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

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 30 nm XUV pulse based on the HGHG scheme and 9 nm soft x-ray pulse based on the EEHG scheme with duration of about 8 fs (FWHM) and peak power of GW level can be generated from a 180 nm UV seed laser with beam waist of 75 μm. This new scheme can provide an optional operation mode for the existing seeded FEL facilities to meet the requirement of short-pulse FEL.