Two-color x-ray free-electron laser by photocathode laser emittance spoiler

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Multispectral x-ray pump-probe experiments call for synchronized two-color free-electron lasers (FEL).This mode often implies a laborious setup or an inefficient use of the undulator.We report on a simple and noninvasive approach tested at SwissFEL for a two-color x-ray delivering almost 60% of the pulse energy compared with a single color.In this new method, a ps UV pulse is overlapped to the photocathode drive laser increasing the beam emittance, which locally inhibits the FEL process.This scheme permits highstability in energy and spectrum and the control of the two-color duration and intensity ratio.It enables shot-to-shot switching between one and two-color FEL and, since not associate to beam losses, it is compatible with high repetition-rate and high average-power FELs.