Generation of femtosecond extreme ultraviolet pulses using low-energy electron beams for a pump-probe experiment

The surface recombination process and molecular dynamics are generally on the order of tens of femtoseconds; therefore, the research and development of an accelerator-based intense and short-pulse generation scheme are needed for pump-probe experiments, which are widely utilized tools for investigating fast molecular dynamics. Here, we propose an echo-enabled harmonic generation (EEHG)-technique-based free electron laser (FEL) scheme that uses a low-energy beam ( KE∼200MeV). The proposed scheme is designed to generate short-pulse soft extreme ultraviolet radiation at ∼80nm, with a pulse duration of 3 fs for the full width at half maximum. An electron injector consisting of a photo-cathode-based S-band radio frequency electron-gun, solenoid magnets, and three S-band accelerating columns was designed and optimized using a multiobjective particle swarm optimization method. For the EEHG-FEL section, the narrow bands of electrons produced by a second modulator and a few-cycle laser pulse with a linear momentum compaction at the second chicane had a perfect upright position at the top of the current modulation produced by the first modulator, which enhanced the peak current by a factor of approximately 30 %. In this scheme, two conventional lasers with wavelengths of 5.2μm and 800 nm were adopted to enhance the high bunching factors by generating microbunching structures. The saturated output power of the proposed FEL was approximately 4.97 MW.