DPB Outstanding Paper Award: Two-color X-ray FEL by photocathode laser emittance spoiler

A novel and noninvasive method for high-energy two-color x-ray FEL emission is demonstrated at SwissFEL. In the experiment, a laser emittance spoiler pulse is overlapped with the primary photocathode laser pulse to locally spoil the beam emittance and inhibit the FEL emission, ultimately resulting in X-ray emission at two wavelengths. High spectral stability and the possibility to independently control the duration and the intensity ratio between the two-color X-ray pulses is demonstrated. The laser emittance spoiler enables shot-to-shot selection between one and two-color FEL emission and further, it is compatible with high repetition-rate FELs, as it does not contribute to beam losses.