National Institute for Fusion Science

In the Large Helical Device (LHD) at the National Institute for Fusion Science development of methods for measuring the condition of high-temperature high-density plasmas confined in the magnetic field is being undertaken. Through applying a method developed in the LHD to an experimental device in the United States we found a new confinement state. Here we introduce this new research result.

Aiming to achieve fusion energy, research in confining a high-temperature, high-density plasma in the nested magnetic field container is being conducted around the world. Plasma confined in the nested magnetic field container has a high temperature in the core area, but the temperature falls extending outward from the core area. In achieving fusion energy, exceptional heat insulation performance that raises the core temperature is required. Objects nearby and convenient that have exceptional heat insulation performance include thermos bottles and feather futons. In fusion research, this heat insulation performance is called “confinement performance.” When confinement performance is good, from being able to maintain a high-temperature plasma within a small radius we can make the fusion reactor smaller and reduce construction costs.

At the National Institute for Fusion Science, as a method for finding areas where confinement performance inside the plasma is high we have proposed the “momentary heating propagation method” and have researched its effectiveness in the LHD. This method should instead be called the hammering sound test method. Like being able to investigate the condition of the metal after listening to the sound after striking by a hammer, we heat the plasma momentarily and from the speed at which the heat propagates we investigate confinement of the plasma. Because the speed at which the heat spreads slows when the confinement is good, from the propagation speed we can estimate the confinement performance.

In the LHD experiments, using this momentary heating propagation method we have investigated the confinement of the area called the “magnetic island.” In the nested magnetic field container which confines the plasma, the cross-sections resemble tree rings, and in those cross-sections appears a crescent area that resembles the knot of the grain. As that area is shaped like an island in a river, it is called the “magnetic island.” In LHD experiments, magnetic islands having high confinement performances that are seven times more than the area outside the magnetic island have been observed.

This time, we applied the momentary heating propagation method to the DIII-D device of General Atomics in San Diego, California, in the United States. As one result from collaboration research by a Japan-United States group, we discovered a “particularly exceptional magnetic island” whose confinement performance was forty times greater than for area outside the magnetic island. Moreover, we also were able to measure a phenomenon that came and went (self-regulated oscillation), in the “good magnetic island” whose confinement performance was five times greater and in the “particularly exceptional magnetic island.” That is, the magnetic island is in the condition in which it is difficult for temperature change to propagate (five times the confinement performance) and in the condition in which it is still more difficult to propagate (40 times the confinement performance).

The discovery of self-regulated oscillation means that there are diverse plasma confinement conditions. Regarding diversity, the heat flux (the quantity that heat propagates per unit time and per unit area) that responds to the temperature gradient indicates not one but, rather, two values. The value of the heat flux that propagates through objects, such as a copper rod, indicates one value if the temperatures at both ends are determined. Instead of deciding upon one value, plasma shows especially complicated behaviors. In the future, in order to maintain magnetic islands whose confinement performance is exceptional it will be necessary to determine how to clarify this. At present, in order to establish guidelines, we are examining closely the phenomena of self- regulated oscillations of magnetic islands through both theory and experiment.

In this way, by applying to an American experimental device a method developed on the LHD, expected results were achieved that will greatly contribute to fusion research in the future. These research results were published in the academic journal Scientific Reports, which is published by the Nature Publishing Group in England. And as an important result of the Japan-United States Collaboration Research Group this was widely reported by newspapers and internet news services.