The PDF file contains the front matter associated with SPIE Proceedings Volume 11038, including the title pate, copyright information, table of contents, and author and conference committee lists.

In X-ray Free-Electron Lasers (FELs), intense and coherent pulses are generated via amplification of the undulator radiation from micro-bunched electron pulses. The initial radiation is spontaneous and intrinsically stochastic, thus causing shot-to-shot fluctuations in the intensity, pointing, and spatiotemporal profile of the X-ray beam. In this work, we use deep neural networks to investigate the fluctuations in X-ray beam profiles, thereby obtaining statistical information on the lasing process. A supervised model was built to classify X-ray images, and an unsupervised one to study the distribution of beam profiles. We have found that round-shaped profiles appear more often with increasing monochromator bandwidth, suggesting that some round-shaped images can be superpositions of higher-order modes. Our results also suggest that the X-ray beam continues to evolve past the FEL saturation length towards a round-shaped beam profile.

FLASH, the soft X-ray Free Electron Laser in Hamburg provides soft X-ray pulses with high brilliance, short pulse duration and high repetition rate for user experiments. Photon beam parameters can be chosen almost independently for FLASH1 and FLASH2 by the users. The experimental hall Kai Siegbahn is designed to offer up to 6 new beamlines. Currently, three beamlines are in operation, which provide advanced focusing systems and extensive photon diagnostics for user experiments. The variable micro-focus beamline FL24 at FLASH2 offers a Kirkpatrick Baez optical system with bendable mirrors allowing different focus positions for user provided endstations. Measurements realized with a Hartmann wavefront sensor show that the Kirkpatrick Baez optics produce a focus size down to 3 µm, depending operating wavelength and desired focal position. The focus measurements were compared with imprinting techniques and wavefront propagation simulations, obtaining a good agreement between the three methods. Therefore, the wavefront propagation code was proposed for the design of the new monochromator beamline FL23 which may be used for a variety of experimental techniques to study fields such as molecular excitations, magnetism or surface effects. The monochromator is designed to work in the soft X-ray range covering a spectral range from 1-20 nm with a spectral resolution of approximately 2000. The two-grating pulse-length compensating design will provide a pulse duration in the range of 20 to 100 fs.

To advance the frontiers of ultrafast X-ray science, Shanghai soft X-ray free-electron laser (SXFEL) user facility was proposed to develop which is now under construction as planned. The SXFEL facility provides ultraintense femtosecond X-ray pulses from 0.1 keV to 1 keV in a repetition rate of 50 Hz. The first phase of the SXFEL includes two beamlines and 5 experimental end-stations. The SXFEL user facility provides some advanced X-ray measurement tools in the research fields of biomaterial, condensed matters, atomic and molecular physics, and novel functional materials by means such as coherent diffraction imaging, time-resolved resonant X-ray scattering, and single-pulse time-resolved x-ray emission spectroscopy etc. In this presentation, I will report the latest status of the beamlines at Shanghai SXFEL.

Once the Free Electron Laser became a reality, all the users were more than happy to use it, without complaining, or being worried, about the quality of the wavefront or, if the minimum spot was not that small. But, with the increase of the number of sources and, therefore, competition, this is no longer the case. The need of higher and higher performance in terms of optics, is becoming paramount. Ultra precise optics are available and it is feasible to preserve the wavefront at level for which, experiments realized out of focus, may be possible, using the entire beam. Also, it is possible to have diffraction-limited spots in the experimental chamber. But, having good mirrors alone is not enough and it is important to figure out how good those mirrors and mirror systems must be. The presentation will focus on three main topics; Mirror specifications (why one have to overcome the Marechal criterion to preserve the wavefront at the level needed for more advanced experiments); Beam performance at LCLS after initial upgrade, proving the need of specifying properly the optics; R&D efforts on various prototypes, measured in both the metrology laboratory and with Synchrtoron Radiation, to prepare the optics for the high repetition rate (or high heat load) FEL operation.

In this work we present the application of a 2D single grating wavefront sensor to align and characterize the 100 nm focus at the Coherent X-ray Imaging (CXI) endstation at the Linac Coherent Light Source (LCLS). The results agree well with a model of the system, indicating that the mirrors perform as designed when alignment is optimized. In addition, a comparison with the imprint technique confirms the validity of the results, which showed that wavefront-based alignment resulted in negligible astigmatism. Analysis of the retrieved focus profile indicates that intensities <10²¹ W=cm² are achievable with currently available LCLS beam parameters and optimal mirror alignment.

Spectroscopic experiments requiring the knowledge of the specific state of light polarization for synchrotron radiation, high-order laser harmonics and free-electron lasers are increasing in the recent years. Correspondingly, there has been an increasing demand for systems to measure polarization in the extreme ultraviolet (XUV) and soft X-ray (SX) regions. Polarization can be measured at a specific wavelength through the use of polarizers based on multilayer coatings, that are almost routinely available. The measurement on a broad band requires to operate at grazing incidence with metallic coatings. It is here presented the design and performances of a reflection polarimeter to be used in the 50-100 eV region with transition metal-coated optics. With respect to standard polarimeters, based on gold-coated optics, the presented coatings gives higher flux and higher contrast between phase and reflectivity responses for s- and p-polarized light. The optical and mechanical design of the polarimeter and the estimated throughput are discussed.

The free-electron laser FLASH2, a variable gap undulators line, has opened new scientific possibilities for users at DESY in the Hamburg area [1]. 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 [2]. 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 [3]. 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 [4]. 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 [5] 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, (2017). 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, Germany. 4. M. Mehrjoo, et al, “Single-Shot Determination of Focused FEL Wave Fields using Iterative Phase Retrieval," Opt. Express,25, (2017). 5. N. Burdet. et al, “Evaluation of partial coherence correction in X-ray ptychography ," Opt. Express, 5, (2015).

The Icarus camera system, combining a sensor developed by Sandia and readout electronics by LLNL, provides 0.5 Hz bursts of four frames with 3 ns separation. The sensor has 1024×512 25 μm pixels and is 25 μm thick. The system was developed for single line-of-sight measurements at the ns time scale for electrons and X-rays at facilities such as NIF. We report on initial tests of the Icarus system with hard X-rays pulse pairs with nanosecond time spacings at the LCLS, a newly available beam mode. We describe noise, gate profiles, gain, cross-talk, persistence, linearity, and quantum efficiency for the first version of the sensor. We present evidence of the suitability of the system for science measurements at a free electron laser with an X-ray pump X-ray probe experiment. We expect further developments of the technology to allow use of 350 ps bunch separation from the LCLS accelerator and, with a pulsed delay tube like DIXIE, to eventually reach sub-25 ps time-resolved X-ray imaging of processes such as plasma evolution.

For the LCLS-II X-ray instruments, we have developed laser power meters as compact X-ray power monitors. A calibration of the responsivity of the power meters was carried out against a silicon photodiode with synchrotron radiation and a gas monitor detector with FEL X-rays. A manipulator with two power meters was installed in various locations at the LCLS. In the LCLS front end, the power meters were compared with the gas detectors, which are calibrated by the electron energy loss method. The agreement between the power meters and the gas detectors was better than 20% at 1500 eV with the pulse energy measured by the gas detectors higher than that from the power meters. In the AMO instrument, the power meters evaluated the improvement in beamline transmission caused by the oxygen plasma cleaning of the Kirkpatrick-Baez mirrors. Measurements were also conducted one and two years later to observe the effect of further contamination of the optical surfaces. Finally at the SXR instrument, the power meters determined the pulse energy at the sample for a beamtime, where the X-ray intensity was an important parameter.

The physics and chemistry of liquid solutions play a central role in science, and our understanding of life on Earth. Unfortunately, key tools for interrogating aqueous systems, such as infrared and soft X-ray spectroscopy, cannot readily be applied because of strong absorption in water. Here we use gas dynamic forces to generate free-flowing, sub-micron, liquid sheets which are 2 orders of magnitude thinner than anything previously reported. Optical, infrared and X-ray spectroscopies are used to characterize the sheets, which are found to be tunable in thickness from over 1 micron down to less than 20 nanometers, which corresponds to fewer than 100 water molecules thick. At this thickness, aqueous sheets can readily transmit photons across the spectrum, leading to potentially transformative applications in infrared, X-ray, electron spectroscopies and beyond. The ultrathin sheets are stable for days in vacuum, and we demonstrate their use at free-electron laser and synchrotron light sources.

Results of single-shot temporal characterization of ultra-short XUV pulses generated by the free-electron laser FLASH will be presented. Using a THz field-driven streaking setup, the limits of this technique were investigated by employing different pulse durations from sub 10 fs to 350fs and different XUV wavelengths. Limits and possible error sources of the diagnostic method are discussed. Furthermore, the single-shot XUV pulse duration measurement allows the detailed investigation of the interplay between different properties of the strongly fluctuating self-amplified spontaneous emission (SASE) radiation. Correlations between pulse duration, pulse energy, spectral distribution, arrival time and electron bunch shapes are explored in the experimental data as well as in simulations. Finally, an outlook on how such a system can be implemented as online photon diagnostic will be presented.

In this paper, we will present the measurements of diffraction efficiency of commercially available replica reflection gratings which are applied to the realization of monochromators for soft X-rays in the classical and off-plane geometry. The efficiency curves for different blaze wavelengths has been measured at the Circular Polarization beamline of the ELETTRA Synchrotron (Italy) and simulated with GSolver, which is a full vector implementation of a class of algorithms known as Rigorous Coupled Wave Analysis (RCWA). Aim of the measurements is the comparison between the two diffraction geometries when used for the realization of instruments with ultrafast response. The advantages and drawbacks of both configurations when applied to the spectral selection of ultrafast pulses below 10 nm are discussed.

We present a new and simple method to generate two-color free-electron laser (FEL) pulses. In our scheme an electron bunch is quadratically tilted in the transverse direction and exhibits an energy chirp. The head and tail of the bunch are aligned to the undulator beamline and will lase, while the middle part of the bunch undergoes orbit oscillations and thus is unable to amplify the FEL radiation. Since the beam is energy chirped, the central wavelengths of the SASE amplification at the two aligned parts of the bunch are different. The beam tilt is introduced by a variation of sextupole strength in a bunch compressor section. We present simulation results of this new method for the hard X-ray FEL beamline Aramis at SwissFEL. This includes the general performance and tunability of the scheme, the requirements on the energy chirp and tilt generation, and adverse effects associated with the beam tilt.

The absolute power of the Linac Coherent Light Source pulses has been measured accurately in the hard X-ray beamline by using simultaneously two detectors: an X-ray Gas Monitor Detector (XGMD) in tandem with a radiometer. From these measurements, we were also able to characterize and calibrate in details our sets of beamline attenuators, in addition to extending an absolute calibration for our beamline intensity monitors. Similarly, we demonstrate that commercial optical power meter has a response in the hard X-ray regime, that can be cross-correlated with the absolute power of the LCLS beam.

The Dalian coherent light source is a free-electron laser facility based on the principle of high-gain harmonic generation, which can generate lasers with full coherence, short pulse and high peak power in 50~150nm. Advanced laser tools can be provided for exploring molecular ionization and dissociation in nature. The main function of the beamline for Dalian coherent light source is to transmit the laser efficiently to the end experimental station and perform optical diagnosis such as light intensity and spectrum. Dalian coherent light source beam line consists of Beam Define Aperture (BDA), gas monitor detector (GMD), Beam Position Monitor (BPM), Photon Block (PB), gas absorber (GA), profile monitor (PRF),on-line spectrum and several different functional mirrors including a focusing mirror and three deflecting mirrors. In this paper, the basic principles and mechanical structure of each functional module are briefly described.