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22 September 2015 Wigner distribution measurement of the spatial coherence properties of FELs
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Free-electron lasers deliver VUV and soft x-ray pulses with the best brilliance available and a high degree of spatial coherence. Users of such facilities have high demands on phase and coherence properties of the beam, for instance when working with coherent diffractive imaging. Thus, detailed knowledge of these parameters is of great importance and provides the possibility for advanced machine studies.

The Wigner distribution function (WDF) describes the entire propagation properties of an electromagnetic beam including all information on its spatial coherence. It can be reconstructed from beam profiles taken at different positions along its propagation direction. Here, we present measurements of the WDF conducted at the Free-electron laser FLASH at DESY. As a result, we derive the entire four-dimensional mutual coherence function, the coherence lengths and the global degree of coherence. Additionally, we provide an estimation of the possible error that our algorithm might produce for the derived quantities.

In comparison to existing studies that characterize the photon beam of FLASH, we find significantly lower values for the global degree of coherence. This difference cannot be explained by our error estimation. We explore the possible reasons for this discrepancy and their effect on the value of the global degree of coherence.
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Tobias Mey, Bernd Schäfer, Klaus Mann, Barbara Keitel, Elke Plönjes, and Marion Kuhlmann "Wigner distribution measurement of the spatial coherence properties of FELs", Proc. SPIE 9589, X-Ray Lasers and Coherent X-Ray Sources: Development and Applications XI, 95890Q (22 September 2015);

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