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- M. Dohlus
DESY, Hamburg
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X-ray Free Electron Lasers (FEL) make use of the principle of Self-Amplified-Spontaneous-Emission (SASE) where electron bunches interact in an undulator with their own co-propagating radiation. They do not require optical resonators and their frequency is therefore not limited by material properties as the reflectivity of mirrors. The performance of X-ray SASE FELs depends exponentially on the beam quality of the electron bunch. Therefore effects in the beamline before the undulator are as important as particle-field interactions of the FEL-SASE process. Critical components are the low emittance electron source, accelerating sections, the bunch compression system and the undulator. Due to the high peak currents and small beam dimensions space charge (SC) effects have to be considered up to energies in the GeV range. Coherent synchrotron radiation (CSR) drives not only the FEL but is also emitted in dispersive sections as bunch compressors. SC, CSR, and wake fields affect significantly longitudinal beam parameters (peak current, correlated and uncorrelated energy spread) and the transverse emittance. Start-to-end simulations use a sequence of various tracking codes (with or without SC, CSR and wake fields) and FEL programs. Usually the particle or phase space information has to be carefully converted for each transition from one tool to another. Parameter studies need many simulations of the complete system or a part of it and beyond that, calculations with several random seeds are necessary to consider the stochastic nature of SASE-FEL process.
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Slides
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