Tunable frequency comb from echo-enabled harmonic generation free electron laser

Abstract

We propose a novel scheme for the generation of a tunable soft x-ray frequency comb in a seeded free-electron laser (FEL) via the chirped pulse beating technology. Our research shows that harnessing the chirped pulse laser to imprint a submillimeter-scale energy modulation in the phase space of the relativistic electron beam together with the process of echo-enabled effect, multichromatic output soft x-ray radiation with a series of longitudinal modes can be amplified in an echo-enabled harmonic generation (EEHG) free electron laser, which finally forms a comb-liked shape in the frequency spectrum with the mode spacing frequency at terahertz (THz) band. Based on the theoretical and numerical analyses of the modulation process, the electron beam maintains a nanometer-scale microstructures due to the echo effect, while the beam obtains an energy modulation at submillimeter-scale from the chirped pulse beating laser. This leads to a series of longitudinal modes in the downstream radiator. Three-dimension time-dependent simulation results show that a frequency comb with a central wavelength of 8.9 nm can be generated directly with the help of chirped beating laser. The frequency spacing between the frequency lines on the radiation spectrum can be tuned easily by changing the parameters of chirped beating laser.