The Optical Design of the Soft X-ray Self Seeding at LCLS

After the successful demonstration of the hard X-ray self-seeding at LCLS1, an effort to build a system for working in the soft X-ray region is ongoing. The idea for self-seeding in the soft X-ray region by using a grating monochromator was first proposed by Feldhauset. al2. The concept places a grating monochromator in middle of the undulators and selects a narrow bandwidth "seed" from the SASE beam produced by the upstream section of undulators, which is then amplified to saturation in the downstream section of the undulators. The seeded FEL beam will have a narrower bandwidth approaching the transform limit. The challenge is to accommodate a monochromator and refocusing system as well as the electron beam magnetic chicane into a very limited space. The Soft X-raySelf Seeding system replaces only a single undulator section of similar to 4 m. Theoverall project and the expected FEL performances are described elsewhere3. Here we present the detailed optical design solution, consisting of a fixed incidence angle toroidal blazed grating with variable groove density, a rotating plane mirror (the only required motion for tuning the energy) to redirect the selected monochromatic beam onto an exit slit, and two more mirrors, one sphere and one flat, to focus and overlap the ` seed' onto the electron beam in the downstream undulators.