Simulations clarify the mechanism of coupled plasma fluctuations

In nature, phenomena in which multiple fluctuations occur in a coupled manner are frequently observed. For example, in large earthquakes, cases of them occurring consecutively in adjacent regions have been reported. When multiple fluctuations occur in this coupled way, compared to a single fluctuation, the coupled ones release more energy, leading to larger-scale phenomena.

In fusion plasmas, fluctuations caused by energetic particles exist and are known to degrade the confinement of energetic particles.

On the other hand, these fluctuations are expected to play a role in transferring the energy of the energetic particles to fusion fuel ions for heating. Thus, fluctuations caused by energetic particles are an important issue in fusion research, and among them, fluctuations that occur in a coupled manner are particularly noteworthy because they can develop into large-scale phenomena.

At the ASDEX Upgrade device in Germany, two fluctuations occurring in a coupled manner were observed. Although these fluctuations were attributed to energetic particles, the underlying mechanism of their coupling remained unclear.

Researchers from the National Institute for Fusion Science (NIFS) and the Max Planck Institute for Plasma Physics (IPP) have collaborated on a simulation study to clarify the physical mechanism by which two fluctuations occur in a coupled manner.

The research is published in the journal Scientific Reports.

NIFS has developed a code named “MEGA” to simulate plasma fluctuations caused by energetic particles. This is called a “hybrid simulation” because it performs coupled and simultaneous calculations for particles and fluid.

The MEGA code has been applied to experimental devices in Japan and overseas, with its effectiveness demonstrated through comparisons with various experimental results.

This time, Assistant Professor Hao Wang of NIFS and others conducted a simulation using MEGA on a supercomputer and succeeded in reproducing the phenomenon observed in the ASDEX-Upgrade device, in which two fluctuations occur in a coupled manner.

In the simulation, the first fluctuation at a frequency of 103 kHz was initially caused by energetic particles, followed by the generation of a second fluctuation with a frequency of 51 kHz, which developed into a larger amplitude than the first. This simulation result is consistent with the experimental results.

To understand the generation mechanism of the second fluctuation, researchers investigated the time evolution of the distribution function of energetic particles. This describes how many particles with what velocities and energies exist at each location in the plasma.

The shape of this distribution function can strongly influence the development of fluctuations and conversely, fluctuations can deform the distribution function.

Assistant Professor Hao Wang and his collaborators performed a detailed analysis of the simulation results. They found that as the first fluctuation grew, the distribution function of energetic particles was significantly deformed, and this deformation caused the second fluctuation to occur. In other words, they revealed that the two fluctuations occurred in a coupled manner via the deformation of the energetic particle distribution function.

Significance and future work

Achieving fusion energy requires energetic particles generated by fusion reactions to heat plasma and sustain these reactions. For this purpose, it is crucial to efficiently confine these energetic particles within the plasma. Coupled fluctuations can lead to significant losses of energetic particles.

By utilizing the knowledge of the physical mechanism clarified in this study, it will be possible to contribute to the development of methods to suppress the coupled generation of fluctuations. Furthermore, this physical mechanism may allow us to generate the second fluctuation—which is difficult to excite directly—from the first.

This could contribute to heating fuel ions. Moreover, even in space plasmas, although the types of waves differ, coupled fluctuations caused by energetic particles have been observed. The analysis method of the energetic particle distribution function developed in this study is also expected to be applicable to space plasmas.

In the future, the researchers plan to conduct simulations that calculate both energetic particles and fuel ions to investigate the role of the latter and the energy transfer to them in coupled energetic particle-driven fluctuations.