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October 20, 2014
Investigating the Light from Tungsten: The Condition of Tungsten in a High-Temperature Plasma

In fusion plasma experiment devices beginning with the Large Helical Device (LHD), as well as the International Thermonuclear Experimental Reactor (ITER), plasma is confined in a magnetic field container and made to float in the vacuum so that the high-temperature plasma does not directly touch the vessel wall. Conversely, in order to terminate plasma that has moved from the core area to outside the magnetic field container, the shapes of the magnetic field lines are devised, and a structure that can guide the plasma to the heat absorbing divertor board while sufficiently lowering the plasma temperature is composed. For this purpose we are advancing in research on reducing the heat load to the divertor board. The divertor board must be made of a strong material that has a high melting point. The best candidate is tungsten. However, because a very minute amount of tungsten enters the core of a high-temperature plasma as impurities shaved off when plasma particles collide, collecting basic data is being advanced in order to investigate those effects. Here we introduce research which investigates the light that tungsten emits in high-temperature plasma.

Tungsten is used in light filaments, and is the representative heat-resistant material that has the highest melting point among metals. For that reason, in ITER tungsten is planned to be used for the divertor board. But because it emits light having entered into the plasma as an impurity, and as light emits energy and cools the plasma, tungsten may be a dangerous presence for the necessary fusion for the high-temperature plasma. Thus, research that investigates where and how much light tungsten emits in the plasma, and how to prevent tungsten from entering into the plasma is one of the keys for realizing fusion.

Thus, in order to investigate in what condition tungsten is found inside the plasma, we designed a theoretical model that calculates the light from tungsten. When tungsten enters plasma, electrons are torn away and become ionized ions. But according to the theoretical model, depending upon the electron temperature of the plasma, we can calculate how tungsten ions shine in the LHD plasma. Next, we placed a minute amount of tungsten into a parvule called a pellet, injected the pellet into the LHD plasma, and measured the light emitted from the tungsten (in this case, shortwave light invisible to the human eye that is called extreme ultraviolet light). At this time, when we separate the observed light into wavelengths and take spectroscopic measurements, we learn the spectrum of wavelength of light that the tungsten emitted. Comparing that to the spectrum calculated by the theoretical model, we investigated the ionization condition of tungsten ions in the plasma.

Through this research, we learned the condition of tungsten ions in the plasma through spectroscopic measurements, how tungsten is moving in the plasma, whether tungsten moves toward the plasma’s center, and other issues. We can advance our research on that movement to the next stage. Utilizing this research, we will contribute to the realization of fusion, including at ITER, in the future.