New Insights into Coupled Fluctuations in Fusion Research

Researchers from the Max Planck Institute for Plasma Physics (IPP) and the National Institute for Fusion Science (NIFS) have worked together on a simulation study to elucidate the physical mechanism by which two fluctuations occur in a coupled manner, according to a study published in Scientific Reports.

Background

Multiple fluctuations occurring in a coupled manner are a common phenomenon in nature. For instance, there have been documented instances of major earthquakes happening one after the other in nearby areas. Larger-scale events happen when several fluctuations occur in this coupled manner because the coupled ones release more energy than a single fluctuation. The confinement of energetic particles is known to be weakened by energetic particle-induced fluctuations that occur in fusion plasmas.

These oscillations are expected to play a role in transferring energy from energetic particles to fusion fuel ions, aiding in heating. As a result, energetic particle-induced fluctuations are a key challenge in fusion research. Coupled-mode fluctuations, in particular, are of special interest due to their potential to grow into large-scale phenomena.

Two coupled fluctuations were reported at the ASDEX Upgrade device in Germany. While energetic particles were identified as the cause of these fluctuations, the fundamental mechanism of their coupling was not understood.

Results

NIFS created an algorithm called “MEGA” to model plasma fluctuations generated by energetic particles. This is known as a “hybrid simulation” because it performs coupled and simultaneous calculations on particles and fluids. The MEGA code has been used to test experimental equipment in Japan and abroad, and its usefulness has been demonstrated by comparing to diverse experimental outcomes.

This time, Assistant Professor Hao Wang of NIFS and a team of researchers ran a MEGA simulation on a supercomputer, and were successful in recreating the behavior observed in the ASDEX-Upgrade device, in which two fluctuations occur simultaneously. In the simulation, energetic particles generated an initial fluctuation at a frequency of 103 kHz. This was followed by a second fluctuation at 51 kHz, which eventually grew larger than the first. These simulation results are in line with the experimental findings.

To further understand the second fluctuation’s generation mechanism, researchers looked at the time evolution of the distribution function of energetic particles. This describes the number of particles, their velocities, and energy at each position in the plasma. The shape of this distribution function has a significant impact on the development of fluctuations, which, in turn, can modify the distribution function.

Assistant Professor Hao Wang and collaborators conducted a thorough investigation of the simulation results. They discovered that when the first fluctuation increased, the distribution function of energetic particles became dramatically distorted, causing the second fluctuation to arise. In other words, they discovered that the two fluctuations happened simultaneously via the distortion of the energetic particle distribution function.

Significance and Future Work

To achieve fusion energy, energetic particles produced by fusion processes must be used to heat and sustain plasma. For this reason, it is critical to effectively confine these energetic particles within the plasma. Coupled fluctuations can cause large losses of energetic particles. Using the understanding of the physical mechanism elucidated in this study, researchers will be able to contribute to the development of strategies to inhibit the coupled formation of fluctuations.

Furthermore, this physical mechanism may enable people to generate the second fluctuation, which is difficult to stimulate directly, from the first. This could help to heat fuel ions. Although the types of waves in space plasmas differ, coupled fluctuations driven by energetic particles have been observed. The analytical approach to the energetic particle distribution function proposed in this study is designed to be applicable to space plasmas.

In the future, researchers intend to run simulations that include both energetic particles and fuel ions to look into the role of the latter and the energy transfer to them in coupled energetic particle-driven fluctuations.

This study was partially funded by MEXT as a "Program for Promoting Research on the Supercomputer Fugaku (Exploration of Burning Plasma Confinement Physics, JPMXP1020200103)", JSPS KAKENHI Grant Nos. JP18K13529, JP18H01202, JP21H04973, and "PLADyS", JSPS Core-to-Core Program, A. Advanced Research Networks.

This study was also partially financed by the National Institutes of Natural Sciences' Promoting Research by Networking among Institutions program (Grant Number 01422301) and NINS's International Research Exchange Support Program. Furthermore, this work was largely carried out under the scope of the EUROfusion Consortium, which was supported by the European Union through the Euratom Research and Training Programme (Grant Agreement No. 101052200 - EUROfusion).

With the assistance and approval of the NIFS Collaboration Research program (NIFS19KNXN397, NIFS20KNST156, NIFS21KNST196, and NIFS22KIST025), the JFRS-1 supercomputer system at the Computational Simulation Centre of International Fusion Energy Research Centre (IFERC-CSC), and the Supercomputer Fugaku provided by the RIKEN Center for Computational Science (Project IDs: hp200127, hp210178, hp220165), numerical calculations were conducted on the NIFS "Plasma Simulator" (NEC SX-Aurora TSUBASA) of the National Institute for Fusion Science (NIFS).

Journal Reference:

Wang, H., et. al. (2025) Nonlinear excitation of energetic particle driven geodesic acoustic mode by resonance overlap with Alfvén instability in ASDEX Upgrade. Scientific Report. doi.org/10.1038/s41598-024-82577-3