SASE Free-electron laser (FEL) radiation arises from shot noise in the electron bunch, which is amplified along the undulator section and results in X-ray pulses consisting of many longitudinal modes. The output bandwidth of FELs can be decreased by seeding the FEL process with longitudinally coherent radiation. In the hard x-ray region, there are no suitable external seeding sources. Self-seeding represents a viable alternative. The X-ray beam is separated from the electrons using a magnetic chicane, and then monochromatized. The monochromatized X-rays serve as a narrowband seed, after recombination with the electron bunch, along the downstream undulators. This scheme generates longitudinally coherent FEL pulses.
Geloni et al. have proposed monochromatization based on Forward Bragg Diffraction (FBD), which introduces a delay of the narrowband X-rays pulse of the order of femtoseconds that can be matched to the delay of the electron bunch due to the chicane. The FBD process produces a small transverse displacement of the X-ray beam, which may result in the loss of efficiency of the seeding process. Preliminary results from an experiment performed at Cornell High Energy Synchrotron Source (CHESS) seem to confirm the predicted transverse displacement, which is therefore to be taken into account in the design of self-seeding infrastructure for optimizing the FEL performance.