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FLASH, the soft X-ray free-electron laser (FEL) in Hamburg provides high-brilliance ultrashort femtosecond pulses at MHz repetition rate for user experiments. For many spectroscopic and dynamical studies in various research fields a small FEL energy bandwidth and ultrashort pulses are a prerequisite. In order to preserve a short pulse duration and still monochromatize the FEL radiation, the new pulse-length preserving monochromator beamline FL23 at FLASH2 uses a double-grating design. While the first grating disperses the radiation and an intermediate slit reduces the spectral bandwidth, the second grating compensates the introduced pulse front tilt, thereby preserving the ultrashort photon pulses.
The beamline is designed to work in the soft X-ray regime covering the spectral range between 1.3 nm and 20 nm with a spectral resolving power of approximately 2000 [1]. To maximize the transmission at the high energy end where the pulse elongation is not so critical the beamline can also be operated in a single grating configuration. A flexible microfocusing is provided at the experiment by a bendable Kirkpatrick-Baez mirror system – similar to the one used at the FL24 beamline at FLASH. A femtosecond optical laser synchronized to the FEL will be provided for pupm-probe experiments. The beamline concept and design has been developed using ray tracing simulations and confirmed by wavefront propagation simulations [2].
In the presentation, the pulse-length preserving double monochromator beamline concept will be introduced, the different operation modes and the expected photon parameters at the experimental station will be discussed and the first commissioning results will also be shown.
[1] L. Poletto, F. Frassetto, G. Brenner, M. Kuhlmann and E. Plönjes, Double-grating monochromatic beamline with ultrafast response for FLASH2 at DESY, Special Issue (PhotonDiag2017), J. Synchrotron Rad. 25, 131-137 (2018); https://doi.org/10.1107/S1600577517013777
[2] M. Ruiz-Lopez, L. Samoylova, G. Brenner, M. Mehrjoo, B. Faatz, M. Kuhlmann, L. Poletto and E. Plönjes, Wavefront-propagation simulations supporting the design of a time-delay compensating monochromator beamline at FLASH2, J. Synchrotron Rad. 26, 899-905 (2019); https://doi.org/10.1107/S160057751900345X
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