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).
Interaction of short-wavelength free-electron laser (FEL) beams with matter is undoubtedly a subject to extensive investigation in last decade. During the interaction various exotic states of matter, such as warm dense matter, may exist for a split second. Prior to irreversible damage or ablative removal of the target material, complicated electronic processes at the atomic level occur. As energetic photons impact the target, electrons from inner atomic shells are almost instantly photo-ionized, which may, in some special cases, cause bond weakening, even breaking of the covalent bonds, subsequently result to so-called non-thermal melting. The subject of our research is ablative damage to lead tungstate (PbWO4) induced by focused short-wavelength FEL pulses at different photon energies. Post-mortem analysis of complex damage patterns using the Raman spectroscopy, atomic-force (AFM) and Nomarski (DIC) microscopy confirms an existence of non-thermal melting induced by high-energy photons in the ionic monocrystalline target. Results obtained at Linac Coherent Light Source (LCLS), Free-electron in Hamburg (FLASH), and SPring-8 Compact SASE Source (SCSS) are presented in this Paper.
At the Lawrence Livermore National Laboratory (LLNL) we have engineered a silicon prototype sample that can be used to reflect focused hard x-ray photons at high intensities in back-scattering geometry.1 Our work is motivated by the need for an all-x-ray pump-and-probe capability at X-ray Free Electron Lasers (XFELs) such as the Linac Coherent Light Source (LCSL) at SLAC. In the first phase of our project, we exposed silicon single crystal to the LCLS beam, and quantitatively studied the x-ray induced damage as a function of x-ray fluence. The damage we observed is extensive at fluences typical of pump-and-probe experiments. The conclusions drawn from our data allowed us to design and manufacture a silicon mirror that can limit the local damage, and reflect the incident beam before its single crystal structure is destroyed. In the second phase of this project we tested this prototype back-reflector at the LCLS. Preliminary results suggest that the new mirror geometry yields reproducible Bragg reflectivity at high x-ray fluences, promising a path forward for silicon single crystals as x-ray back-reflectors.
The recent success of the X-ray Free Electron Lasers has generated great interests from the user communities of a wide range of scientific disciplines including physics, chemistry, structural biology and material science, creating tremendous demand on FEL beamtime access. Due to the serial nature of FEL operation, various beam-sharing techniques have been investigated in order to potentially increase the FEL beamtime capacity. Here we report the recent development in using thin diamond single crystals for spectrally splitting the FEL beam at the Linac Coherent Light Source, thus potentially allowing the simultaneous operation of multiple instruments. Experimental findings in crystal mounting and its thermal performance, position and pointing stabilities of the reflected beam, and impact of the crystal on the FEL transmitted beam profile are presented.
The Linear Coherent Light Source (LCLS), a free electron laser operating from 250eV to10keV at 120Hz, is opening windows on new science in biology, chemistry, and solid state, atomic, and plasma physics1,2. The FEL provides coherent x-rays in femtosecond pulses of unprecedented intensity. This allows the study of materials on up to 3 orders of magnitude shorter time scales than previously possible. Many experiments at the LCLS require a detector that can image scattered x-rays on a per-shot basis with high efficiency and excellent spatial resolution over a large solid angle and both good S/N (for single-photon counting) and large dynamic range (required for the new coherent x-ray diffractive imaging technique3). The Cornell-SLAC Pixel Array Detector (CSPAD) has been developed to meet these requirements. SLAC has built, characterized, and installed three full camera systems at the CXI and XPP hutches at LCLS. This paper describes the camera system and its characterization and performance.
A recently demonstrated single-shot measurement of the relative delay between x-ray FEL pulses and optical laser pulses has now been improved to ~10 fs rms error and has successfully been demonstrated for both soft and hard x-ray pulses. It is based on x-ray induced step-like reduction in optical transmissivity of a semiconductor membrane (Si3N4). The transmissivity is probed by an optical continuum spanning 450 - 650 nm where spectral chirp provides a mapping of the step in spectrum to the arrival time of the x-ray pulse relative to the optical laser system.
Results of coherent diffractive imaging experiments performed with soft X-rays (1-2 keV) at the Linac Coherent
Light Source are presented. Both organic and inorganic nano-sized objects were injected into the XFEL beam
as an aerosol focused with an aerodynamic lens. The high intensity and femtosecond duration of X-ray pulses
produced by the Linac Coherent Light Source allow structural information to be recorded by X-ray diffraction
before the particle is destroyed. Images were formed by using iterative methods to phase single shot diffraction
patterns. Strategies for improving the reconstruction methods have been developed. This technique opens
up exciting opportunities for biological imaging, allowing structure determination without freezing, staining or
The recent commissioning of a X-ray free-electron laser triggered an extensive research in the area of X-ray ablation of
high-Z, high-density materials. Such compounds should be used to shorten an effective attenuation length for obtaining
clean ablation imprints required for the focused beam analysis. Compounds of lead (Z=82) represent the materials of first
choice. In this contribution, single-shot ablation thresholds are reported for PbWO4 and PbI2 exposed to ultra-short
pulses of extreme ultraviolet radiation and X-rays at FLASH and LCLS facilities, respectively. Interestingly, the
threshold reaches only 0.11 mJ/cm2 at 1.55 nm in lead tungstate although a value of 0.4 J/cm2 is expected according to
the wavelength dependence of an attenuation length and the threshold value determined in the XUV spectral region, i.e.,
79 mJ/cm2 at a FEL wavelength of 13.5 nm. Mechanisms of ablation processes are discussed to explain this discrepancy.
Lead iodide shows at 1.55 nm significantly lower ablation threshold than tungstate although an attenuation length of the
radiation is in both materials quite the same. Lower thermal and radiation stability of PbI2 is responsible for this finding.