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August 3, 2015
Data Analysis of Waves Inside a Plasma: Cooperating with Basic Plasma Experiments

For the realization of fusion energy, in the plasma experiment devices around the world beginning with the Large Helical Device (LHD) research is being conducted for producing and maintaining high-temperature plasmas. In research to this point, it has become clear that causes for the lowering of a plasma’s temperature include the “fluctuations” and the “waves” that develop in a plasma. Here, we introduce research that, in seeking to suppress the generation of waves, is advancing in cooperation with basic plasma experiments.

In nature there are various types of waves, such as the waves on the water’s surface, the waves of earthquakes, sunlight, and others. Further, speaking in general regarding the waves of water, there are many sizes of waves, such as the small and large waves in the sea and the ripples in the bathtub. We know that there are many waves of various types and sizes in a high-temperature plasma, too. When these waves appear, the plasma will be disturbed, and the plasma that has been successfully heated will cool.

In order to suppress the emergence of waves, first we investigate in detail the characteristics of the waves that have appeared. Then we must ascertain the cause of the wave’s appearance. For this reason we have installed various diagnostics to the LHD. (For details, please see Research Update 213 and 248. ) Because there is information regarding these various waves in the signals that have been measured, this research is extremely complicated. Thus we use computers to analyze the data. However, analyzing the data is complicated, and developing a system for a large device such as the LHD is a Herculean task. Using a basic experimental device whose size is of a smaller scale and whose structure is simple, various types of basic research, such as measurement methods, are being undertaken. For example, basic research that uses a “linear-type device” confining plasma by using straight magnetic field lines is advancing at the National Institute for Fusion Science, Tohoku University, Nagoya University, Kyushu University, and elsewhere.

In these basic experiments using this linear-type device, a method of analysis for the complicated data including the numerous waves appearing in the plasma has been established. At present, using this method, we are conducting data analysis by utilizing computers regarding waves in high-temperature plasma in the LHD. From the complicated experimental data obtained from diagnostics, we extract information for each wave after first removing the noise. Then we determine the natural law regarding the waves. From this data analysis we are becoming better able to identify the nature of problematic waves that lower the temperature of the plasma. For example, in an LHD plasma, we observed for the first time in the world a new species of wave with a large amplitude and a large wavelength. We also learned that a large wavelength wave connects directly the high-temperature core region of the plasma to the low-temperature edge, and this lowers the temperature of the plasma’s core. In this way, by analyzing complicated experimental data we were able to obtain a full image of a wave from limited information and clarify the influences that a wave imparts upon plasma temperature. From now, by clarifying the physical mechanism of the observed wave, and by searching for operational methods that stop the generation of those waves we will contribute in realizing fusion power.