High resolution gratings for the soft X-ray

Abstract

I discuss methods of obtaining high spectral resolution for soft X-ray and extreme ultraviolet radiation employing reflection gratings at grazing incidence. Resolution is optimized when the geometrical aberrations, caused by a large optical aperture, are comparable to the physical diffraction limit. The simplest method of aberration correction, that of under-illuminating the grating, is experimentally shown to result in a great sacrifice in the throughput and only a modest gain in resolution for a fixed slit monochromator employing a conventional spherical grating. The use of a plane grating with varied groove spacing to reduce or eliminate some aberrations is discussed, in the geometry of converging incident radiation. It is shown that spectrometers and monochromators employing such gratings are tunable in wavelength using fixed slits and a fixed beam direction, are spatially stigmatic without sacrifice of spectral resolution, and benefit in practical ways due to spectra which are perpendicular to the diffracted beam. Performances are discussed in the context both of prior results and present experiments using laser produced plasmas, and of future applications using the soft X-ray radiation emitted from synchrotrons.