Figure 2: This shows the restoring force against the growth of fluctuations. When the amplitude is lower than the threshold value, the amplitude approaches zero and is stable (black). Separating from the center, when the amplitude exceeds the threshold value, the amplitude abruptly grows red. Credit: National Institutes of Natural Sciences
In the Large Helical Device (LHD) at the National Institute for Fusion Science in Japan, researchers have developed the high-energy heavy ion beam probe, in order to perform potential measurement inside plasma that was generated in the LHD and have clarified the mechanism of this new phenomenon.
Seeking to achieve nuclear fusion, research on plasma of >100,000,000 C is being conducted around the world. In a magnetically confined plasma, sometimes there abruptly occurs the excitation of fluctuations with large amplitude, which leads to a possible plasma loss. Such a phenomenon influences the performance of the nuclear fusion reactor. Because there is the possibility of damage to the surrounding construction material, clarifying the mechanisms that lead to excitation, predicting excitation, and avoiding excitation are important issues.
On the other hand, in cosmic plasma, too, similar abrupt phenomena occur, and among them the appearance of solar flares is well known. However, in either case, why large events abruptly occur is not well understood. At present, this is an unsolved problem.
Examining the experimental data in detail, they achieved the result in which before the excitation of this abrupt fluctuations occurred there was generated a separate fluctuation, which triggered the process, and a result which indicates abrupt large amplitude fluctuations had been obtained.
The researchers constructed a new theoretical model for explaining this phenomenon. When they conducted confirmations through numerical simulations they successfully reproduced the experimental results. From this, they were able to discover the heretofore unknown phenomenon of abrupt excitation of fluctuations, to clarify the mechanism, and to predict excitation. They proved that when the stimulus from outside is beyond a certain level, the physical mechanism exists in a high-temperature plasma that excites abrupt and large amplitude fluctuations, and they clarified the conditions necessary for excitation. Phenomena that possess this type of quality are called subcritical instability.
As an example of the phenomenon in which large amplitude fluctuations abruptly is excited, in a magnetically confinement plasma, there are collapse phenomena such as sawtooth oscillation and disruption which degrades plasma performance, and in cosmic plasma there is the abrupt occurrence of solar flares. As candidates for causing these abrupt phenomena, the existence of subcritical instability was indicated theoretically. Through this research, it has been proven for the first time that such an instability exists in geodesic acoustic waves, which are in a plasma, and we successfully predicted the occurrence of this phenomenon. These results are expected to be indicators in addition to advancing our understanding of numerous abrupt phenomena that are widely observed. The abrupt excitation of fluctuations gives indications of the possibility of plasma heating that these fluctuations contribute to. Moreover, research in a confined plasma that can clarify the occurrence mechanism of abrupt phenomena and predict occurrence will contribute greatly to future nuclear fusion research and the development of science and technology, eg avoiding damage to the nuclear fusion reactor and suppressing damage from magnetic storms. http://www.eurekalert.org/pub_releases/2016-01/nion-aep011116.php
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