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Register a new userToward a single mode Free Electron Laser for coherent hard x-ray experiments
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The fluctuations of the longitudinal coherence length expected from the worlds first hard X-ray Free Electron Laser, the Linac Coherent Light Source, are investigated. We analyze, on a shot-to-shot basis, series of power spectra generated from 1D-FEL simulations. We evaluate how the intrinsic noise in the spectral profile of the X-ray beam reflects on its longitudinal coherence length. We show that the spectral stability of the LCLS beam will allow coherent X-ray experiments with a reasonable acquisition time. We also propose a scheme to deliver single-mode X-ray radiation using a narrow bandpass monochromator.
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X-ray Absorption Spectroscopy (XAS) is a widely used X-ray diagnostic method. While synchrotrons have large communities of XAS users, its use on X-Ray Free Electron Lasers (XFEL) facilities has been rather limited. At a first glance, the relatively narrow bandwidth and the highly fluctuating spectral structure of XFEL sources seem to prevent high-quality XAS measurements without accumulating over many shots. Here, we demonstrate for the first time the collection of single-shot XAS spectra on an XFEL, with error bars of only a few percent, over tens of eV. We show how this technique can be extended over wider spectral ranges towards Extended X-ray Absorption Fine Structure (EXAFS) measurements, by concatenating a few tens of single-shot measurements. Such results open indisputable perspectives for future femtosecond time resolved XAS studies, especially for transient processes that can be initiated at low repetition rate.
The resolution function of a spectrometer based on a strongly bent single crystal (bending radius of 10 cm or less) is evaluated. It is shown that the resolution is controlled by two parameters, (i) the ratio of the lattice spacing of the chosen reflection to the crystal thickness and (ii) a single parameter comprising crystal thickness, its bending radius, and anisotropic elastic constants of the chosen crystal. Diamond, due to its unique elastic properties, can provide notably higher resolution than silicon. The results allow to optimize the parameters of bent crystal spectrometers for the hard X-ray free electron laser sources.
Resonant elastic X-ray scattering has been widely employed for exploring complex electronic ordering phenomena, like charge, spin, and orbital order, in particular in strongly correlated electronic systems. In addition, recent developments of pump-probe X-ray scattering allow us to expand the investigation of the temporal dynamics of such orders. Here, we introduce a new time-resolved Resonant Soft X-ray Scattering (tr-RSXS) endstation developed at the Pohang Accelerator Laboratory X-ray Free Electron Laser (PAL-XFEL). This endstation has an optical laser (wavelength of 800 nm plus harmonics) as the pump source. Based on the commissioning results, the tr-RSXS at PAL-XFEL can deliver a soft X-ray probe (400-1300 eV) with a time resolution about ~100 fs without jitter correction. As an example, the temporal dynamics of a charge density wave on a high-temperature cuprate superconductor is demonstrated.
Free-electron lasers (FELs) opened a new window on imaging the motion of atoms and molecules. At SLAC, FEL experiments are performed at LCLS using 120 Hz pulses with 10^12 to 10^13 photons in 10 fs (billions of times brighter than at the most powerful synchrotrons). Concurrently, users and staff operate under high pressure due to flexible and often rapidly changing setups and low tolerance for system malfunction. This extreme detection environment raises unique challenges, from obvious to surprising, and leads to treating detectors as consumables. We discuss in detail the detector damage mechanisms observed in 7 years of operation at LCLS, together with the corresponding damage mitigation strategies and their effectiveness. Main types of damage mechanisms already identified include: (1) x-ray radiation damage (from catastrophic to classical), (2) direct and indirect damage caused by optical lasers, (3) sample induced damage, (4) vacuum related damage, (5) high-pressure environment. In total, 19 damage mechanisms have been identified. We also present general strategies for reducing damage risk or minimizing the impact of detector damage on the science program. These include availability of replacement parts and skilled operators and also careful planning, incident investigation resulting in updated designs, procedures and operator training.
X-ray free-electron lasers (XFELs) as the world`s most brilliant light sources provide ultrashort X-ray pulses with durations typically on the order of femtoseconds. Recently, they have approached and entered the attosecond regime, which holds new promises for single-molecule imaging and studying nonlinear and ultrafast phenomena like localized electron dynamics. The technological evolution of XFELs toward well-controllable light sources for precise metrology of ultrafast processes was, however, hampered by the diagnostic capabilities for characterizing X-ray pulses at the attosecond frontier. In this regard, the spectroscopic technique of photoelectron angular streaking has successfully proven how to non-destructively retrieve the exact time-energy structure of XFEL pulses on a single-shot basis. By using artificial intelligence algorithms, in particular convolutional neural networks, we here show how this technique can be leveraged from its proof-of-principle stage toward routine diagnostics at XFELs, thus enhancing and refining their scientific access in all related disciplines.
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