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High gain Free-electron lasers (FELs) are driven by linear accelerators that are currently capable of delivering electron bunches in high-repetition rates from kHz and up to several MHz. Short wavelength FELs are commonly operated in a self-amplified spontaneous emission (SASE) scheme which is, due to its stochastic nature, limited in temporal coherence. External seeding and harmonic generation techniques have been shown to be valid solutions for the generation of fully coherent and stable radiation up to the soft x-ray. However, the required seed lasers in the UV regime and shorter wavelengths cannot generate pulses with sufficient energy at the repetition-rates accessible with superconducting accelerators, hence the seed laser can become the limiting factor which determines the maximum repetition-rate for future seeded FEL based on superconducting linacs. In this paper, we address the need for generation of fully coherent radiation with higher average flux and we propose two different schemes which overcome the limitations and enable the generation of seeded radiation at high-repetitionrates. In one case, an optical cavity is used instead of the seed laser source. It retains a seed laser pulse and recirculates it to seed electron bunches in high-repetition-rates. In the other case, an optical klystron scheme is exploited in the seeding process: a low-intensity seed laser initiates the process in the first modulator and then, a dispersive section increases the density modulation and enables coherent emission in the second modulator which increases further the energy modulation. The resulting energy modulation is several times larger than the one possible with a standard seeding scheme and same laser intensity. Reducing the requirements in seed laser power, the proposed setup makes the use of seed lasers at higher repetition-rates or shorter wavelengths possible. Here, we show simulation results for the optical klystron scheme and we discuss experimental possibilities.
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