Beam characterization of FLASH from beam profile measurement by intensity transport equation and reconstruction of the Wigner distribution function

Beam parameters of the free-electron laser FLASH @13.5 nm in two different operation modes were determined from beam profile measurements and subsequent reconstruction of the Wigner distribution function behind the ellipsoidal focusing mirror at beamline BL2. 40 two-dimensional single pulse intensity distributions were recorded at each of 65 axial positions around the waist of the FEL beam with a magnifying EUV sensitized CCD camera. From these beam profile data the Wigner distribution function based on different levels of averaging could be reconstructed by an inverse Radon transform. For separable beams this yields the complete Wigner distribution, and for beams with zero twist the information is still sufficient for wavefront determination and beam propagation through stigmatic systems. The obtained results are compared to wavefront reconstructions based on the transport of intensity equation. A future setup for Wigner distribution measurements of general beams is discussed

High-harmonic generation wave front dependence on a driving infrared wave front

With high-harmonic generation (HHG), spatially and temporally coherent XUV to soft x-ray (100 nm to 10 nm)table-top sources can be realized by focusing a driving infrared (IR) laser on a gas target. For applications such ascoherent diffraction imaging, holography, plasma diagnostics, or pump–probe experiments, it is desirable to havecontrol over the wave front (WF) of the HHs to maximize the number of XUV photons on target or to tailor the WF.Here, we demonstrate control of the XUV WF by tailoring the driving IR WF with a deformable mirror. The WFsof both IR and XUV beams are monitored with WF sensors. We present a systematic study of the dependence of theaberrations of the HHs on the aberrations of the driving IR laser and explain the observations with propagationsimulations. We show that we can control the astigmatism of the HHs by changing the astigmatism of the drivingIR laser without compromising the HH generation efficiency with a WF quality fromλ/8 toλ/13.3. This allows usto shape the XUV beam without changing any XUV optical element

Single-shot ptychography at a soft X-ray free-electron laser

Ptychograhy is a scanning coherent diffraction imaging technique capable of providing images of extended samples withdiffraction-limited resolution. However, ptychography experiments are time-consuming due to their scanning nature which alsoprevents their use for imaging of dynamical processes. Recently, setups based on two con-focal lenses were proposed toperform single-shot ptychography in the visible regime by measuring the diffraction pattern produced by multiple overlappingbeams in one shot. However, this approach cannot be extended straightforwardly to X-ray wavelengths due to the application ofrefractive optics. In this work, we demonstrate a novel and nascent single-shot ptychography setup utilizing the combination ofX-ray focusing optics with a two-dimensional beam-splitting diffraction grating. It allows single-shot imaging of extended samplesat X-ray wavelengths. As a proof of concept, we performed single-shot ptychography in the XUV range at the free-electronlaser FLASH and obtained a high-resolution reconstruction of the sample

FLASH2020+ Project Progress: Current installations and future plans

The FLASH2020+ project has started to transform the FLASH facility to broaden the facility profile and meet demands of future user experiments. In a nine-month lasting shutdown until August 2022 the linear accelerator of the FLASH facility has, among others, been upgraded with a laser heater, new bunch compressors and new modules. The latter results in an energy upgrade to 1.35 GeV allowing to reach sub 4 nm wavelength. In the following 14-month lasting shutdown starting mid 2024 the FLASH1 FEL beamline will be completely rebuild. The design is based on external seeding at MHz repetition rate in burst mode allowing for coherent tuneable FEL radiation in wavelength and polarization by installation new APPLE-III undulators. Post compression of the beam downstream of the radiators will allow for high quality THz generation and together with the new experimental endstations and pump probe lasers provide a unique portfolio for next generation user experiments

Experimental study of EUV mirror radiation damage resistance under long-term free-electron laser exposures below the single-shot damage threshold

The durability of grazing- and normal-incidence optical coatings has been experimentally assessed under free-electron laser irradiation at various numbers of pulses up to 16 million shots and various fluence levels below 10% of the single-shot damage threshold. The experiment was performed at FLASH, the Free-electron LASer in Hamburg, using 13.5 nm extreme UV (EUV) radiation with 100 fs pulse duration. Polycrystalline ruthenium and amorphous carbon 50 nm thin films on silicon substrates were tested at total external reflection angles of 20° and 10° grazing incidence, respectively. Mo/Si periodical multilayer structures were tested in the Bragg reflection condition at 16° off-normal angle of incidence. The exposed areas were analysed post-mortem using differential contrast visible light microscopy, EUV reflectivity mapping and scanning X-ray photoelectron spectroscopy. The analysis revealed that Ru and Mo/Si coatings exposed to the highest dose and fluence level show a few per cent drop in their EUV reflectivity, which is explained by EUV-induced oxidation of the surface

The PERCIVAL detector: first user experiments

The PERCIVAL detector is a CMOS imager designed for the soft X-ray regime at photon sources. Although still in its final development phase, it has recently seen its first user experiments: ptychography at a free-electron laser, holographic imaging at a storage ring and preliminary tests on X-ray photon correlation spectroscopy. The detector performed remarkably well in terms of spatial resolution achievable in the sample plane, owing to its small pixel size, large active area and very large dynamic range; but also in terms of its frame rate, which is significantly faster than traditional CCDs. In particular, it is the combination of these features which makes PERCIVAL an attractive option for soft X-ray science