In situ wavefront compensation is a promising method to realize a focus size of only a few nanometers for x-ray beams. However, precise compensation requires evaluation of the wavefront with an accuracy much shorter than the wavelength. Here, we characterized a one-dimensionally focused beam with a width of 7 nm at 20 keV using a multilayer mirror. We demonstrate that the wavefront can be determined precisely from multiple intensity profiles measured around the beamwaist. We compare the phase profiles recovered from intensity profiles measured under the same mirror condition but with three different aperture sizes and find that the accuracy of phase retrieval is as small as 12.
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