In phenomena occurring in nature and in society, it is often commented that "Something may happen at any time, but it is difficult to predict when that something may occur." Solar flares, volcanic eruptions, extreme rainfall, and economic crises are examples of this. Such cases are called sudden phenomena. Prediction of sudden phenomena is one of the important issues in physics. By comparing physical processes related to sudden phenomena by researchers from various fields, new research is being initiated in which we fully understand sudden phenomena and what we understand leads to predictions of those phenomena.

In the fusion research, too, understanding sudden phenomena is an important research topic. In order to achieve fusion energy, it is necessary to stably confine high-temperature high-density plasma in the magnetic field. However, as plasma reaches higher temperatures and densities it becomes unstable, and the sudden phenomena in which portions of the plasma are lost are frequently observed. In the stable confinement of plasma, it is necessary to predict and avoid sudden phenomena. However, causes of sudden phenomena are not yet clear, and it was not possible to conduct predictions.

At the National Institute for Fusion Science, the research group led by Professor Katsumi Ida and Assistant Professor Tatsuya Kobayashi clarified for the first time in the world the trigger mechanism (that is, the changes that appear immediately before) by which the sudden phenomenon appears in a plasma confined in the magnetic field on the Large Helical Device. Through this finding, it has become possible to predict sudden phenomena of a plasma immediately prior to its appearance.

In LHD plasma, deformations of the plasma occur before the sudden phenomenon occurs. This deformation is called the resonant mode, and it extends throughout the plasma. (See the top image.) This research group clarified that the resonant mode is not linked to the sudden phenomenon. Rather, the research group clarified that the resonant mode performs a role similar to the "intermittent release of tension" which avoids the occurrence of the sudden phenomenon. Further, they discovered that when this resonant mode stops suddenly for a short period of time, similar to "the quiet before the storm," (see the middle image) in the next moment a change called the non-resonant mode grows, and the sudden phenomenon, in which suddenly a portion of the plasma is lost, occurs. This non-resonant mode is concentrated locally (see the bottom image). By observing this plasma deformation due to the non-resonant mode, the occurrence of the sudden phenomenon can be predicted immediately beforehand. In this way, by skillfully seizing the "trigger" that occurs immediately before the sudden phenomenon, they learned that it was possible to make predictions immediately prior to the phenomenon.

These results are believed to provide important guides for research regarding sudden phenomena observed in nature and in society. The results also will be presented at the symposium "Science of Sudden Phenomena" of The Physical Society of Japan, which will be held at the Tokyo University of Science from March 22 to March 25, 2018.

Terms

Sudden phenomenon:

The phenomena that is expressed as "This will not be a surprise no matter when it occurs, but we do not know when it will occur." Its prediction is difficult. In the sudden phenomena, the state in which energy continues to accumulate (metastable) but nothing happens moves to a different stable state by releasing energy triggered by some event. It is contrasted with the intermittent phenomenon that occur intermittently when energy is accumulated in a fixed quantity.

In the intermittent phenomenon, as represented by the intermittent spring (springs that discharge boiling water and vapor in a fixed period), if we calculate the necessary amounts of energy required (amount of heat necessary to spout hot vapor and water), then we can predict quite accurately when the intermittent phenomenon will occur.

On the other hand, in the sudden phenomenon, using a volcano as an example, there is no doubt that energy accumulates through the accumulation of magma. However, it is extremely difficult to predict by what timing the volcano will erupt. The change that occurs immediately before the sudden phenomenon is called the "trigger."

Resonant mode and non-resonant mode:

Resonant mode is the phenomenon resonant with the external force (energy) having some frequency. A pendulum and a swing are good examples. The special characteristics of these phenomena are highlighted by the fact that the amplitude oscillation increases when increasing the power provided from outside. In contrast, non-resonant mode is the phenomenon that does not have any resonant frequency and even does not oscillate, and suddenly grows. This is similar to fracture.