Enhanced high-gain harmonic generation for x-ray free-electron laser

The harmonic generation free-electron laser (HG-FEL) is capable of producing temporally and spatially coherent pulses in the vacuum ultraviolet and shorter wavelength regions. In this letter an easy-to-implement scheme that significantly enhances the performance of HG-FEL and extends its short-wavelength range is proposed. By adding a short phase-reversed modulator after the dispersive section, the electron energy spread is reduced and simultaneously the electron microbunching is enhanced. Three-dimensional simulations demonstrate the energy spread reduction and the bunching enhancement and show that more powerful higher harmonic radiation can be obtained