<title>Compound refractive lenses: high-quality imaging optics for the X-FEL</title>

Abstract

Compound refractive lenses (CRL) for hard x-rays are genuine imaging devices like glass lenses for visible light. They are ideally suited for both full field and scanning microscopy with hard x-rays in the range from 2 to 100keV. They are robust and can withstand the heat load of the white beam of an ESRF undulator source. In full field microscopy, resolutions down to 300nm have been achieved so far using aluminium lenses. Resolutions below 100nm are expected for beryllium lenses currently under development. For scanning microbeam techniques, a monochromatic microbeam of 550nm by 5.5micrometers with a 1.1 10<SUP>10</SUP>ph/s (gain 1120) has been achieved with aluminium lenses at a third generation undulator source. For beryllium as lens material, a flux up to two orders of magnitude higher is expected. At planned FEL beamlines, the source size and distance from the source are favorable to microbeams produced by compound refractive lenses, and a diffraction limited microbeam is expected both horizontally and vertically. For beryllium lenses the diffraction limit can be below 100nm. A typical FEL beam size of approximately 1mm at the experiments hutch ideally matches the aperture of compound refractive lenses. Estimates of the heat load on the CRL as well as expected photon fluxes and micro beams sizes are given.