The Solar Upper Transition Region Imager (SUTRI) was proposed to observe for the first time the Ne VII line at 46.5nm emitted from the upper solar transition region. As the key optical elements of the SUTRI, Sc/Si multilayer reflective mirrors are developed to offer high spectral selectivity and high reflectivity at 46.5nm with a normal-incidence angle. To avoid spectral contamination, the reflective bandwidth of the multilayer is required less than 4nm, which is achieved when the Sc layer thickness ratio is tuned to above 0.65. Meanwhile, the mechanical property, resistivity to thermal cycling, and temporal stability of the Sc/Si multilayer are characterized. The addition of an ultrathin Si layer (thickness of 0.6nm) in each Sc layer decreased the crystallization of Sc and flatted the interface, resulting in the enhancement of the mechanical property of the Sc/Si multilayer with new structure. After three times of thermal cycling, the temperature from 5°C to 40°C, the surface morphology of the new Sc/Si multilayer remained unchanged. The grazing incidence reflectometer test results showed that the periodic structure and thickness of the new multilayer were still similar after 2 years of storage. The optimized Sc/Si multilayer has a d-spacing of 24.55nm and a Sc thickness ratio of 0.72, achieving the extreme ultraviolet reflectivity of 28% at 46.1nm obtained from the laboratory-based reflectometer. The optimized Sc/Si multilayer mirrors have been applied in the SUTRI and received bright solar images at 46.5nm.
A laboratory-based reflectometer designed for characterizing the reflectivity of optical coatings in 30- to 200-nm wavelength range was recently developed at IPOE. An RF-produced gas-discharge light source is applied to generate characteristic lines. The light source is mounted on a grazing incident monochromator with a 146-deg deviation angle between the incident and diffracted arms. By precisely adjusting the toroidal grating inside the monochromator chamber, monochromatic lights are acquired through the exit slit. A collimator mirror and two sets of collimation slits with 2 mm × 2 mm dimension are utilized for reducing the divergence of the beam incident on the sample. A high-precision six-axis translation stage, which allows a heavy sample with a maximum diameter of 100 mm, is used to control positions of the samples and the detector. A chopper disk used both for incident light intensity monitor and signal modulation is placed with an incidence angle of 70 deg relative to the incident light beam. The configuration, adjustment process, and test results of the reflectometer are presented in detail. The experimental reflectivity results for Al / LiF / MgF2 film obtained from our laboratory and BESSY-II Synchrotron as well as Hefei Synchrotron Light Source are given and compared for demonstrating the reliability of the system.
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