The single shot spectrometer (SSS) is located in the FEE before the HOMS mirrors. The device is a transmissive hard x-ray single shot spectrometer based on cylindrically bent thin silicon crystal (a 3 mm2 Si wafer with 10 um thickness mechanically bent). It provides a shot by shot measurement of the SASE spectrum by absorbing a portion of the incident beam and reflecting a range of wavelengths fullfilling the bragg condition due to the curvature of the crystal. Currently the spectrometer has 4 crystals available with different cuts/curvature:
Si(100) flat
Si(100) bent to 100 mm radius
Si(100) bent to 50 mm radius
Si(110) bent to 50 mm radius
which can also work at higher reflections and at Bragg angles between 30 and 60 degrees.
Cu, Ni and Stainless steel foils can be inserted in the beam upstream the spectrometer for calibration.
At 120 Hz, the FEE SSS crystals get damage at high fluences, specially at relatively low energies. Even at 7 keV with 5 mJ there is slow damage.
For more information see here.