<title>Microcapillary refractive lens as a possible optical element for focusing free-electron laser x-rays</title>

Abstract

We have fabricated and tested short focal-length compound refractive lenses for X-rays (CRLs) and considered its application for focusing coherent beams. The lens is designed in the form of glass capillary filled by micro-air-bubbles embedded into epoxy. The interface between the bubbles formed 90 to 196 spherical bi-concave microlenses with curvature radius equals to the capillary one. When compared with CRLs manufactured using other methods, the micro-bubble lenses have shorter focal lengths with higher transmission for moderate energy X-rays (e.g. 7 - 12 keV). The lenses are inexpensive and are ideally suited for focusing X-rays generated by high power single pulsed operation coherent X-ray sources with Source size 50-100 microns. We used beamline 2-3 at the Stanford Synchrotron Radiation Laboratory (SSRL) to measure focal lengths between 100-150 mm and absorption apertures between 90 to 120 pm. Transmission profiles were measured giving, for example, a peak transmission of 27 % for a 130-mm focal length CRL at 8 keV. The focal-spot sizes were also measured yielding, for example, an elliptical spot of 5 × 14-μm² resulting from an approximate 80-fold demagnification of the 0.44 × 1.7 mm² source.