Temporal and spatial coherence properties of free-electron-laser pulses in the extreme ultraviolet regime

The average temporal (longitudinal) and spatial (transverse) coherence of free-electron-laser pulses in the extreme ultraviolet at FLASH is measured by interfering two time-delayed partial beams directly on a CCD camera. Wavelengths between $\ensuremath{\lambda}=32\text{ }\text{ }\mathrm{nm}$ and $\ensuremath{\lambda}=8\text{ }\text{ }\mathrm{nm}$ are examined. A decrease of the coherence time for the fundamental wavelengths from ${\ensuremath{\tau}}_{c}=(6\ifmmode\pm\else\textpm\fi{}0.5)\text{ }\text{ }\mathrm{fs}$ at 32 nm to ${\ensuremath{\tau}}_{c}=(2.9\ifmmode\pm\else\textpm\fi{}0.5)\text{ }\text{ }\mathrm{fs}$ at 8 nm is measured. At $\ensuremath{\lambda}=8\text{ }\text{ }\mathrm{nm}$ the fundamental wavelength and the third harmonic of 24 nm are compared to each other. For 8 nm radiation as third harmonic of 24 nm a coherence time of ${\ensuremath{\tau}}_{c}=(2.5\ifmmode\pm\else\textpm\fi{}0.5)\text{ }\text{ }\mathrm{fs}$ is observed. The spatial coherence of 24 and 8 nm fundamental pulses are found to be very similar. The visibility decreases to 50% of the maximum visibility at about 3.2 mm overlap of the partial beams, which corresponds to 42% of the beam diameter at a distance of 90 m from the exit of the undulator. These results are analyzed in terms of the Gaussian Schell model resulting in six contributing modes to the total radiation. In addition, the correlation of the visibility between the fundamental radiation at 24 nm and its third harmonic at $\ensuremath{\lambda}=8\text{ }\text{ }\mathrm{nm}$ is investigated for identical shots.