Study: Self Seeding Crystal Angles Study
Shifts :1
Purpose: Measure the Self Seeding diamond crystal initial orientation and verify the Self Seeding Crystal
model. Advantages of the study:1) having an absolute measurement of FEL wavelength on the
spectrometer as function of the crystal angles, without needing to actually do an absolute
measurement. 2) faster tune up for the desired wavelength for users. 3) faster calibration of user
spectrometers (and maybe more reliable) than SASE scan. 4) better understanding of the crystal model.
An additional task is to check reproducibility of crystal angles by repeating some scans going in opposite
directions or going far back and then to the set-point for each value.
Details: The measurements of the reflections crossings give information on the crystal initial orientation
in the holder and on the offsets between the crystal angles being set and the real machine state. We
want to reproduce maps [see Figure] of the Self Seeded lines as function of the pitch and yaw angles to
determine for which pair of angles the crossing between different lines occurs, thus giving information
on the output wavelength for any pair of angles.
Relevant maps can be made at the FEE spectrometer energies of 9.1 keV and 8.4 keV.
The procedure consists in having an HXR self-seeded beam sent to FEE spectrometer, fix one of the
crystal angles and the beam energy, scan the other angle and record the spectra. Repeat for other beam
energies and same angles to cover all the energy range relevant for the crossings. This has to be done
because for each angle scan we can see only that falls within FEL amplification bandwidth (~30 eV),
despite the spectrometer range is wider. Each figure will take approximately 5 crystal scans each 3
minutes long, for ~16 figures (~4 hours of acquisitions). Time will be required to set up the beam, to
change energy between 8.4 and 9.1, and to find the spectrometer signal on the camera. Reliably finding
the dispersed signal on FEE spectrometer screen for the energies needed is a prerequisite for this study.
Relevant scans at 9.05 keV pitch =74 , 64.5, 46.1 (scanning yaw); at 8.5 keV Yaw=2 (scanning pitch
around 54.6), 8.33 keV Yaw=-2 (scanning pitch around 56.5), 8.46 keV Yaw=-2.5, (scanning Pitch around
68) 8.48 keV Yaw=-2 (scanning pitch around 75.4).