When observed over a long period of time, there are phenomena that will occur certainly some day and for which it is difficult to accurately when an event will occur. For example, it is clear that the energy of volcanoes is accumulated as magma illuviates. However, it is difficult to predict by what timing the eruption event will take place. Such a phenomenon is called a “sudden phenomenon.” Earthquakes, sudden downpours of rain, solar flares, auroras, and economic crisis, too, are examples that may be raised. The sudden phenomenon is considered to be a transition phenomenon from the condition (metastable) in which no event happens although energy continues to accumulate, to the other stable condition due to a trigger that causes some change. If we can clarify the trigger, then it may become possible to predict the trigger immediately prior to the phenomenon. Here, we will introduce research that focuses on the sudden trigger phenomenon of plasma in the Large Helical Device.
In order to achieve fusion energy, it is necessary to confine a stable plasma at high temperatures in the magnetic field. However, based upon plasma experiments to date, we have learned that distortions have appeared in high-temperature plasmas confined in the magnetic field and at a particular time there suddenly are instances in which a portion of the plasma is lost. In order to stabilize and confine the plasma, clarifying how this sudden phenomenon is triggered is a very important topic. Thus, in the LHD we have carefully observed plasma distortions in detail, and have investigated in detail how sudden phenomena emerge.
In the LHD, distortions emerge in plasma from much before the occurrence of a sudden phenomenon. Because this distortion emerges throughout the doughnut-configuration plasmas, there are distortions on the inner side and on the outer side. And those distortions are changing in time. Further, the radius of the cross-section is approximately 60 centimeters and the size of this distortion is as much as 0.5 centimeters. This time, we learned that the plasma distorts outside by 1~2 centimeters just after the disappearance of the distortion throughout the plasma and one ten-thousandth of a second later a sudden phenomenon in which the distorted part of the plasma is lost occurs. That is, we clarified that the changes in the distortion on one part of the outer side of the plasma are the trigger of the sudden phenomenon. And we realized that if we can capture this trigger, then it would be possible to predict an event immediately before the sudden phenomenon in which one part of the plasma would be lost appears.
We presented this research at “The Science of Sudden Phenomena” symposium of the Physical Society of Japan held at Tokyo University of Science on March 22, 2018. At this symposium, active discussions of research relating to various types of sudden phenomena, such as volcanic eruptions, downpours of rain, solar flares, and other such phenomena were held. Sudden phenomena which may be observed in plasma experiments does not extend to directly influencing our daily lives. However, that result, the trigger which we have clarified in research, has gained wide attention through the possibility of predicting natural disasters beforehand. Next, together with seeking to achieve the generation of fusion energy, we will aim to acquire knowledge that also contributes to research on various types of sudden phenomena, and we will deepen our understanding of sudden phenomena in plasma.
The image above shows outward and inward distortions born in a plasma in red and in blue, respectively. To the extent that the color is dark the distortion is large. From quite some time from before the sudden phenomenon called plasma loss occurs, a distortion appears throughout the plasma (left image). Immediately before the sudden phenomenon, the change in which the distortion concentrates in the part of the plasma (right edge) appears (right image). This change becomes the trigger of the sudden phenomenon. (The images are reproduced from Parity, vol. 33:1 (2018), from page 40.)