Intensity interferometry measurements were carried out to study the spatial coherence properties of a Free-Electron Laser (FEL) in the Self-Amplified Spontaneous Emission (SASE) mode in the hard X-ray regime. Statistical analyses based on ensemble averages of the spatial intensity correlation function were performed on a large number of pulses, overcoming challenges associated with the FEL beam being non-stationary in time and highly collimated. The second-order intensity correlation functions consistently show deviations from unity, reminiscent of the classical Hanbury-Brown and Twiss effect. They also exhibit a slow decaying spatial dependence at length-scales larger than the width of the beam, indicating a high degree of spatial coherence. These measurements are consistent with the behavior of a highly brilliant but chaotic source obeying Gaussian statistics as expected for a SASE FEL. Our study could be used to devise an in-line diagnostic capable of providing quasi real-time feedback for understanding and tuning the FEL process.
We present a method for the propagation of partially coherent radiation using coherent mode decomposition and
wavefront propagation. The radiation field is decomposed into a sum of independent coherent modes. Each mode
is then propagated separately using conventional wavefront propagation techniques. The summation of these
modes in the plane of observation gives the coherence properties of the propagated radiation. As an example,
we analyze propagation of partially coherent radiation transmitted through a single circular aperture.
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.