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Dispersion geometry of the spectrometer. The cylindrically bent crystal membrane diffracts dispersively a small percentage of the FEL beam to form the spectrograph, which is recorded on a scintillator-based detector positioned normal to the diffracted beam.
(a) Schematics of the experimental setup. (b) Example of SASE spectra for 20 consecutive pulses recorded using the full-range Si(111) spectrometer. (c) The full-range Si(111) spectrum and (d) the high-resolution Si(333) spectrum of the same individual pulse (red) overlayed with the Si(111) measurement (gray).
Dispersion calibration of the spectrometer for both the Si(111) and Si(333) geometry. The red and blue curves are fitted lines using Eq. (1). The inset on the lower right shows the observed line shape with the red data points measured with the Si(111) spectrometer and blue data points Si(333). The red line is a Gaussian fit to the line shape measured by Si(111). The blue line is the theoretical line shape of the Si(111) rocking curve for comparison to the higher resolution Si(333) measurement, showing good agreement.
Spectra of an individual FEL pulse reordered simultaneously by the Si(111) and Si(333) spectrometers while operating in the self-seeded mode. The red crosses are measurement obtained using the Si(333) spectrometer while the black circles are from the Si(111) spectrometer measurement. The dashed lines are Gaussian fits to the central peak, yielding FWHM values of 0.65 eV and 0.45 eV, respectively.
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