Seeding with a harmonic optical klystron resonator configuration in a high repetition rate free electron laser

The generation of highly coherent radiation in a high repetition rate free electron laser driven by superconducting linear accelerator has become topic growing interest. External seeding schemes like high-gain harmonic (HGHG) and echo-enabled are proven to be able generate the extreme ultraviolet x-ray range. However, current systems with sufficient power modulate beam is limited kilohertz Recently, achieve at rate, an optical resonator scheme been introduced recirculate modulator seed bunches. In this paper, two klystron configurations either starting or from shot noise studied. With as source, efficiency comparatively can significantly enhanced when compared standard single-stage HGHG. Simulation results show that coherent, stable, pulses water window range could generated proposed schemes. Some practical considerations including energy chirp effects density on mirrors discussed.