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November 16, 2015
Investigating Plasma’s “Thread-like” Behavior by Computer Simulation: Microscopic Effects in Edge Plasma

Inside a vacuum vessel of a high-temperature plasma experiment device such as the Large Helical Device (LHD), we confine high-temperature high-density plasma by generating a magnetic field container. However, in reality, some of the hot plasma in the container gradually leak to the edge. This plasma that has leaked is called edge plasma. Further, in recent experiments, thread-like plasma nodules (plasma blobs) that follow the magnetic field lines and are ejected intermittently from the core area have been observed. Plasma blobs, too, are thought to be a source of edge plasma. In order to realize fusion power, controlling edge plasma is also one of the important issues. In this Research Update, we will introduce numerical simulation research on plasma blobs, which also is important in studying edge plasma.

Edge plasma is usually considered to move along the magnetic field lines. For this reason, at the end of the magnetic field lines is placed a metal board called the “divertor plate.” The edge plasma’s particles are guided to hit the divertor plate so as not to hit the vacuum vessel wall. If particles from the plasma hit the vacuum vessel wall, the wall’s metal atoms are expelled and the plasma temperature falls. However, the plasma blob, depending on the curvature of the magnetic field line, moves across the magnetic field line, and heads for the vacuum vessel wall at a speed of several kilometers per second. This speed is slower than the speed of the edge plasma following the magnetic field line toward the divertor plate (typically approximately several tens kilometers per second). However, because the distance to the vacuum vessel wall is much shorter than the distance of the magnetic field line to the divertor plate, plasma particles may strike the wall before they arrive at the divertor plate. In order to control this movement of plasma blob, first, it is necessary to understand the properties of blobs. Because plasmas are complicated, in understanding them numerical simulation is indispensable.

Simulations of plasma blob to date have focused on the blob’s behavior as a whole, and did not calculate the movements of individual plasma particles that compose blobs. By this method, we could not incorporate the effects caused by individual particles, that is, we could not incorporate the microscopic effects. For that reason, for example, because we cannot calculate the microscopic electric field called the “plasma sheath” that is formed when plasma particles hit the divertor plate, there occurred a problem that we could not confirm whether the electric current circuit inside the blob was being correctly reproduced or not. Because the electric current circuit becomes important when investigating the behavior of the plasma blob in the magnetic field, the current circuit must be investigated through a simulation which includes microscopic effects. Thus, using the supercomputer “Plasma Simulator” at the National Institute for Fusion Science, we are conducting a large-scale simulation for calculating the movement of the numerous plasma particles that compose a blob and the temporal evolution in the electric field formed by these particles. From this simulation, because we also can calculate the sheath, which is a microscopic electric field, consistently, we have confirmed the existence of an electric current that flows spirally inside the plasma blob. Further, it has become possible to investigate in detail the potential structure inside the blob. Inside the blob, when cross-sections are viewed, there are both areas where the electric potential is high and where the electrical potential is low. It becomes clear that according to the areas of high and of low electric potential, the plasma’s temperature, too, is high in some areas and low in other areas.

The plasma simulator was updated in June, 2015, with the calculation performance improved by more than eight times compared to the earlier one. In the future, utilizing this new plasma simulator, we will perform still larger-scale simulations, and examine in even greater detail the movements of a plasma blob. Further, we aim to achieve the ability to predict the behaviors of edge plasma including the plasma blob, and we aim to contribute to the realization of fusion energy.