Sub-micron focusing of soft x-ray free electron laser beam

A multilayer-coated 27-cm focal length parabola, optimized to reflect 13.5 nm wavelength at normal incidence, was used in multiple FLASH experiments and focused the beam to a sub-micron beam size. The intensity of the beam was measured indirectly from the depths of craters left by the FLASH beam on PMMA-coated substrates. Comparing simulated and experimental shapes of the craters we found the best match for a wavefront error of 0.45 nm, or λ/30. We further estimated that the FWHM of the focal spot was 350 nm and that the intensity in the focus was 10 18 W/cm 2 . The sub-micron FLASH beam provided extreme intensity conditions essential for warm dense matter experiments. The same optic was used in multiple experiments and survived the beam. However, after the first measurements, which took place over several days, the optical surface was contaminated. This contamination reduced the mirror reflectivity, which was partially recovered by oxygen plasma cleaning. However, even the partially cleaned multilayer-coated optic is still diffraction limited and can focus the beam in future experiments to a sub-micron beam size.