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Abstract: Self-seeding schemes, consisting of two undulators with a monochromator inbetween, aim to reduce the bandwidth of SASE X-ray FELs. We recently proposed anew method of monochromatization exploiting a single crystal inBragg-transmission geometry for self-seeding in the hard X-ray range. Astraightforward extension is to use such kind of monochromator setup with two-or more- crystals arranged in a series to spectrally filter the SASE radiationat two closely-spaced wavelengths within the FEL gain band. This allows for theproduction of doublet -or multiplet- spectral lines. Applications involve anyprocess with a large change in cross section over a narrow wavelength range, asin multiple wavelength anomalous diffraction. Here we consider the simultaneousoperation of the LCLS hard X-ray FEL at two closely spaced wavelengths. Wepresent simulation results for the LCLS baseline, yielding fully coherentradiation shared between two longitudinal modes. Mode spacing can be easilytuned within the FEL gain band. The XFEL output intensity contains anoscillating -mode-beat- component whose frequency is related to the frequencydifference between the pair of longitudinal modes considered. At saturation oneobtains FEL-induced modulations of energy loss and energy spread in theelectron bunch at optical frequency, which can be converted into densitymodulation at the same optical frequency using a weak chicane installed behindthe baseline undulator. Powerful coherent radiation can then be generated withthe help of an optical transition radiation OTR station. We also consider howthe doublet structure of the XFEL generation spectra can be monitored by anoptical spectrometer. The OTR coherent radiation pulse is naturallysynchronized with the X-ray pulses, and can be used for timing the XFEL to highpower conventional lasers with femtosecond accuracy for pump-probeapplications.



Author: Gianluca Geloni, Vitali Kocharyan, Evgeni Saldin

Source: https://arxiv.org/







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