Beam parameters of FLASH beamline BL1 from Hartmann wavefront measurements

We report on online measurements of beam parameters in the soft X-ray and extreme ultraviolet (EUV) spectral range at the free-electron laser FLASH. A compact, self-supporting Hartmann sensor operating in the wavelength range from 6 to 30 nm was used to determine the wavefront quality of individual free-electron laser (FEL) pulses. Beam characterization and alignment of beamline BL1 was performed with λ13.5nm/90 accuracy for wavefront rms (wrms). A spot size of 159 μm (second moment) and other beam parameters are computed using a spherical reference wavefront generated by a 5 μm pinhole. Beam parameters are also computed relative to a reference wavefront created by a laser-driven plasma source of low coherence, proving the feasibility of such a calibration and reaching λ13.5nm/7.5 wrms accuracy. The sensor was used for alignment of the toroidal focusing mirror of beamline BL1, resulting in a reduction of wrms by 25%, and to investigate wavefront distortions induced by thin solid filters.