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February 23, 2015
Controlling Convection: Numerical Simulation Using Supercomputers

When we raise the temperature of gas, electrons separate from atoms and ions are born, and each ion and electron take on a plasma condition and move about freely. In this way, high-temperature plasma becomes an ionized gas from ions and electrons. Its quality is determined by the movement of individual particles from ions and electrons. On the other hand, when observing the movement of groups of particles, plasma also exhibits its quality as a fluid. That representative movement is convection. Ions and electrons are particles that bear electricity. In magnetic field confinement fusion experimental devices such as the Large Helical Device (LHD), because ions and electrons are confined in the magnetic field the convection of plasma is an extremely complicated phenomenon. Here, we introduce research regarding the high-temperature plasma convection phenomenon using supercomputers.

In the natural world, there are various turbulent flows. Let us take an example from nearby. When we attempt to boil water, pour water into a kettle, and light a fire, the bottom area where the fire is burning grows hot, but the upper portion remains cold. As a result, the heated water causes an upward convection and, moreover, this convection becomes a turbulent flow, with the hot water and the cold water mixing together efficiently. This convection occurs because gravity acts vertically.

This type of convection phenomena also occurs in a high-temperature plasma that is confined in a doughnut-shape through the LHD’s strong magnetic field. In the LHD, we heat the core of a doughnut-shape plasma. As a result, similar to water inside a kettle, convection occurs. Because the plasma is being confined by the magnetic field, and is under a condition in which gravity is working toward the center. The doughnut’s center corresponds to the water in the lower part of the kettle and the doughnut’s surface corresponds to the water in the upper part of the kettle.

In the future fusion power generation, because we wish to heat the center area as much as possible it is necessary to control this convection. A plasma confined in the magnetic field differs from the water inside a kettle, and forms the related structure to the magnetic field. Then, when we control this structure, the generation of a convection is suppressed, and spontaneously a phenomenon appears in which the escape of heat in the center area is not permitted.

In order to investigate such a phenomenon, we developed for the first time in Japan a numerical simulation code that simultaneously solves the movement of a light electron having a negative electric charge together with a heavy ion having a positive electric charge. For numerically simulating simultaneously ions and electrons whose weight are several thousandth part of ions, a computer of extremely high performance is necessary. Using the National Institute for Fusion Science supercomputer, conducting calculations that continued for more than one month, we succeeded for the first time in grasping the characteristics of a convection that appears in the LHD when plasma is under high pressure. This convection that appears under high pressure did not appear under low pressure, and was born of a new physics mechanism that fundamentally differs from what had been clarified to date. Further, we discovered that this new convection is suppressed being caused by the structure of the direction followed in the doughnut. Because this new convection control mechanism appears when the temperature of a plasma confined in the magnetic field is high, this is considered to be an important mechanism in moving toward the realization of the future fusion power generation.