11:00 AM - 1:00 PM
[PCG18-P05] Development of multilayer-coated mirrors for the next generation of the inner magnetospheric imaging
Keywords:EUV, magnetospheric imaging, inner magnetosphere
Inner magnetospheric imaging in the EUV spectral region contributed to our present understanding of plasmasphere formation, emerges of fine structures and solar wind interaction. Remote sensing technique in EUV has become known as a promising tool in geophysics.
We have strong motive in extension of the inner magnetospheric imaging in the past, on Kaguya, Nozomi, International Space Station and IMAGE (NASA) missions. A multilayer coated mirror reflecting the EUV radiation at the normal incidence is still providing solutions to overcome the technical difficulties, i.e., existing optics has less effective collection of EUV photons. Therefore, multilayer coated mirrors had been used for inner magnetosphere imaging, solar physics, and astrophysics. The multilayer coating consisting of alternating layers of typically molybdenum and silicon (Mo/Si) achieved the highest and fairly stable reflectivity (approximately 20%) at 30.4 nm (He II), and therefore it has been most often employed in the space missions. The thickness of each layer should be determined by the trade-off as follows: it maximizes the constructive interference of the incidence reflected at each interface and it minimizes the total absorption to enable more interfaces to contribute to the absolute reflectance increase.
Recently, we have shown the performance of the new multilayer coated mirror, consisting of 40 pairs of Mg and SiC. Its peak reflectivity achieved 30% or higher. Mg is a preferable material for optical constants to achieve higher reflectivity. We decided to use this coating for a small satellite mission. However, we had to realize that the Mg/SiC multilayer mirror has decisive disadvantage. The degradation of the reflectivity occurred under the condition of feverish temperature and/or high humidity. The interface between two materials seemed to become unstable, the thickness of the layer changes due to interfacial tension, and then the reflectance decreased dramatically (diffusion problem). As a result, we were forced to use a conventional multilayer mirror.
For space missions, science payload might have to survive hot and/or humidity environments before the launch as well as very cold condition in space. The development of robust instruments against environmental changes is mandatory for us.
More recently, we have designed two kinds of new coatings, B4C/Mg2Si and B4C/AlSi. We use the metal alloys of AlSi (or Mg2Si) instead of using bulk of Al (or Mg) to solve diffusion problems. We will show the experiment results of these coatings with a polarization dependence (S- and P- polarization) considered. We will also present microscope investigations by Transmission Electron Microscope (TEM) and Scanning electron microscope (SEM) to witness what really happened in the structure of the coatings.
We have strong motive in extension of the inner magnetospheric imaging in the past, on Kaguya, Nozomi, International Space Station and IMAGE (NASA) missions. A multilayer coated mirror reflecting the EUV radiation at the normal incidence is still providing solutions to overcome the technical difficulties, i.e., existing optics has less effective collection of EUV photons. Therefore, multilayer coated mirrors had been used for inner magnetosphere imaging, solar physics, and astrophysics. The multilayer coating consisting of alternating layers of typically molybdenum and silicon (Mo/Si) achieved the highest and fairly stable reflectivity (approximately 20%) at 30.4 nm (He II), and therefore it has been most often employed in the space missions. The thickness of each layer should be determined by the trade-off as follows: it maximizes the constructive interference of the incidence reflected at each interface and it minimizes the total absorption to enable more interfaces to contribute to the absolute reflectance increase.
Recently, we have shown the performance of the new multilayer coated mirror, consisting of 40 pairs of Mg and SiC. Its peak reflectivity achieved 30% or higher. Mg is a preferable material for optical constants to achieve higher reflectivity. We decided to use this coating for a small satellite mission. However, we had to realize that the Mg/SiC multilayer mirror has decisive disadvantage. The degradation of the reflectivity occurred under the condition of feverish temperature and/or high humidity. The interface between two materials seemed to become unstable, the thickness of the layer changes due to interfacial tension, and then the reflectance decreased dramatically (diffusion problem). As a result, we were forced to use a conventional multilayer mirror.
For space missions, science payload might have to survive hot and/or humidity environments before the launch as well as very cold condition in space. The development of robust instruments against environmental changes is mandatory for us.
More recently, we have designed two kinds of new coatings, B4C/Mg2Si and B4C/AlSi. We use the metal alloys of AlSi (or Mg2Si) instead of using bulk of Al (or Mg) to solve diffusion problems. We will show the experiment results of these coatings with a polarization dependence (S- and P- polarization) considered. We will also present microscope investigations by Transmission Electron Microscope (TEM) and Scanning electron microscope (SEM) to witness what really happened in the structure of the coatings.