December 15, 2014
Destroy the Symmetry Slightly and Skillfully Confine the Plasma

Seeking the realization of the future fusion energy, in the magnetic field confinement device we are progressing in research on confining high-temperature, high-pressure plasma in the magnetic field container. In order to confine plasma the “symmetry” of the configuration of the magnetic field container plays an important role. When the symmetry is high, the performance of the plasma confinement, too, is high. But when the confinement condition changes for some reason, there is worry that the plasma will suddenly flare and damage the vessel. For that reason, we are conducting research in which, beforehand, we slightly destroy one part of the symmetry and stabilize the plasma confinement condition. Here, we introduce research in heightening the plasma confinement technique through control of the magnetic field symmetry.

What is symmetry? Let us consider symmetry in an origami figure. When a square origami paper is folded at the middle of two sides, the two corners overlap. If we fold the paper in diagonals, the remaining corners will overlap. The squares in this way have the same shape with regard to the two fold lines (axis). We call this “line symmetry.” Further, the same shape will emerge by rotating the paper 90 degrees around the axis that is perpendicular to the paper in the middle of the square. That is, this is also called “rotating symmetry” in a limited sense. In the fusion plasma magnetic field confinement device, it is important for the configuration of the magnetic field that confines the plasma to have this type of symmetry. The tokamak confinement device has a doughnut-shape magnetic field configuration. Even if the plasma revolves around the axis in the middle of the doughnut, its shape does not change. This is an important feature of the doughnut-type device. The configuration of the plasma confinement magnetic field in the Large Helical Device (LHD) is in the shape of a twisted doughnut. And because the shape does not change when it revolves around the axis by a certain number of degrees, we say that there is twisted symmetry, that is, “helical symmetry” in a limited sense. A magnetic field device that has a highly symmetrical configuration can well confine plasma because it does not have a weak aspect toward a plasma that seeks to escape outside.

However, recently, experiments that purposefully attempt to destroy such magnificent symmetry are gathering attention. When symmetry is good and we confine plasma quite well, when the confinement conditions collapse slightly and the plasma escapes to outside the confinement, the escaped plasma unexpectedly has a large amount of energy and hits the vacuum vessel. We now know that in some cases it damages the vacuum vessel. When a volcano’s magma chamber is closed tightly, the volcano may eject much energy at the time of an eruption. Thus, when we slightly tear the symmetry of the magnetic field that confines the plasma and the plasma little by little attempts to escape to a pre-determined place, the quantity of plasma that suddenly escapes at the time of the collapse of the confinement condition becomes smaller and can be skillfully stopped in the vessel.

How, then, does one destroy symmetry? If one can add from outside a magnetic field that is not symmetrical, that is good. Because a plasma moves unpredictably when receiving influence from an external magnetic field, it is difficult to predict what will happen to the magnetic field inside the plasma. If the symmetry is destroyed too fully, as everything will be destroyed it is necessary to know how small an external magnetic field will be satisfactory. Because this is an extremely important yet also difficult problem, a conference at which specialists discuss only this problem is held every year. In the LHD, we are using a new diagnostic device that can investigate internal conditions of the plasma. And we are moving forward with preparations for research of the changes in the plasma shape caused by an external magnetic field. This diagnostic device affects the shape of the plasma from two symmetrical directions. If there is symmetry in the plasma, two images will match, and if the symmetry is torn, different images will be photographed. From the differences in the two images we will estimate the condition of the tear in the symmetry. Working toward enhancing plasma control methods that will link to the future fusion reactor, we are planning tests of the diagnostic device and international experiments involving members from Japan, the United States, and the Republic of Korea who conduct experiments on devices including tokamaks.