Elucidating the mechanism of coupled plasma fluctuations through simulation

Time evolution of the frequency spectrum of distribution functions and fluctuations of high-energy particles. In the initial state, the energy particle distribution function is mountain-like. There is a slope in the first region of variation where it starts to grow, and as the shape of the mountain changes, the slope of the distribution function becomes steeper in the second region of variation. It has become clear that the deformation of the energy particle distribution function due to the first fluctuation excites the second fluctuation, and that the two fluctuations occur in conjunction. Credit: Institute for Fusion Science
In nature, phenomena in which multiple fluctuations occur in conjunction are frequently observed. For example, it has been reported that large earthquakes occur consecutively in adjacent areas. When multiple fluctuations occur in tandem like this, they release more energy and cause a larger-scale phenomenon than a single fluctuation.
In fusion plasmas, fluctuations caused by high-energy particles exist and are known to worsen the confinement of high-energy particles.
On the other hand, these fluctuations are expected to play a role in transferring the energy of high-energy particles to fusion fuel ions for heating. As described above, fluctuations caused by high-energy particles are an important issue in nuclear fusion research, and among these fluctuations, coupled fluctuations are attracting particular attention because they have the potential to develop into large-scale phenomena.
Two interlocking fluctuations were observed in the German ASDEX upgrade equipment. These fluctuations were thought to be caused by high-energy particles, but the underlying mechanism of their binding remained unclear.
Researchers from the National Institute for Fusion Science (NIFS) and the Max Planck Institute for Plasma Physics (IPP) collaborated on a simulation study to elucidate the physical mechanism by which the two fluctuations occur in conjunction.
This research will be published in the scientific journal Scientific Reports.
NIFS has developed a method code-named “MEGA” that simulates plasma fluctuations caused by high-energy particles. This is called a “hybrid simulation” because it performs coupled and simultaneous calculations of particles and fluids.
The MEGA code has been applied to experimental equipment both domestically and internationally, and its effectiveness has been demonstrated by comparison with various experimental results.
This time, NIFS Assistant Professor Hao Wang and his colleagues conducted a simulation using MEGA on a supercomputer, and succeeded in reproducing the phenomenon in which two fluctuations occur in conjunction, as observed by the ASDEX-Upgrade instrument.
In the simulation, a first fluctuation with a frequency of 103 kHz was first caused by a high-energy particle, followed by a second fluctuation with a frequency of 51 kHz, which developed to a larger amplitude than the first fluctuation. The simulation results are consistent with the experimental results.
To understand the mechanism by which the second fluctuation occurs, the researchers investigated the time evolution of the distribution function of high-energy particles. This tells us how many particles are present at each location in the plasma, with what speed and energy.
The shape of this distribution function can have a significant impact on the progression of fluctuations, and conversely, fluctuations can deform the distribution function.
Assistant Professor Hao Wang and his colleagues conducted a detailed analysis of the simulation results. They found that as the first fluctuation becomes larger, the distribution function of the high-energy particles deforms significantly, and this deformation causes the second fluctuation. In other words, we revealed that the two fluctuations occur in conjunction through the deformation of the energy particle distribution function.
Significance and future efforts
To achieve fusion energy, high-energy particles produced by fusion reactions must heat the plasma to sustain these reactions. For this purpose, it is important to efficiently confine these high-energy particles within the plasma. The combination of fluctuations can result in a significant loss of energetic particles.
By utilizing the knowledge of the physical mechanisms revealed in this research, it will be possible to contribute to the development of methods to suppress the occurrence of coupled fluctuations. Additionally, this physical mechanism could generate a second fluctuation that is difficult to excite directly from the first fluctuation.
This can contribute to heating of the fuel ions. Coupled fluctuations due to high-energy particles have also been observed in space plasma, although the types of waves are different. The analysis method for energetic particle distribution functions developed in this research is also expected to be applied to space plasma.
In the future, the researchers plan to conduct simulations that calculate both energetic particles and fuel ions to investigate the role of the latter in coupled energetic particle-driven fluctuations and the energy transfer to them.
Further information: Hao Wang et al, Nonlinear excitation of high-energy particle-driven geodesic acoustic modes by resonant overlap with Alfvén instability in the ASDEX upgrade, Scientific Reports (2025). DOI: 10.1038/s41598-024-82577-3
Provided by National Institutes of Natural Sciences
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