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).
An accurate transmission measurement of thin foils (usually made of elemental metals and/or semiconductors), which routinely are used as attenuators in soft x-ray beamlines, end-stations and instruments, represents a long standing problem over the wide experimentation field with photon beams, see for example [1-4]. Such foils are also frequently utilized for blocking long wavelength emission, i.e., UV-Vis-IR radiation, from plasma and high order harmonic sources, whilst soft x-rays emitted from the source pass through the foil with only a slight attenuation. Despite the enormous amount of data available in the literature, e.g., Henke’s tables , measurements made on real foils often provide surprising results. In this study, a procedure based on the ablation imprints method is utilized for determination of soft x-ray filter transmission, namely the f-scan technique [6,7]. This technique combines the GMD (Gas Monitor Detector) pulse energy measurement and attenuation of the beam by foils (made of different metallic/semiconducting elements of varying thickness) with areas of ablation imprints created on a suitable target, e.g. PMMA – Poly(methyl methacrylate). The results show only a partial overlap with transmission values found in Henke’s tables. Nevertheless, a good agreement with transmission values determined by conventional radiometry techniques at synchrotron radiation beamlines has been found. Such a difference between the experimentally obtained values and transmissions calculated for a pure element is usually explained by spontaneous formation of oxidized layers on the filter surface and in the near-surface layer and their possible alteration by intense FEL radiation. The first results obtained with Al, Nb, Zr and Si filters at FLASH/FLASH2 (Free-electron LASer in Hamburg tuned to 13.5 nm) facilities will be shown and discussed in this presentation.
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2. E. M. Gullikson, P. Denham, S. Mrowka, J. H. Underwood: Absolute photo absorption measurements of Mg, Al, and Si in the soft x-ray region below the L2,3 edges, Phys. Rev. B 49, 16 283 (1994).
3. R. Keenan, C. L. S. Lewis, J. S. Wark, E. Wolfrum: Measurements of the XUV transmission of aluminium with a soft x-ray laser, J. Phys. B 35, L449 (2002).
4. A. Joseph, M. H. Modi, A. Singh, R. K. Gupta, G. S. Lodha: Analysis of soft x-ray/VUV transmission characteristics of Si and Al filters, AIP Conf. Ser. 1512, 498 (2013).
5. B. L. Henke, E. Gullikson, J. C. Davis: X-ray interactions: photoabsorption, scattering, transmission, and reflection at E = 50-30000 eV, Z =1-92, At. Data Nucl. Data Tables 54, 181 (1993).
6. J. Chalupsky et al.: Spot size characterization of focused non-Gaussian X-ray laser beams, Opt. Express 18, 27836 (2010).
7. J. Chalupský, T. Burian, V. Hájková, L. Juha, T. Polcar, J. Gaudin, M. Nagasono, R. Sobierajski, M. Yabashi, J. Krzywinski: Fluence scan: an unexplored property of a laser beam, Opt. Express 21, 26363 (2013).