Focusing XFEL SASE pulses by rotationally parabolic refractive x-ray lenses

摘要

Using rotationally parabolic refractive х-ray lenses made of beryllium, we focus hard x-ray free-electron laser pulses of the Linac Coherent Light Source (LCLS) down to a spot size in the 100 nm range.We demonstrated efficient nanofocusing and characterized the nanofocused wave field by ptychographic imaging [A.Schropp, et al., Sci.Rep.3, 1633 (2013)] in the case of monochromatic LCLS pulses produced by a crystal monochromator that decreases the LCLS bandwidth down to △E/E = 1.4 ? 10~(-4).The full spectrum of LCLS pulses generated by self-amplified spontaneous emission (SASE), however, fluctuates and has a typical bandwidth of a few per mille (△E/E ≈ 2 ? 10~(-3)).Due to the dispersion in the lens material, a polychromatic nanobeam generated by refractive x-ray lenses is affected by chromatic aberration.After reviewing the chromaticity of refractive x-ray lenses, we discuss the influence of increased bandwidth on the quality of a nanofocused SASE pulse.