The free-electron laser FLASH2, a variable gap undulators line, has opened new
scientific possibilities for users at DESY in the Hamburg area . The current pulsed radiation at
the FLASH facility primarily relies on the SASE process. Thus, the beam characteristics may
differ drastically from pulse to pulse; requiring single-shot photon diagnostics and characterization
of the photon beam parameters.
The beamline FL24 at FLASH2 is equipped with a set of bendable Kirkpatrick-Baez (KB)
mirrors which can strongly focus the beam down to a few micrometers. As a key parameter for
many experiments, understanding of the focus characteristics and variations is demanded by users.
The current instrumentation at the beamline FL24 has foreseen a dedicated Hartmann-Wavefront
Sensor (HWS) to run the online, highly focused, single-shot beam characterization within the
operating wavelength range of the FLASH2 . However, a critical issue linked to the success
of the current HWS is the assumption of high transverse coherence of the radiation. We observed a slight difference between the retrieved focuses by the HWS and those measured with the imprints method. We attribute the observed difference to the low-degree of the transverse coherence. Recently, we performed a non-destructive Young’s double-pinhole experiment, at the beamline FL24, which proved the variation of the degree of transverse coherence (25-50% deviation from the full coherence) correlated to the various machine parameters .
Advances in the Fresnel Diffractive Imaging (FDI) have promoted the FEL pulse characterization by reconstructing partially coherent wave fields. This approach was successfully applied to characterize highly transverse coherent,
focused pulses at the beamline BL2 at the FLASH1 line . We have extended the application of the FDI method, at the beamline FL24, to characterize the transverse partially coherent pulses, in a single-shot basis, and estimate a measure of the degree of the transverse coherence.
Summarily, we report on the results of our previous pulse and transverse coherence characterization
experiments, and discuss the feasibility of each method as an on-line photon diagnostic. Furthermore,
our future plan to apply the partially coherent ptychography method  for the wave field
characterization will be discussed providing the results of start-to-end simulations.----------------------------------------
References and links:
1. B. Faatz, et al. “The FLASH Facility: Advanced Options for FLASH2 and Future Perspectives," Applied Science 7,
2. B. Keitel, et al. “Hartmann wavefront sensors and their application at FLASH," Special Issue (PhotonDiag2015), J.
Synchrotron Rad. 23, (2016).
3. T. Wodzinski, et al. “Coherence measurements with double pinholes at FLASH2," PhotonDiag 2018, Hamburg,
4. M. Mehrjoo, et al, “Single-Shot Determination of Focused FEL Wave Fields using Iterative Phase Retrieval," Opt.
5. N. Burdet. et al, “Evaluation of partial coherence correction in X-ray ptychography ," Opt. Express, 5, (2015).