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内容記述 |
Objective soft X-ray flat-field spectrograph employing a laminar-type bilayer coated varied-line-spacing (VLS) spherical grating was designed to improve detection limit and sensitivity of soft X-ray flat-field spectrograph in a region of 250?550 eV. As a design criterion, spectral flux, SF, [T. Hatano et al., Appl. Opt., 60, 4993-4999 (2021)], which is proportional to the amount of optical flux incident onto a detector and correlated with detection sensitivity, was used to be maximized. To enhance reflectivity with the coating design, Au/Ni bilayer coating was investigated to optimize incidence angle and thickness of Ni layer. This is based on the consideration that, in an energy region of over 400 eV, refractive indexes of Au (bottom layer), Ni (top layer), and vacuum are increased from the bottom to top of layers and a supplemental enhancement of reflectivity can be expected by optimizing the thickness of the top layer. Thus, the thickness of Ni and incidence angle was chosen to be 8.0 nm and 86.00°, respectively. To maintain dispersion and spectral resolution of the grating used at an incidence angle of 87.07° as previously designed, groove density was increased to 1500 lines/mm from 1200 lines/mm of our previous design. Finally, a holographic varied-line-spacing spherical grating was designed assuming aspherical-wavefront-recording configuration. The numerical simulation results showed that the spectrograph employing newly designed grating with laminar-type grooves and Au/Ni bilayer coating exhibited 2?18 times higher spectral flux as well as an improved spectral resolution comparing with those obtained with the previously designed gratings and spectrographs. |
