Two-bunch seeding of soft x-ray free electron lasers

Seeded free electron lasers (FELs) demonstrate a good performance and are successfully used in different user experiments in extreme ultraviolet and soft x-ray regimes. In this paper, a simple modification of the seeding scenario is proposed relying on the generation of two closely spaced bunches with very different properties: a low-current seeding bunch and a high-current bunch that amplifies coherent radiation, produced by the seeding bunch. This approach eliminates different limitations and mitigates some harmful effects in the standard scenario. In particular, one can generate very high harmonic numbers with a moderate laser power in a simple high-gain harmonic generation (HGHG) scheme. Alternatively, in case of moderate harmonic numbers, one can strongly reduce the required laser power thus simplifying the design of high repetition rate seeded FELs. An influence of beam dynamics effects (like nonlinearities of the longitudinal phase space of electron beams, coherent synchrotron radiation, longitudinal space charge, geometrical wakefields, and microbunching instabilities) on properties of the output radiation (spectrum broadening, pedestals, and stability) can be to a large extent reduced in the proposed scheme. In this paper, we illustrate the operation of the two-bunch seeding scheme in HGHG configuration with realistic start-to-end simulations for the soft x-ray user facility free electron laser in Hamburg. We show that nearly Fourier-limited multigigawatt pulses can be generated at 4 nm using the present compact design of the undulator system. With several thousand pulses per second, this can be a unique source for photon science. Published by the American Physical Society 2024