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Free-electron lasers (FELs) seeded with external lasers hold great promisefor generating high power radiation with nearly transform-limited bandwidth insoft x-ray region. However, it has been pointed out that the initial seed lasernoise will be amplified by the frequency up-conversion process, which maydegrade the quality of the output radiation produced by a harmonic generationscheme. In this paper, theoretical and simulation studies for laser phase erroramplification in seeded FEL schemes with slippage effect taken into account arepresented. It is found that, the seed laser imperfection experienced by theelectron beam can be significantly smoothed by the slippage effect in themodulator when the slippage length is comparable to the laser pulse length.This smoothing effect allows one to preserve the excellent temporal coherenceof seeded FELs in presence of large laser phase errors. For ultra-short UV seedlasers with FWHM around 16 fs, the slippage length in a modulator with ~30undulator periods is typically comparable to the laser width; for longer seedlaser pulses with FWHM around 80 fs, the slippage length can be made comparableto the laser width using a modulator tuned at the sub-harmonic of the UV seedlaser. Three-dimensional simulations have been carried out for a soft x-rayfacility using seed laser pulses with large frequency chirp and the resultsshow that the sensitivity of the bandwidth of the seeded FEL to the initialfrequency chirp can be significantly reduced by a proper design of themodulator such that the slippage length is comparable to the laser width. Ourstudies show that the tolerance on laser phase error for generating nearlytransform-limited soft x-ray pulses in seeded FELs is much looser thanpreviously thought and fully coherent radiation at nanometer wavelength may bereached with current technologies.

Slippage effect on laser phase error amplification in seeded harmonic generation free-electron lasers