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March 9, 2016
Discovery of a New Abrupt Phenomenon in High-temperature Plasma in the Large Helical Device: Abrupt Generation of Large Amplitude Waves and Clarification of the Mechanism

In the Large Helical Device (LHD) a new “abrupt” phenomenon of plasma has been discovered. In this phenomenon, inside a high-temperature plasma “abrupt”ly a wave with a large amplitude is generated. Further, we have succeeded in clarifying the mechanism of this phenomenon. Here, we introduce research in this new phenomenon.

In order to achieve fusion energy it is necessary to stabilize and confine high-temperature, high-density plasma. However, because a plasma is an aggregation of numerous ions and electrons, similar to a fluid such as water which is an aggregation of water molecules, inside a plasma various waves are generated. Depending upon the wave generated, the energy and plasma particles escape to outside the plasma, and the temperature and the density fall. For that reason, we are advancing with research for understanding under what conditions are what types of waves generated inside a plasma.

In the LHD, in order to investigate the waves that are generated inside a plasma, in observing “geodesic acoustic waves,” which are one type of sound wave, we discovered the new phenomenon in which a wave typically thought not to be generated abruptly was generated accompanying a large amplitude. Upon closely investigating the experimental data, we learned that when the frequency of the wave originally being observed reaches a certain value, abruptly the wave with one-half of the frequency increases. That is, the wave originally present became the cause, and when that stimulus exceeded a certain level the wave that was thought would not be generated unexpectedly was generated as a large amplitude wave.

Here, in order to consider the mechanism by which the wave was generated, please recall rolling a ball in a valley or on a mountain. In the valley, when one rolls the ball, it merely rolls back and forth. Considering the changing of the placement of the ball as the wave amplitude, the wave amplitude is small and stable. On the other hand, when made to move but slightly the ball on a mountain will roll downhill from the summit. This is unstable resembling the condition in which the wave amplitude rapidly grows. If the mountain summit is concave, what happens then? The ball is stable when in the concave area. But when somehow released from the concave area the ball will suddenly roll downhill. In this way, we call this situation in which the stimulus from outside exceeds a certain level and abruptly becomes unstable a “subcritical instability.” To date, subcritical instability has been identified theoretically as a candidate for causing this abrupt phenomenon.

In joint research with Kyushu University, for this newly discovered phenomenon we have developed a new theoretical model based upon subcritical instability. When we have undertaken a computer simulation based upon this, we have succeeded well in reproducing the experimental results. In this way, we have discovered heretofore unknown abrupt-wave-excitation phenomena, and we have succeeded in clarifying those mechanisms. Further, we were able to indicate that the subcritical instability, which heretofore was known only as theory, actually exists in a high-temperature plasma. Currently, we are investigating what types of influence this abrupt phenomenon has upon the confinement of high-temperature, high-density plasmas.

These results are expected to contribute to a broad range of research topics. The abrupt phenomena have been observed not only in LHD plasma but also in plasmas in other devices. In the tokamak experimental device, which is one of the methods for the confinement of fusion plasma, in the short time of approximately 1 millisecond there occurs the abrupt phenomenon in which the plasma disappears. Further, regarding the abrupt phenomenon of space plasma, there is the solar flare. Predicting the generation of this phenomenon is an important issue. However, this problem of why does sudden generation occur remains unsolved after many years of discussion. The subcritical instability which the LHD proved exists may be a link to solving this issue.