Around the world, research is advancing to efficiently confine fusion plasma and harness its immense energy for power generation. However, it is known that turbulence occurring at various scales within the plasma causes the release of plasma energy and constituent particles, degrading the confinement performance. Elucidating this physical phenomenon and suppressing performance degradation is critically important. Particularly in the high-temperature plasma experiments currently conducted worldwide, micro-scale (just a few centimeters) turbulent eddies forming at various locations within the plasma significantly impact this confinement performance degradation. While it was known that suppressing this micro-scale turbulence could improve performance "to a certain extent", the reason why further improvement could not be achieved remained unclear. In addition, theoretical simulation studies predict that in future fusion power reactors, turbulence smaller than micro-scale will interact and exert influence. While experimental verification was anticipated, it has not been achieved due to the extremely precise measurement technology required.
To investigate the characteristics of two different scales of turbulence, A research group led by Professor Tokihiko Tokuzawa and Project Professor Katsumi Ida of the National Institute for Fusion Science, graduate student Tatsuhiro Nasu of the Graduate University for Advanced Studies, and Professor Shigeru Inagaki of Kyoto University prepared precise measurement instruments tailored to the size of each turbulent eddy. As shown in Figure 1, we simultaneously observed the same location in the Large Helical Device (LHD) plasma to examine how the strength of each turbulence varied. For the smaller-scale turbulent eddies in particular, we enabled simultaneous observation from two directions to capture changes in the eddies' deforming variations. By measuring their degree of deforming, we could determine the state of affected electric field, which was a key factor defining the background force flow at that location. As a result, we discovered that when the strength of the larger-scale turbulence (the blue eddies in Figure 2) suddenly decreased, that of the smaller-scale turbulence (the red eddies in Figure 2) conversely increased. Furthermore, we found that these smaller-scale turbulent eddies exhibited reduced deformation. This experimental result can be explained by a theoretical model suggesting that smaller-scale turbulent eddies can be stretched by the electric field generated by larger-scale ones. In other words, it is thought that the smaller-scale turbulent eddies, which were strongly stretched and suppressed by the larger, began to grow once the latter weakened. It was then speculated that the growth of this smaller-scale turbulence might be the factor causing confinement improvement to stop at "a certain extent," despite the reduction in the previously mysterious micro-scale turbulence.
Future burning plasma targeted for realization in the International Thermonuclear Experimental Reactor (ITER) will rely primarily on plasma heating mechanisms driven by alpha particles generated from fusion reactions. The smaller-scale turbulent eddies measured in this study are believed to be more strongly excited than those currently observed, exerting a greater influence on plasma transport and confinement. Consequently, experimental verification of these smaller-scale turbulent eddies is currently being actively pursued worldwide. Recognizing this challenge early, our research group pioneered the development of measurement techniques. We not only succeeded in elucidating the turbulent response but also established a method to verify the extent of the eddies' elongation, leading to this world-first discovery.
Recent theoretical and simulation studies using huge-scale super-computers have suggested the possibility of newly discovered cross-scale interactions between micro-scale and finer-scale turbulence. This discovery represents the first experimental observation of this phenomenon and is expected to accelerate the refinement of theoretical models. Furthermore, it is anticipated to contribute to improving the performance of future fusion reactors based on these models. From an academic perspective, the interaction between turbulence at different scales and abrupt structural changes in turbulent eddies has been a subject of study not only in laboratory fusion plasmas but also in cosmic plasmas. Detailed experimental observations obtained in the high-temperature plasma of the LHD are expected to contribute to the understanding of plasma physics in other fields as well.
