Aperiodic reflection diffraction gratings for soft X-ray radiation and their application

We briefly review the works concerned with the development and experimental use of the spectral instruments based on aperiodic reflection gratings whose spacing varies monotonically across the aperture according to a prescribed law (VLS gratings). The review considers the employment of VLS-grating instruments intended for the spectroscopy of laboratory and astrophysical plasmas, including the diagnostics of relativistic laser plasmas, for measuring X-ray laser linewidths, for recording the high-order harmonics of laser radiation, the radiation of fast electric discharges and other laboratory X-ray sources, as well as in reflectometry, X-ray fluorescence analysis and microscopy with the use of synchrotron radiation and laser-plasma radiation, and in emission spectroscopy combined with an electron microscope. Also discussed are achievements in the recently undertaken design and development of special-purpose VLS spectrometers intended for the investigation of the electronic structure of different materials and molecules by the spectroscopic technique of resonant inelastic X-ray scattering of synchrotron radiation. We describe flat-field grazing-incidence spectrometers with concave VLS gratings, which are compatible with modern CCD detectors, as well as plane VLS gratings, which are the key elements of scanning high- and ultrahigh-resolution spectrometers/monochromators with a constant deviation angle and stigmatic (imaging) spectrometers, which are also compatible with CCD detectors.