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TY - UNPB AU - Yang, X. AU - Giannessi, L. ED - Chin, Yong Ho ED - Zhao, Zhentang ED - Petit-Jean-Genaz, Christine ED - Schaa, Volker RW TI - Ultrashort Pulse Generation and Superradiance in FELs J2 - Proc. of FLS2018, Shanghai, China, 5-9 March 2018 C1 - Shanghai, China T2 - ICFA Advanced Beam Dynamics Workshop T3 - 60 LA - english AB - The generation of a single X-ray isolated spike of radiation with peak power at multi-GW level and femtosecond temporal duration represents an almost unique opportunity in time resolved diffraction imaging of isolated molecules or non-periodic structures, for the possibility of acquiring single shot images before the Coulomb explosion of the sample takes place. Such a condition is met by an FEL operating in superradiant regime when the pulse has a self-similar shape deriving from the combined dynamics of saturation and slippage of the radiation over fresh electrons. We show that the peak of the pulse moves at superluminal speed similarly to the case of pulses propagation in anomalous dispersive active media and it is followed by a long pedestal resulting from the complex dynamics occurring in the tail after saturation. We analyze the dynamical conditions leading to the formation of the main pulse and the following tail and we study the correlation of the tail structure with the longitudinal phase space of the e-beam, providing recipes to partially suppress it. PB - JACoW Publishing CP - Geneva, Switzerland ER -