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October 28, 2013
Observing an Open Seam in the Magnetic Field Container: An Experiment in the Transport of Heat

In the Large Helical Device (LHD) we confine high-temperature high-density plasma in the magnetic field “container” produced by the superconducting coils. Because the magnetic field lines container is in a nested state in the stratified, multiple layers, we are not only attempting to prevent the plasma that is floating from making contact with the device’s wall. We are also maintaining the core plasma at a high temperature. Further, the shape of the magnetic field container changes in accord with the condition of the plasma, and in some instances greatly controls the performance of the plasma. Here, regarding the magnetic field container, which greatly influences the plasma confinement, we will introduce research that examines the changes in a plasma’s shape.

Seeking the realization of fusion energy, the National Institute for Fusion Science is advancing its research on high-temperature high-density plasma. In order to confine such plasma, a magnetic field container is being used. This magnetic field container is in the shape of a doughnut that does not have a beginning. In order to confine plasma it is furthermore necessary that the magnetic field lines be twisted. Thus in the magnetic field container from the core area to the boundary are innumerable layers one upon another in a nested state.

Plasma does not move freely between one layer and the next layer. Due to the multi-layer structure of the magnetic field lines it is possible to confine plasma. Further, in moving from the core toward the boundary the pitch angle of the twist changes at each layer. But when the pitch angles of the twist of the magnetic field lines are close it becomes easier for layers next to one another to connect. As a result, it is not possible to form a multi-layered structure, and the magnetic field container loosens and becomes disturbed. Thus the magnetic field container strengthens when the changes in the pitch angle of the twist are great.

The magnetic field container is produced by the electrical current that flows from the superconducting magnet. However, because plasma is composed from ions and electrons that have an electric charge, when these particles move about, an electrical current flows within the plasma, and thus the magnetic field appears. Next, the magnetic fields made by the plasma form one atop another in the magnetic field container and the shape of the container ends up changing. How the change occurs varies, and the container’s edge sometimes falls and marks that resemble an island sometimes appear between the layers. When such changes are numerous this brings poor plasma confinement performance.

Thus, we attempted to observe through experiments the unraveling of the magnetic field container in the core area. Of course, because we cannot directly see the form of the magnetic field container inside the plasma, we periodically heated only the core part of the plasma for short intervals and measured how the oscillating heat propagates to the plasma edge. In cases when the speed of the heat propagation is extremely fast, because we cannot adequately confine the plasma we think that this is a condition in which the magnetic field container is not forming a nested structure. When investigated through this method, in conditions for plasma such as that in which the electrical current flows within the plasma in a certain direction, results were achieved which suggest that an area where the temperature does not rise appears, and thus the unraveling of the magnetic field container.

Despite heating the core area, there are times when the temperature of the plasma’s core area does not rise. In such instances, it may be thought that there are areas where the magnetic field container is unraveling. We will carefully examine the conditions of the plasma in which such events occur and aim to establish a method for high-temperature and high-density plasma production necessary for fusion energy in the future.