to be useful in plasma control based on velocity distribution function information. Plasma with few collisions is not only found in magnetically confined plasma but are also commonly found in astronomical objects, such as the Sun and auroras. Detailed measurements of the plasma phase space distribution will be useful in these systems as well.

Journal Information
Publication: PNAS
Title: Detection of bifurcation in phase-space perturbative structures across transient wave-particle interaction in laboratory plasmas
DOI: 10.1073/pnas.2408112121