National Institute for Fusion Science

In the Large Helical Device (LHD) plasmas whose temperatures exceed 100,000,000 degrees are confined by a magnetic field. Because the behavior of such plasmas is extremely complicated, utilizing computer simulations we are carrying out reproductions and predictions from experiment results. At that time, as a result of interactions of fast microphenomena in an extremely narrow space in complex ways, plasmas are observed as a total phenomenon (macrophenomenon) that extends over a wide area, one is asked to link the microphenomena and the macrophenomena, which have different temporal and spatial scales, and solve this issue. In this report we will introduce a new simulation method that can solve the multi-time and multi-space phenomena of such plasma.

Human beings emerge from organs having various functions, and those organs are formed from cells. The cells come from the molecules and the atoms, and if seriously considered, from elementary particles. On the other hand, when human beings gather they form companies and societies, and, further, countries. In this way, in the case when the special characteristic of size and the scale of active time for a society, a unit, an organ, a cell, an atom, and an elementary particle vary, it is said that layers exist. Further, in this way, in the world of nature phenomena of varying scales are often seen in multi-layered phenomena that are interwoven in complex ways.

The phenomena at each layer do not occur independently. Because they compose one overall phenomenon while together affecting each other, it is not simple to understand a multi-layer phenomenon. A simple sum total of each layer does not express the whole. For that reason, by investigating only the smallest layer one cannot understand the behavior of the whole. Even if we investigate only the elementary particles and cells, we can easily imagine that we will not understand the changes in people, much less the changes in society.

Plasma is a treasure house of multi-layer phenomena. In the high-temperature plasma confined by the LHD magnetic field, each movement of a particle is at the extremely small scale of 10 micrometers. The fluctuations and turbulence that emerge at such a microscale influence the confined plasma’s performance throughout the device. On the other hand, the structure of the plasma that extends throughout the device also greatly influences the behavior of the microscale fluctuations and turbulence. That is, each is a cause and each is also a result.

Regarding this type of complicated multi-layer phenomenon, in a supercomputer, through computer simulations that reconstruct a phenomenon virtually, until recently simulations were conducted that separate layers into individual layers and treat each phenomenon independently. Thus at the National Institute for Fusion Science we have developed a new simulation that connects and solves from macrolayers to microlayers. Through this new method, we separate the continuous virtual space that was created inside the supercomputer into macrolayers and microlayers and we conduct calculations using different equations that are appropriate for expressing each layer’s phenomena. Then without separating these macrolayers and microlayers, together we exchange necessary information and move forward in the full simulation without contradictions. We have called this multi-layer simulation.

We applied this multi-layer simulation method to the phenomenon called magnetic reconnection, which performs the fundamental role in nuclear fusion plasma. Now, in the magnetic reconnection we are continuing to explain how each layer is intertwined. In the future, using the multi-layer simulation method we will reproduce a complete plasma of a nuclear fusion reactor through a supercomputer. Moving forward in promoting design research for the nuclear fusion power plant through predictions of its behavior is anticipated.