Design of a plane grating with multilayer coating for the soft X-ray tomography beamline

An alternate multilayer plane grating has been designed for the soft X-ray tomography beamline. At this beamline, a grating with 600 line/mm and Au coating has already been set up to serve the energy range from 260 eV to 1200 eV. To fit the original optical design, a varied-line-spacing plane grating optimized at 2473 eV will be used in this beamline. The groove depth is 2.5 nm and its duty cycle is 0.5. The B4C/Mo alternate layers with period 30 are selected to enhance diffraction efficiency in the energy range 1100 eV -- 2600 eV. The varied-line-spacing plane grating paired with a matched multilayer plane mirror constitutes a monochromator to provide energetic photons. Based on the designed multilayer conditions, the calculated diffraction efficiency of alternate multilayer varied-line-spacing plane grating is about 0.24 at 2473 eV; the calculated photon flux is about 2.3 × 1011 photons/s. The resolving power is about 1670; the openings of the entrance and exit slits are 25 and 50 μm respectively.