The SXRSS Beam Overlap Diagnostics PRD provides the physics requirements for the overlap diagnostics as part of the SXRSS project for the LCLS undulator. This document covers the overlap diagnostics to be installed after girders 9 and 12 of the LCLS undulators. The SXRSS project replaces the LCLS undulator segment 9 with a 4-magnet chicane and a grating monochromator to enable self-seeding capability for soft x-rays at LCLS. Both the electron beam via the chicane and the x-ray seeding beam via the grating and mirrors can get displaced from their original path through the downstream undulators. Optimal performance of the self-seeding requires that both beams overlap for one gain length (approximately m) downstream of the chicane system within a small fraction (approximately 5 im) of their beam sizes (approximately 30 im) and that both beams resume the original trajectory for normal SASE operation. Although the absolute electron beam orbit can easily be recovered to im accuracy with the existing BPM system and the optimum relative overlap of both beams can be achieved by maximizing the FEL output at the seed wavelength, initial commissioning requires a diagnostics to set and verify the relative overlap to 10 im accuracy to ensure a detectable signal of the seeded FEL. The existing undulator layout makes it feasible to add an overlap diagnostic device to each of the two long break sections between undulators 9 & 10 and 12 & 13, respectively. This more than satisfies the one gain length overlap requirement.
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