The physics of x-ray free-electron lasers

Abstract

X-ray Free-electron Lasers (X-ray FELs) give us for the first time the possibility to explore structures and dynamical processes of atomic and molecular systems at the Ångstrom-femtosecond space and time scales. They generate coherent photon pulses with time duration of a few to 100 femtosecond, peak power of 10 to 100 GW, over a wavelength range extending from about 100 nm to less than 1 Å. Using these novel and unique capabilities new scientific results are being obtained in atomic and molecular sciences, in areas of physics, chemistry and biology. In this paper we review the physical principles, the theoretical models and the numerical codes on which X-ray FELs are based, starting from a single electron spontaneous undulator radiation to the FEL collective instability of a high density electron beam, strongly enhancing the electromagnetic radiation field intensity and its coherence properties. We present also a short review of the main experimental properties of X-ray FELs, and discuss the results of the most recent research to improve their longitudinal coherence properties, increase the peak power and generate multicolor spectra.

Interplay between relativistic energy corrections and resonant excitations in x-ray multiphoton ionization dynamics of Xe atoms

“…For Xe at 1.5 keV, however, the highest charge of the experimental CSD exceeded by far the theoretical prediction of the direct one-photon ionization limit [16]. To explain this * koudai.toyota@cfel.de † sangkil.son@cfel.de ‡ robin.santra@cfel.de discrepancy, it has been proposed that multiple resonant excitations followed by Auger-like decays, combined with the broad energy bandwidth of SASE XFEL pulses [6], can drive further ionization beyond the direct one-photon ionization limit. This is called resonance-enabled or resonance-enhanced x-ray multiple ionization (REXMI) [15,16] and this mechanism has been implemented in Refs.…”

“…A series of decay processes, a socalled decay cascade, can occur if the hole is formed in a deep inner shell [1][2][3][4][5]. The interaction with x rays becomes rather complex when the sample is exposed to the unprecedentedly high fluence generated by x-ray free-electron lasers (XFELs) [6][7][8][9]. Beyond the one-photon saturation fluence, which is the inverse of the single-photon ionization cross section [10], a single atom can absorb more than one photon sequentially after or during decay cascades, and then it becomes a highly charged ion.…”

Femtosecond single-shot timing and direct observation of subpulse formation in an infrared free-electron laser

“…FEL theory has now been developed so far as to quantitatively predict the length and shape of the generated optical pulses. A classical treatment of the relativistic electrodynamics based on Maxwell-Lorentz equations emerged to be a valid description of the energy exchange between particles and optical field [16,17]. Whereas the underlying physical process for the optical power growth is already captured by a 1D analysis, the study of the pulse profile requires 2D simulations [18].…”

High-accuracy wavefront sensing for x-ray free electron lasers

“…The rapid evolution of hard x-ray FELs [1][2][3], with femtosecond pulse durations, has enabled a wide range of previously impossible dynamical studies of atoms, molecules, clusters, and materials in the physical and life sciences [4]. In addition to the photon energy, pulse duration, and spectral characteristics, it is important to have an accurate knowledge of the single pulse x-ray wavefront, which affects focal plane intensity and profile, spot size, and spatial resolution, as well as centroid location within the focal plane.…”