The first experimental proof of the propagation of plasma turbulence
In seeking to achieve fusion energy, research on magnetic field confinement of high-temperature plasma is being conducted around the world. In a high-temperature plasma, there is a temperature gradient. When the temperature gradient becomes steep, turbulence is generated. Because the high-temperature regions and the low temperature areas are mixed due to the turbulence, the core temperature cannot be effectively raised. Thus, research on the generation and suppression of turbulence is being performed around the world on experimental devices of magnetically confined plasma (tokamaks and helical devices). It has been very difficult to distinguish where and how turbulence propagates in a plasma. Researchers have predicted that turbulence expands into other regions, a phenomenon called "turbulence propagation," but this has not been observed experimentally.
The research group of Professor Katsumi Ida and Professor Tatsuya Kobayashi of the National Institutes of Natural Sciences (NINS) National Institute for Fusion Science (NIFS) and United States collaborators conducted research on turbulence propagation in the Doublet III-D tokamak of General Atomics, in the United States. They used the so-called "heat pulse modulation method" in the Large Helical Device (LHD) at NIFS to measure turbulence in a special region called the "magnetic island," where turbulence theoretically should not be generated because there is no temperature gradient. They discovered that the turbulence exists in the magnetic island, and propagates faster toward the center of the magnetic island, which is called the O-point, than the modulated temperature change. Thus, the researchers observed turbulence propagation for the first time.
The results have been published in Physical Review Letters. In the future, the researchers will continue to improve high-performance plasma through accumulating further knowledge on the suppression of turbulence.