Abstract
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 the velocity of light in vacuum and is followed by a pedestal resulting from a complex postsaturation dynamics. The tail is constituted by a train of subpulses 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.
Highlights
Free-electron lasers (FELs) delivering brilliant VUV-Xray pulses with femtosecond temporal resolution are providing unique opportunities in the investigation of fast processes in physics, chemistry, and biology [1,2,3,4,5,6]
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 the velocity of light in vacuum and is followed by a pedestal resulting from a complex postsaturation dynamics
Summary
Free-electron lasers (FELs) delivering brilliant VUV-Xray pulses with femtosecond temporal resolution are providing unique opportunities in the investigation of fast processes in physics, chemistry, and biology [1,2,3,4,5,6] These FELs typically operate in single pass, high gain regime; the electrons, interacting with the self-emitted radiation in a long undulator, are periodically modulated at the resonant wavelength and amplify exponentially the radiation with power folding length Lg scaling inversely with the Pierce parameter ρfel [7]. This may have important practical implications, allowing a substantial reduction of the total pulse duration with a time-bandwidth product closer to the Fourier limit
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