National Institute for Fusion Science

In the Large Helical Device (LHD) we are confining high-pressure plasma of high-temperature and high-density by utilizing the pressure of the magnetic field. Because a magnetic field is produced by sending an electrical current through an electromagnet, when considered through economic issues relating to the size of the electromagnet and the cost of electricity, confining high-pressure plasma by the weakest permissible magnetic field is important when seeking to make real the future fusion reactor. However, when plasma pressure rises, at that moment the plasma becomes unstable and plasma confinement performance falls. In the LHD, we are investigating the conditions that cause such instabilities, and we are making advances in research on the stable confinement of high-pressure plasma. Currently, we are using the unique method of intentionally causing instabilities to the point of destroying a plasma. Here, we will introduce research being conducted in the LHD for establishing a technique for stable confinement of plasma through the “destruction” of plasma.

In the magnetic confinement method of the LHD, we are confining high-temperature, high-density plasma using the magnetic field container composed in a nested state in multiple layers. However, when temperature and density rise and plasma pressure rises, fluctuations generated in the plasma grow and cannot be completely suppressed by the magnetic field container, and there are occasions when plasma confinement performance falls. The shape of a magnetic field container is determined by the combination of the coil currents in the electromagnet that surrounds the plasma. Whether or not fluctuations will grow is greatly affected by the shape of this magnetic field container.

In the LHD, from research to date, we have optimized the shape of the magnetic field container so that the growth of fluctuations will be comparatively more difficult, and we are succeeding in confining high-pressure plasmas that are considered necessary for the fusion reactor. In addition, we have discovered the phenomenon that even if fluctuations are generated, if the plasma temperature rises, the growth of fluctuations will halt.

On the other hand, what will happen to a plasma in a magnetic field container in which fluctuations are generated easily? Similar to not discovering a method of treating an illness if people do not become ill, investigating the characteristics of plasma that appear when there is an “unstable” magnetic field in which fluctuations grow easily is important in terms of design plans for the optimal magnetic field in the future fusion reactor. In the LHD it is possible to make magnetic field containers of various shapes by combining coil currents in electromagnets. By slightly changing the coil current in specified electromagnets while creating a plasma, we can make the shape of the magnetic field container gradually change into an “unstable” configuration. In such cases, we have investigated how a plasma will behave. In the midst of changing, fluctuations occur in the plasma, and should those fluctuations have grown large, then the distribution of the plasma’s pressure will have changed in form. However, at this time, the confinement performance of the plasma did not change greatly, and the fluctuations rotated together with the electrons in the plasma. When the magnetic field container takes an all the more unstable shape, the rotation of the fluctuations stops and, at the same time, the fluctuations grow larger. Next, the core plasma pressure falls by more than 60%. This event indicates that, for example, even if the fluctuations had grown in the “unstable” magnetic field container, if the fluctuations were rotating, then they were not growing especially greatly. That is, if by some method the fluctuations can be made to rotate, the possibility that the plasma can be maintained rather than destroyed has been indicated. Such a phenomenon has been similarly observed in other devices, and making the fluctuations rotate has shown us a method for the stable sustainment of plasma.

Growth of instabilities Energy loss The magnetic field pressure A Depiction of a Plasma Confined in a Doughnut-shaped Magnetic Field Container A plasma is being confined in a magnetic field container in which the magnetic field lines are twisting. When fluctuations grow above a certain point, depending upon the setting, they cannot be completely suppressed by the pressure from the magnetic field container, and the plasma’s energy is lost.