Emulation of two-pass gain in a cavity-based XFEL via self-seeding at LCLS

This work presents an experimental study emulating a two-pass gain scenario in a cavity-based X-ray free-electron laser (CBXFEL) using a self-seeding configuration at LCLS. In this “7+7” arrangement, radiation generated by the first seven hard X-ray undulators (HXUs) is spectrally filtered by a high-resolution self-seeding crystal monochromator and used to seed a second set of seven undulators downstream. This setup mimics the regenerative amplification process expected in the CBXFEL cavity, where the seed pulse is recirculated and overlapped with a second trailing electron bunch. By systematically reducing the number of post-crystal undulators (from 13 to 5), we quantified the spectral amplification ratio by comparing the self-seeded peak signal to the SASE background. These results confirm the feasibility of seeding with the initial seven HXUs and provide a valuable benchmark for extrapolating gain in future two-bunch CBXFEL demonstration experiments.