Two-stage EEHG for coherent hard X-ray generation based on a superconducting linac

A two-stage echo-enabled harmonic generation （EEHG） scheme is proposed for a superconducting linac- driven FEL to produce coherent hard X-rays. Electron beams of quite different bunch lengths are separately used in each stage of EEHG, and a monochromator is designed to purify the radiation from the first stage for seeding the second stage. Theoretical analysis and 3D simulations indicate that the proposed scheme can generate high-rep- etition-rate coherent hard X-ray pulses directly from a conventional UV seed laser.