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Using Off-Resonance Laser Modulation for Beam Energy Spread Cooling in Generation of Short-Wavelength Radiation

Various seeding configurations have being proposed for frequency up-conversion of the electron beam density distribution, in which the energy spread, may however hinder the harmonic generation efficiency. In this Letter, a method for cooling the electron beam energy spread by off-resonance seed laser modulation is described, using a transversely dispersed beam and a modulator undulator with proper transverse gradient. With this novel mechanism, it is shown that the frequency up-conversion efficiency can be significantly enhanced. We present theoretical analysis and numerical simulations for seeded soft x-ray free electron laser and storage ring based coherent harmonic generation in extreme ultraviolet spectral region.