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High power, short pulse lengths and full coherence are the main parameters of the second generation free electron lasers like BESSY soft X-ray FEL. To provide radiation with these properties, cascades of HGHG stages are planned. In these stages an energy modulation is imprinted on the electron beam by seeding radiation. A dispersive section converts this energy modulation to a spatial modulation which is optimized for a particular harmonic. The following radiator is tuned to this harmonic and generates radiation with high power. The separation of the electron beam and seeding radiation after the modulation is desirable, as not only the quality of the radiator improves but also the seeding radiation itself can be used for diagnostics. A simple bending magnet leads to the separation but it causes a coupling of the longitudinal and transverse motion and could spoil the longitudinal modulation. Based on an exact linear model, a dispersive chicane is designed for one stage of the BESSY FEL which bends the electron beam without the coupling effects. The linear model will be presented and used to design a chicane. Simulations of the spatial modulation will also be discussed.
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