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June 1, 2015
Long-Term Prediction of the Temperature and Density of Plasma Made Possible: Computer Simulation Research Focusing on the Butterfly Effect

At the National Institute for Fusion Science, using the Large Helical Device (LHD) we are conducting research in magnetic field confinement of plasma whose temperature exceeds 100,000,000 degrees. The characteristics of such plasma are extremely complex. For example, in observing in detail the changes over time in density and temperature, those quantities are changing in an irregular manner. Such irregular fluctuations are one of the causes that produce turbulence in a plasma. Further, turbulence influences importantly the performance of plasma. Here, we will introduce computer simulation research on temporal variation in the temperature and density of plasma by focusing upon fluctuations.

In a computer simulation, we specify the condition at a certain moment and then examine changes over time from that moment. In a simulation involving the temperature and density of a magnetic field confinement plasma, we establish the initial value for the temperature and for the density, we heat a plasma from outside or inject particles that will become fuel, and then we proceed with calculations. In this simulation, we can recreate the irregular changes and plasma turbulence caused by those irregular changes over time in temperature and density. Further, if we undertake calculations over a long duration, we can know the temperature and the density of plasma after that long duration. However, there is an important issue, which is that simply because of the condition at the beginning the result after a long period of time has passed can greatly differ.

This problem in which the result is sensitive to the original condition is related not only to the fluid phenomena of the high-temperature plasma but also to changes in weather, which is called the “butterfly effect.” This term is said to derive from the saying that the extremely small air turbulence caused by the flapping of a butterfly’s wings will affect the weather for a long period of time. This is one cause for tomorrow’s weather report often being accurate and the weather report for one week in the future being difficult to predict. The butterfly effect is caused by turbulence that results from irregular changes over time. And no matter the precision of the calculation, we cannot eliminate that effect. For this reason, we produce many early conditions that vary but slightly, and calculate the time changes of each. If a similar result can be obtained, this will be considered a highly reliable forecast.

In simulations of plasma temperature and density, too, it is necessary to establish several early-stage conditions for each plasma and conduct calculations, and to compare the many results obtained. We develop simulations and methods for analyzing data, and based upon the most appropriate predictions we have become able to conduct research on the qualities of time changes and turbulence in plasma. Further, we are able to investigate how a plasma’s condition changes through external heating and refueling, and examine methods for achieving high-performance plasma.

For achieving fusion energy it is necessary to confine plasma for a long period of time. Thus, long-duration prediction of a plasma is an important topic. In the future, we aim to enhance the reliability of long-duration prediction and to achieve high-performance plasma.