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TY - UNPB AU - Yang, X. AU - Giannessi, L. AU - Mirian, N.S. ED - Schaa, Volker R.W. ED - Decking, Winfried ED - Sinn, Harald ED - Geloni, Gianluca ED - Schreiber, Siegfried ED - Marx, Michaela TI - Post-Saturation Dynamics and Superluminal Propagation of a Super-Radiant Spike in a Free-Electrons Laser Amplifier J2 - Proc. of FEL2019, Hamburg, Germany, 26-30 August 2019 CY - Hamburg, Germany T2 - Free Electron Laser Conference T3 - 39 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 a unique opportunity in time resolved diffraction imaging of isolated molecules or non-periodic structures for 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 for the possibility of simultaneously reducing the pulse duration while increasing the peak power and the pulse energy. We study the dynamics of an isolated spike of radiation in superradiant regime. We show that conditions exist where the pulse moves with a group velocity larger than one and is followed by a pedestal resulting from a complex post-saturation dynamic. The tail is constituted by a train of sub-pulses with both transverse and longitudinal coherence and decaying amplitudes. We analyze the dynamical conditions leading to the formation of the main pulse and the tail. We study the correlation of the tail structure with the longitudinal phase space of the electrons and provide a recipe to partially suppress this tail. PB - JACoW Publishing CP - Geneva, Switzerland ER -