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.
We report experimental demonstration of capturing single-shot X-ray Free-electron Laser (FEL) beam profiles using gas fluorescence. The measurement was carried out at the Linac Coherent Light Source using 7 keV hard X-rays propagating through ambient air. The nitrogen fluorescence emitted upon the passage of the X-ray FEL beam were imaged using a highly sensitive optical setup, and there was sufficient optical yield that single-shot measurements were feasible. By taking two orthogonal and simultaneous images, the beam trajectory could be determined in a nearly non-invasive manner, and is best suited for photon energies in the soft X-ray regime, where such a diagnostic capability has been largely unavailable previously. The integrated intensity of the images could also serve as a non-invasive intensity monitor, complementary to current implementations of gas- and solidbased monitors. High repetition-rate Free-electron Lasers can greatly benefit from such a new diagnostic tool for eliminating potential thermal damages.
After the successful demonstration of the hard X-ray self-seeding at LCLS, an effort to build a system for working in the soft X-ray region is ongoing. The idea for self-seeding in the soft X-ray region by using a grating monochromator was first proposed by Feldhauset. al. The concept places a grating monochromator in middle of the undulators and selects a narrow bandwidth “seed” from the SASE beam produced by the upstream section of undulators, which is then amplified to saturation in the downstream section of the undulators. The seeded FEL beam will have a narrower bandwidth approaching the transform limit. The challenge is to accommodate a monochromator and refocusing system as well as the electron beam magnetic chicane into a very limited space. The Soft X-raySelf Seeding system replaces only a single undulator section of ~ 4 m. Theoverall project and the expected FEL performances are described elsewhere. Here we present the detailed optical design solution, consisting of a fixed incidence angle toroidal blazed grating with variable groove density, a rotating plane mirror (the only required motion for tuning the energy) to redirect the selected monochromatic beam onto an exit slit, and two more mirrors, one sphere and one flat, to focus and overlap the ‘seed’ onto the electron beam in the downstream undulators.
During the last years, scanning coherent x-ray microscopy, also called ptychography, has revolutionized nanobeamcharacterization at third generation x-ray sources. The method yields the complete information on the complex valued, nanofocused wave field with high spatial resolution. In an experiment carried out at the Matter in Extreme Conditions (MEC) instrument at the Linac Coherent Light Source (LCLS) we successfully applied the method to an attenuated nanofocused XFEL beam with a size of 180(h) × 150(v)nm2 (FWHM) in horizontal (h) and vertical direction (v), respectively. It was created by a set of 20 beryllium compound refractive lenses (Be-CRLs). By using a fast detector (CSPAD) to record the diffraction patterns and a fast implementation of the phase retrieval code running on a graphics processing unit (GPU), the applicability of the method as a real-time XFEL nanobeam diagnostic is highlighted.
A hard x-ray free-electron laser (XFEL) provides an x-ray source with an extraordinary high peak-brilliance, a time structure with extremely short pulses and with a large degree of coherence, opening the door to new scientific fields. Many XFEL experiments require the x-ray beam to be focused to nanometer dimensions or, at least, benefit from such a focused beam. A detailed knowledge about the illuminating beam helps to interpret the measurements or is even inevitable to make full use of the focused beam. In this paper we report on focusing an XFEL beam to a transverse size of 125nm and how we applied ptychographic imaging to measure the complex wavefield in the focal plane in terms of phase and amplitude. Propagating the wavefield back and forth we are able to reconstruct the full caustic of the beam, revealing aberrations of the nano-focusing optic. By this method we not only obtain the averaged illumination but also the wavefield of individual XFEL pulses.
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 advent of X-ray Free-electron Laser (FEL) such as the Linac Coherent Light Source (LCLS) has and will continue
to enable breakthroughs and discoveries in a wide range of scientific disciplines including physics, chemistry, structural biology, and material science. It has created high demand on user beamtime that is often left unfulfilled. We report here the fabrication, characterization and X-ray measurements of ultra-thin silicon single-crystal membranes for potentially beam-sharing the LCLS beam. Using a special fabrication process, samples of (111), (110), and (100) orientations were made with thicknesses ranging from 5 to 20 μm. Both high-resolution rocking curves and white-beam topographic data were first obtained using synchrotron X-rays, demonstrating near ideal diffraction qualities. Subsequent tests using the full LCLS FEL beam revealed lattice distortions from beam-induced membrane vibrations, which were then shown to be effectively reduced by ambient air or with smaller membrane dimensions. These findings are paving a way for a practical beam-sharing implementation at LCLS in the near future.
Current and upcoming X-ray sources, such as the Linac Coherent Light Source (LCLS) at the Stanford Linear Accelerator Center (SLAC, USA), the SPring-8 Angstrom Compact Free Electron Laser (SACLA, Japan), or the X-ray Free Electron Laser (XFEL, Germany) will provide X-ray beams with outstanding properties.1, 2 Short and intense X-ray pulses of about 50 fs time duration and even shorter will push X-ray science to new frontiers such as, e. g., in high-resolution X-ray imaging, high-energy-density physics or in dynamical studies based on pump-probe techniques.
Fast processes in matter often require high-resolution imaging capabilities either by magnified imaging in direct space or diffractive imaging in reciprocal space. In both cases highest resolutions require focusing the X-ray beam.3, 4 In order to further develop high-resolution imaging at free-electron laser sources we are planning a platform to carry out high-resolution phase contrast imaging experiments based on Beryllium compound refractive X-ray lenses (Be-CRLs) at the Matter in Extreme Conditions (MEC) endstation of the LCLS. The instrument provides all necessary equipment to induce high pressure shock waves by optical lasers. The propagation of a shock wave is then monitored with an X-ray Free Electron Laser (FEL) pulse by magnified phase contrast imaging. With the CRL optics, X-ray beam sizes in the sub-100nm range are expected, leading to a similar spatial resolution in the direct coherent projection image. The experiment combines different state-of-the art scientific techniques that are currently available at the LCLS. In this proceedings paper we describe the technical developments carried out at the LCLS in order to implement magnified X-ray phase contrast imaging at the MEC endstation.
We report the design of a single-shot transmissive spectrometer for spectral measurements of the Linac Coherent Light
Source (LCLS) operating in the mode of Self-Amplified Spontaneous Emission (SASE). The spectrometer was
constructed using an ultra-thin and bent silicon single-crystal operating in the symmetric Bragg geometry. It was shown
to be capable of recording the single-shot LCLS spectra while transmitting the majority of the incident flux to
downstream experimental stations. The spectrometer used the Si (111) reflection for capturing the full SASE spectrum,
and was then configured the high resolution mode using the Si (333) for resolving single spikes in the power spectral
An inline diagnostics device was developed to measure the intrinsic shot-to-shot intensity and position fluctuations of
the SASE-based LCLS hard X-ray FEL source. The device is based on the detection of back-scattered X-rays from a
partially-transmissive thin target using a quadrant X-ray diode array. This intensity and position monitor was tested for
the first time with FEL X-rays on the XPP instrument of the LCLS. Performance analyses showed that the relative
precision for intensity measurements approached 0.1% and the position sensitivity was better than 5 μm, limited only by
the Poisson statistics of the X-rays collected in a single shot.
We report on the x-ray absorption of Warm Dense Matter experiment at the FLASH Free Electron Laser (FEL) facility at DESY. The FEL beam is used to produce Warm Dense Matter with soft x-ray absorption as the probe of electronic structure. A multilayer-coated parabolic mirror focuses the FEL radiation, to spot sizes as small as 0.3μm in a ~15fs pulse of containing >1012 photons at 13.5 nm wavelength, onto a thin sample. Silicon photodiodes measure the transmitted and reflected beams, while spectroscopy provides detailed measurement of the temperature of the sample. The goal is to measure over a range of intensities approaching 1018 W/cm2. Experimental results will be presented along with theoretical calculations. A brief report on future FEL efforts will be given.
Femtosecond time-resolved small and wide-angle x-ray diffuse scattering techniques are applied to investigate the
ultrafast nucleation processes that occur during the ablation process in semiconducting materials. Following intense
optical excitation, a transient liquid state of high compressibility characterized by large-amplitude density fluctuations is
observed and the build-up of these fluctuations is measured in real-time. Small-angle scattering measurements reveal
the first steps in the nucleation of nanoscale voids below the surface of the semiconductor and support MD simulations
of the ablation process.
The melting dynamics of laser excited InSb have been studied with femtosecond x-ray diffraction. These measurements demonstrate that the initial stage of crystal disordering results from inertial motion on a laser softened potential energy surface. These inertial dynamics dominate for the first half picosecond following laser excitation, indicating that inter-atomic forces minimally influence atomic excursions from the equilibrium lattice positions, even for motions in excess of an Å. This also indicates that the atoms disorder initially without losing memory of their lattice reference.
Perfect crystal monochromators cannot diffract x rays efficiently, nor transmit the high source brightness available at synchrotron radiation facilities, unless surface strains within the beam footprint are maintained within a few arcseconds. Insertion devices at existing synchrotron sources already produce x-ray power density levels that can induce surface slope errors of several arcseconds on silicon monochromator crystals at room temperature, no matter how well the crystal is cooled. The power density levels that will be produced by insertion devices at the third-generation sources will be as much as a factor of 100 higher still. One method of restoring ideal x-ray diffraction behavior, while coping with high power levels, involves adaptive compensation of the induced thermal strain field. The design and performance, using the X25 hybrid wiggler beam line at the National Synchrotron Light Source (NSLS), of a silicon crystal bender constructed for this purpose are described.
Preserving the high source brightness of the third generation of synchrotron radiation facilities will require that thermal and pressure deformations of the monochromator crystals be maintained within a few arc-seconds. Recent experiments at the National Synchrotron Light Source (NSLS) have demonstrated the potential of adaptive crystal optics to cope with high power densities. In this technique the crystals deformations are minimized both by an efficient water-jet cooling and by externally applied pressure loads. Thermal deformation can be reduced further with diamond crystals because of their high thermal conductivity and low coefficient of thermal expansion. In this paper we describe the results achieved by optimization of adaptive crystal optics for diamond crystals.
A brief review of using soft x-ray resonant magnetic scattering in the study of magnetic thin films and multilayers is given. Results from recent studies of thin Fe films and Fe/Gd multilayers are used as examples to demonstrate the information that can be obtained and the unique features of this technique. Comparison is made with related techniques: magneto- optical Kerr effect, Faraday effect, and magnetic circular dichroism.