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Simple generation of two-color FEL pulses using a sextupole magnet

We present a new and simple method to generate two-color free-electron laser (FEL) pulses. In our scheme an electron bunch is quadratically tilted in the transverse direction and exhibits an energy chirp. The head and tail of the bunch are aligned to the undulator beamline and will lase, while the middle part of the bunch undergoes orbit oscillations and thus is unable to amplify the FEL radiation. Since the beam is energy chirped, the central wavelengths of the SASE amplification at the two aligned parts of the bunch are different. The beam tilt is introduced by a variation of sextupole strength in a bunch compressor section. We present simulation results of this new method for the hard X-ray FEL beamline Aramis at SwissFEL. This includes the general performance and tunability of the scheme, the requirements on the energy chirp and tilt generation, and adverse effects associated with the beam tilt.