Design of multi-area grating for soft x-ray flat-field spectrograph.

A soft x-ray flat-field grating with a nominal groove density of 2400 grooves/mm is typically used in wavelengths of 1-6 nm or 1-7 nm for plasma diagnostics. However, measurement of wavelengths even down to 0.6 nm simultaneously is expected in practice. Unfortunately, a grating has a poor spectral image at wavelengths below 1.3 nm. In order to improve the spectral image at lower wavelengths, multi-area gratings (divided perpendicular to the direction of grating grooves) are devised. To reduce the contribution of certain areas with poor spectral image to the final spectral image, the profile parameters of groove on the area are optimized by suppressing spectral efficiency. Theoretical analysis demonstrates that, using multi-area gratings, the spectral image at lower wavelengths could be obviously improved; thus, the spectral resolving power at a wavelength of 0.7 nm is increased from 94 to 398.