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March 10, 2014
The “Flare of My Dreams” in My Mind: Integrated Simulation Research

Experiment-based research that aims at achieving the future fusion energy is moving forward in the Large Helical Device (LHD). Benefitting from recent, rapid advances in supercomputers, we are working too on integrated simulation research that reproduces computer simulations of the complicated behavior of extreme high-temperature plasma that has been observed in LHD experiments. Here we will introduce integrated simulation research that is important in accurately predicting plasma performance in designs for the future fusion energy power plant.

“The Flare of My Dreams” = fusion. This was a word mentioned in a newspaper interview in 1981 by a predecessor who pulled me into nuclear fusion research. Thirty years later, much intellect has come together, and research aimed at realizing fusion energy is achieving many important developments. Together with these developments in research, devices have grown immense, and extreme high-temperature plasma that exceed several tens of millions of degrees is being confined through a powerful superconducting magnetic field. Such extreme high-temperature plasma behaves in complicated manners through various physical processes. As a result, the character of plasma, beginning with its temperature and density, is now observed through experiments. In the past, we moved forward with research while concentrating on physical processes that were most strongly related to the character observed in plasma and comparing theoretical calculations that corresponded to the physical processes. However, because physical processes relate in complicated ways in extreme high-temperature plasma, we combined individual computer programs that related to these various physical processes, and we have recently initiated integrated simulation research that reproduces extreme high-temperature plasma in the computer. Through this approach we have deepened our comprehension of the individual physical processes that are the fount of the complicated behavior of extreme high-temperature plasma. Moreover, this is a research plan for trying to predict the behavior of plasma inside the future fusion reactor.

As this is an extremely large plan, results will accumulate steadily. For example, if we are able to accurately predict “If we introduce a certain amount of heat power, what will the temperature of the plasma become?” we will be able to further accelerate our research. However, unfortunately, research is not that simple. Through the collision of plasma particles, the remarkable transfer of heat inside a plasma, the changes in the magnetic field that confines the plasma, the changes caused by that in the plasma’s heating efficiency, and other factors, the plasma’s temperature is decided by numerous complicated physical processes. Thus, integrating extreme high-temperature plasma theory and physics models with numerical data from large-scale simulations, we are trying to make it possible to achieve highly precise predictions.

In a word, to “integrate” is not simple. We extract important information from the computer programs that correspond to the various physical processes and input these data into another computer. In addition, because it is necessary to conduct that manipulation without using human hands, together with a mutual understanding of the physical processes that are the contents of the calculations, technologies for working with the experiment data and the calculation data are necessary. For that reason, if we do not integrate data based upon a solid understanding, we cannot conduct appropriate calculations. We will move forward with integrated projects with the wide expertise and the high-level skills possessed by the many researchers.

Using integrated packages in computer programs composed heretofore, we have the example of success in replicating the distribution of electron temperatures that have been measured in experiments. However, at present, we have not been able to replicate the distribution of ion temperatures. We are undertaking a review of the model for the transport of ion heat. Examples of comparative evaluations are still few, but while utilizing LHD experiment data we are moving forward with integrated simulation research.

Here, taking the temperature of plasma as an object of analysis, I have introduced integrated simulation research that conducts “fusion in one’s head” through computers. This introduced but one small aspect of the complicated behavior that extreme high-temperature plasma displays. Together with pressing further forward with integration whether or not results from simulation research can be replicated in LHD plasma, building upon accumulated verified research we are aiming to contribute to the improved performance of LHD plasma and to the prediction of the future fusion energy power plant’s plasma performance.