Turbulence permeating the inner part of a plasma magnetic island
The "instantaneous heat propagation method" devised for LHD at the National Institute for Fusion Science was applied to a tokamak device (DIII-D) in the United States. Together with collaborators in the United States, we observed the phenomenon of turbulence propagating into the inner part of the magnetic island. It is well known that turbulence is generated as temperature and density increase in magnetic confinement plasmas for nuclear fusion, but this is the first experimental discovery that turbulence has propagation property.
The "instantaneous heat propagation method" devised for the Large Helical Device (LHD) at the National Institute for Fusion Science was applied to a tokamak device (Doublet III-D) in the United States. Together with collaborators in the United States, we observed the phenomenon of turbulence propagating into the inner part of the magnetic island. It is well known that turbulence is generated as temperature and density increase in magnetic confinement plasmas for nuclear fusion, but this is the first experimental discovery that turbulence has a propagation property.
Research on confining high-temperature plasma with a magnetic field is being conducted worldwide to realize fusion power generation. A heated high-temperature plasma has a temperature gradient, where the temperature is high at the center and low at the outside. As the central temperature becomes higher and the temperature gradient becomes steeper, turbulence is generated, and the increase of central plasma temperature is suppressed. For this reason, research on the generation and suppression of turbulence has been conducted in tokamak and helical plasma experimental devices around the world. Turbulence in plasmas has been observed in many studies. However, it has been challenging to identify where the turbulence is generated. The phenomenon of "turbulence propagation," in which the generated turbulence spreads to other plasma regions, has been predicted theoretically but has never been observed experimentally.
The research team applied the "instantaneous heat propagation method" developed for the Large Helical Device (LHD) to General Atomics' tokamak-type fusion device, the Doublet III-D, and observed the "propagation of turbulence" experimentally for the first time in the world. In order to verify "turbulence propagation," we attempted to make observations in a unique region of the plasma called a magnetic island, where the temperature gradient is zero and no turbulence is generated. As a result, we discovered that turbulence exists in the magnetic island region, where it should not occur. The turbulence is transmitted to the center of the magnetic island before the temperature change. This result experimentally demonstrated that the turbulence could be confined to the location where it originated if the propagation is suppressed. This achievement lead to a new concept of restricting turbulence by suppressing its propagation rather than suppressing its generation itself.
This research was conducted by a research group led by Katsumi Ida, Tatsuya Kobayashi, and Makoto Ohno at the National Institute for Fusion Science in collaboration with Dr. Todd Evans and Dr. George McKee at General Atomics Corporation in the United States.
The research results were published in the Physical Review Letters on 12 June 2018, for a preliminary report. The Plasma Physics and Controlled Fusion on 24 October 2019, for a summary.
- K. Ida et al., "Hysteresis Relation between Turbulence and Temperature Modulation during the Heat Pulse Propagation into a Magnetic Island in DIII-D", Physical Review Letters 120 (2018) 245001.［NIFS Repository]
- K. Ida et al., "On the interplay between MHD instabilities and turbulent transport in magnetically confined plasmas", Plasma Physics and Controlled Fusion 62 (2020) 014008.［NIFS Repository]
- Magnetic island: A nested cage made of a magnetic field must be formed to confine the plasma. The cross-sectional shape of the magnetic cage is concentric, like the annual rings of a tree. In this experiment, a slight magnetic field (perturbation field) was intentionally applied from outside to create a crescent-shaped structure like the grain of a tree. This structure is called a magnetic island because it looks like an island in a river.
- Instantaneous heat propagation method: This method was devised for the large helical device at the National Institute for Fusion Science to investigate the confinement performance of the plasma by applying heat to the plasma instantaneously and measuring the speed at which the heat propagates. This technique resembles a percussion test where one would listen to the sound of metal after it has been struck.