The successful operation of X-ray free-electron lasers (FELs), like the Linac Coherent Light Source or the Free-Electron Laser in Hamburg (FLASH), makes unprecedented research on matter at atomic length and ultrafast time scales possible. However, in order to take advantage of these unique light sources and to meet the strict requirements of many experiments in photon science, FEL photon pulse durations need to be known and tunable. This can be achieved by controlling the FEL driving electron beams, and high-resolution longitudinal electron beam diagnostics can be utilized to provide constraints on the expected FEL photon pulse durations. In this paper, we present comparative measurements of soft X-ray pulse durations and electron bunch lengths at FLASH. The soft X-ray pulse durations were measured by FEL radiation pulse energy statistics and compared to electron bunch lengths determined by frequency-domain spectroscopy of coherent transition radiation in the terahertz range and time-domain longitudinal phase space measurements. The experimental results, theoretical considerations, and simulations show that high-resolution longitudinal electron beam diagnostics provide reasonable constraints on the expected FEL photon pulse durations. In addition, we demonstrated the generation of soft X-ray pulses with durations below 50 fs (FWHM) after the implementation of the new uniform electron bunch compression scheme used at FLASH.
X-ray pump/X-ray probe applications are made possible at X-ray Free Electron Laser (XFEL) facilities by generating two X-ray pulses with different wavelengths and controllable temporal delay. In order to enable this capability at the European XFEL, an upgrade project to equip the soft X-ray SASE3 beamline with a magnetic chicane is underway. In the present paper we describe the status of the project, its scientific focus and expected performance, including start-to-end simulations of the photon beam transport up to the sample, as well as recent experimental results demonstrating two-color lasing at photon energies of 805 eV + 835 eV and 910 eV + 950 eV. Additionally, we discuss methods to analyze the spectral properties and the intensity of the generated radiation to provide on-line diagnostics for future user experiments.Appl. Sci. 2020, 10, 2728 2 of 20 short pulse duration and the high intensity of FEL pulses [5,6]. Recent examples of this type of studies at FELs are related to the dissociation dynamics of molecules, magnetic ordering of material, as well as charge transfer processes [7]. One major challenge in these experiments is related to the inherent temporal jitter of the two independent laser sources and to its precise characterization in order to actually take advantage of the short pulse duration. Until now, only experiments using a seeded FEL in combination with the optical seed laser could demonstrate high temporal resolution reaching the sub-femtosecond time scale [8].New developments at FEL sources have shown that it is also possible to generate two FEL pulses at different photon energies, which enable pump-probe experiments [9][10][11]. This method has two major advantages; first, the temporal jitter is drastically reduced, since both pulses are produced by the same electron bunch and, second, pump and probe pulses are both of high photon energies. This allows, for example, to selectively address different sites in a molecule or different subshells in an atom for inducing a process (pump) as well as for monitoring its evolution (probe). Very soon after the start of operation of the European XFEL discussions were initiated about the possibilities to realize such a two-color pump-probe scheme at the SASE3 Soft X-ray branch of the facility.Already the simplest way of generating two-color pulses at the SASE3 beamline of the European XFEL, in combination with the high-repetition rate capabilities of the facility is expected to enable novel exciting science at the two soft X-ray instruments Small Quantum Systems (SQS) [12,13] and Spectroscopy & Coherent Scattering (SCS) [12,14].In the following we will summarize the scope of the project (Section 2) and the scientific background for the two-color pump-probe (2CPP) set-up at the European XFEL with the help of concrete proof-of-principle experimental proposals (Section 3) followed by the results of simulations for pulse generation and transport (Section 4), while in Section 5 we show the results of recent experiments demonstrating the first two-color lasi...
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