Demonstration of two-color x-ray FEL by laser emittance spoiler

Two-color is an emerging mode of operation in x-ray Free Electron Laser (FEL) having the potential to expand the pump and probe experiments toward fs temporal resolution and to site-selective spectroscopy. A novel and quick to setup method for two-color x-ray generation is presented here. The approach is experimentally demonstrated at the hard x-ray branch of SwissFEL (Aramis) using a laser emittance spoiler(LES). A short laser pulse is overlapped at the peak of the primary photocathode laser causing a local increase of the electron bunch emittance. In this configuration, the x-ray FEL emission occurs only for the two unspoiled parts of the bunch. Due to the fact that the electron bunch acquires an energy chirp along the acceleration, the spoiled beam produces two x-ray pulses of individual duration of few tens of fs having 1% photon energy separation and a time delay controllable up to 100 fs. Different from other techniques, the present method relies on the standard FEL configuration and it does not require a dedicated accelerator and undulator setting. With the LES, we achieved high efficiency, high energy and spectral stability paired to an independent control of the duration and of the relative intensity of the two colors. The LES enables shot-to-shot switching between one and two-color FEL and further, it is compatible with high repetition-rate FELs, as it is not associated to electron beam losses such as in other two-color methods.