In the future fusion reactor we will improve function by coating the surface of metallic materials with ceramic materials, for the purpose of insulating the surface electrically, and of preventing gas permeation and corrosion. Further, in the observation window for plasma, too, will be used ceramics materials through which light passes. Because high-quality ceramics materials are required, in order to adequately demonstrate such functions it is necessary to investigate beforehand the conditions of the required materials. Here, we will introduce the “cathode luminescence (electron beam induced fluorescence measurement)” method which can precisely investigate the conditions of ceramic materials (for example, the regular arrangement of atoms, the number of holes of which space is supposed to be occupied by the atoms, and the presence of impurities) in a short time and research that has applied this method to ceramic materials for the fusion reactor.
Cathode luminescence (abbreviated below as CL), like the display system in the Braun tube color televisions that are rarely seen today, uses an electron beam. In the Braun tube color television, fluorescence materials which emit red, green, and blue light, respectively, are coated on the inner side of the screen where the electron beam strikes. The CL method, as a method for investigating sorts of materials and the condition of various crystals from the color of the light (that is, the light’s wavelength) and from the intensity of the light that are generated when the electron beam strikes material, is being widely used also in semiconductor material research and analysis of minerals.
In the ceramics materials being examined for use in the fusion reactor are various materials. Those materials are required to be tested under an environment that may be expected in the reactor, and it is necessary to analyze that condition. We have begun to apply the CL method also to analysis of those conditions. However, many of the ceramics materials for use in the fusion reactor have not been investigated with regard to light spectra by the electron beam (the distribution of the intensity of light as a function of wavelength), and data that express the relationship between the light spectrum and the crystal condition were few, which are necessary for the analysis of the material condition.
First, we are moving forward with constructing a new database. Because the color of the light emitted when the electron beam strikes differs depending upon the material, we are investigating in what color will the light shine, and to what color will that color change when the crystal’s condition changes. We are collecting data relating to these processes. To date, for example, erbium oxide, which is used to insulate the inner piping (see number 267), emits purple, green, and red lights together. But we have learned that when the arrangement of the atoms becomes disordered, red-colored light will weaken. Further, similarly, yttrium oxide, which is a candidate for the coating material, ordinarily shines in a mix of purple, blue, and green colors. But when the order of the atoms becomes disturbed, ultraviolet radiation and the light of the orange and red colors are strengthened.
It is possible to observe this type of light spectrum in short blocks of time. Although the material has the same name, ceramic materials may vary in quality when purchased from different companies because the condition of the crystal changes depending upon the firing condition (baking temperature, length of time, etc.). By utilizing the CL method and the database, we can promptly evaluate that quality. Further, one can also examine over a short period of time the condition of ceramics materials that one has produced experimentally. In the future, in order to be able to utilize the CL method in the test for evaluating conditions of materials under the condition of the fusion reactor, we will widen further the targets for observation of the light spectrum (that is, make still more materials and crystals the object of investigation), collect and store data, and enrich the database.