Scientists at Tohoku University have found that electromagnetic waves, produced by high-energy particles, carry heated plasma energy through a process known as Landau damping. The observation of this process is recorded as the first in the world.
The team measured the time variation of the plasma velocity profile in the Large Helical Device (LHD), at the National Institute for Fusion Science, in order to observe the wave process.
In fusion power generation, it is essential that the high-energy particles generated by a fusion reaction in plasma energy heat it to sustain further fusion reactions. The key to the self-heating process of plasma is whether it can be heated by waves.
A paper summarising the results of the new discovery was published in Communications Physics.
A new method of measuring the plasma heating process
Until now, there has been no method to directly measure the heating process of plasma energy caused by electromagnetic waves, so it was unknown whether the process actually existed.
In order to capture the process, the researchers worked to develop a new measurement system. To directly measure the heating process, it is necessary to determine the time variation of the velocity distribution, which indicates which velocity particles are present and the proportion of them.
To make this discovery, the team injected high-speed atoms into the plasma while using a method to measure the velocity distribution of particles in plasma energy, known as high-speed charge exchange spectroscopy.
Considered to be too difficult in previous studies, the researchers were able to carry out ultrahigh-speed treatment, which enabled them to measure the time variation of the velocity distribution of plasma particles at an ultrahigh-speed of 10 kHz (10,000 times per second).
Using the LHD to investigate the self-heating of plasma
In the LHD, experiments are being conducted to investigate the self-heating of plasma energy, using a high-speed particle beam that simulates particles from nuclear fusion reactions. In order to simulate the self-heating of the plasma energy, a newly developed measurement system was used to record the time variation of plasma velocity distribution in detail.
As a result, the team found that plasma energy is heated due to the slowing down of the high-speed particle beam and the distortion of the velocity profile of the plasma particles, caused by the generation of electromagnetic waves inside it.
The reasons for the distortions in the velocity profile were discovered to be that the energy of the particle beams was transferred to the electromagnetic wave through the Landau damping process, and the energy of the electromagnetic wave was transferred to the particles of plasma energy.
Professor Katsumi Ida, who led the study, explained: “For the self-heating of plasma in fusion power generation, it is not enough for high-energy particles to collide with plasma ones and heat them, so heating by other processes is also necessary. This result, which shows that electromagnetic waves generated inside the plasma can heat it, provides important knowledge for fusion research.
“Furthermore, it will contribute to the study of the Earth’s magnetosphere, where particle acceleration occurs by a similar process, and will promote future interdisciplinary research.”
Overall, the researchers concluded that the electromagnetic waves carried the energy of the high-speed particle beam to the plasma and heated it.