Generation of a few femtoseconds pulses in seeded FELs using a seed laser with small transverse size

Abstract

We propose a simple method to generate a few femtosecond pulses in seeded FELs. We use a longitudinal energy-chirped electron beam passing through a dogleg where transverse dispersion will generate a horizontal energy chirp, then in the modulator, a seed laser with narrow beam radius will only modulate the center portion of the electron beam and then short pulses at high harmonics will be generated in the radiator. Using a representative realistic set of parameters, we show that 30nm XUV pulse based on the HGHG scheme and 9nm soft x-ray pulse based on the EEHG scheme with duration of about 8fs (FWHM) and peak power of GW level can be generated from a 180nm UV seed laser with beam waist of 75μm. This new scheme can provide an optional operation mode for the existing seeded FEL facilities to meet the requirement of short-pulse FEL.