Advances in Understanding Energetic Ion Transport by Magnetic Field Oscillations

We have shown that the magnetic field oscillations caused by energetic beam ions induce the transport of energetic ions exciting the oscillations. In this study, we developed a new detector to measure high-energy neutrons with high sensitivity and observed that energetic particles that are not the cause of the oscillation also escape from the plasma. This achievement has advanced our understanding of energetic ion transport due to magnetic field oscillations, which is a concern in future fusion reactors.

This is a detector for detecting high-energy neutrons with high sensitivity, which was newly developed in collaboration with the National Institute for Fusion Science and the National Institute of Technology, Toyama College. About 500 optical fibers are embedded in the head to detect high-energy neutrons.

In future fusion burning plasmas, there is a concern that energetic alpha particles may escape due to magnetic field oscillations caused by the pressure of alpha particles, which is the main heating source of the plasma. In the Large Helical Device (LHD), the energetic ion confinement study has been intensively studied to understand a way to reduce the energetic ion transport due to the magnetic field oscillations. The steep gradient of energetic beam ions pressure sometimes generates magnetic field oscillations. We reported that the magnetic field oscillations are caused by the pressure of the energetic ions and up to about half the beam ions escape from the plasma due to the oscillation, thereby reducing heating efficiency, "Investigating the cause of the disturbance in maintaining the high-ion temperature state".

This research group focused on how the magnetic field oscillations affect the confinement of energetic particles produced by a fusion reaction between deuterium and deuterium, which is not the source of the magnetic field oscillations. To investigate this effect, we increased the sensitivity of the detector, which measures a small number of high-energy neutrons produced by the reaction between energetic particles and bulk ions, by a factor of about 50. The amount of reduction of these energetic particles by the magnetic field oscillation was proportional to the cube of the magnitude of the magnetic field oscillation, and the maximum reduction was found to be 30%. This achievement has greatly advanced our understanding of the effect of magnetic field oscillations generated by energetic ions on the confinement of energetic ions, which is a concern in future fusion plasmas.

This research was carried out by a research group led by Kunihiro Ogawa and Mitsutaka Isobe at the National Institute for Fusion Science, in collaboration with Professor Takata at the National Institute of Technology, Toyama College, Professor Murakami at Kyoto University, and Dr. J. Jo at the Korea Institute of Fusion Energy.

The results of this research were selected as an oral presentation at the 28th International Atomic Energy Agency Fusion Energy Conference and published in the journal Nuclear Fusion on August 17, 2021.