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April 1, 2016
Optical Devices Made of Ceramics that Cause the Phenomenon that the Father of Electricity “Faraday” Discovered: Toward Accurate Measurements of Plasma

Does everyone know of Michael Faraday (1791-1867)? He was an English scientist, and is known as “the father of electricity” for having discovered the law of electromagnetic induction, which is the principle of motors and generators. In high school physics classes is taught Fleming’s right-hand rule, which was formulated by John Ambrose Fleming years later to easily apply the Faraday’s principle. This is: “electric current” (the middle finger), “magnetic field” (the index finger), and “force” (the moving direction of conducting material) (the thumb). To put it another way, when the conductor (the thumb) moves in the magnetic field (the index finger) the electromotive force is generated inside the conductor and the electric current (the middle finger) flows. In our lives today, we receive the benefits of trains and electrical appliances. For these we can thank Faraday. By the way, Fleming’s left-hand rule, too, which is used to explain the ability to confine plasma through the magnetic field, is identified as “electric current” (the middle finger), “magnetic field” (the index finger), and “force” (the thumb). The “force” at this moment is the force which the charged particles receive.

Faraday left numerous scientific achievements. An important discovery relating to light is the “Faraday effect.” Light has the character of waves, and it oscillates in various directions. From among the oscillations in those various directions, we call light bearing only one oscillation direction “polarized light.” The polarized sunglasses that we use when fishing at sea are equipped with a filter constructed so as not to permit only polarized light that oscillates in a specific direction, thus preventing light reflected at the sea surface. Faraday discovered that when “polarized light” passes through glass placed in the magnetic field, the direction of the polarized light, that is, the direction of the oscillation turns round. This phenomenon was called the Faraday effect, and was an important discovery which indicated that there was a connection between light and the magnetic field which were thought to be unrelated to each other. At the same time, this became an important discovery in order for humans to manipulate the polarized light. As was introduced above, when we change the direction of polarized light using the Faraday effect before the polarized sunglasses which is used to avoid the light reflected from the sea, we become able to block light or to transmit light.

In research on fusion plasma, we use laser light in order to diagnose the plasma. In manipulating that laser light we use the Faraday effect. At the time when Faraday conducted experiments, he used lead glass and caused the Faraday effect. In order to use laser light that is used in nuclear fusion plasma research, lead glass is not appropriate. It is weak with regard to heat, and breaks easily. Recently, using Japanese technology we became able to produce transparent ceramics, that is, transparent pottery. There is much transparent glass, but exceptional technology is necessary for producing transparent pottery. At the National Institute of Fusion Science, in collaboration with private enterprises and with research institutes in Japan and abroad, using these technologies for producing transparent pottery, we produced optical devices that cause the Faraday effect. Because pottery is very strong against heat and because it is possible to add various ingredients, the performance of optical devices is improving greatly by using technologies of pottery.

To explain in further detail, the material that we used for optical devices is terbium-gallium-garnets. Terbium and gallium have the atomic numbers of 65 and element 31, respectively. It is known that this material causes the Faraday effect. But by turning this into transparent pottery, we have improved heat conductivity (the ease of transmitting the heat inside the material), the linear expansion coefficient (the percentage to which the material expands due to heat), the temperature dependence of the refraction index (the rate of change that light inside a material changes the direction according to the temperature), and the magnitude of the Faraday Effect. Using the transparent terbium-gallium-garnet pottery, we developed new optical devices.

Through the new optical devices that have been developed, we have learned that beyond the glass materials used heretofore we can perform polarization operations with laser light more than twenty times more powerful than previous. This means that when we repeatedly inject the laser light, with the energy the same at each injection, we can increase the number of injections by more than 20 times by shortening the time intervals. We can increase the repetition of plasma measurements, improve the time resolution, and make possible still sharper examinations of the plasma conditions that change moment by moment using laser light having more than twenty times repetition. In this way, we will enable further development of plasma research by improving the performance of the optical devices that cause the phenomenon that Faraday, the father of electricity, discovered, using pottery technologies.