Focusing XFEL SASE Pulses by Rotationally Parabolic Refractive X-Ray Lenses

Using rotationally parabolic refractive x-ray lenses made of beryllium, we focushard x-ray free-electron laser pulses of the Linac Coherent Light Source (LCLS) down to aspot size in the 100 nm range. We demonstrated efficient nanofocusing and characterizedthe 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 thatdecreases the LCLS bandwidth down to ∆E/E = 1.4 · 10 −4 . The full spectrum of LCLSpulses generated by self-amplified spontaneous emission (SASE), however, fluctuates and hasa typical bandwidth of a few per mille (∆E/E ≈ 2 · 10 −3 ). Due to the dispersion in the lensmaterial, a polychromatic nanobeam generated by refractive x-ray lenses is affected by chromaticaberration. After reviewing the chromaticity of refractive x-ray lenses, we discuss the influenceof increased bandwidth on the quality of a nanofocused SASE pulse