Postsaturation dynamics and superluminal propagation of a superradiant spike in a free-electron laser amplifier

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.

Few-Cycle Infrared Pulse Evolving in FEL Oscillators and Its Application to High-Harmonic Generation for Attosecond Ultraviolet and X-ray Pulses

“…It should be noted that Figure 4 suggests an evolution of a superluminal pulse, which moves with a group velocity larger than the velocity of light in vacuum during the linear growth at the early stage of the macropulse. Such superluminal FEL pulse also appears in FEL amplifiers of superradiant regime [18,19].…”

A self-consistent numerical approach to track particles in FEL interaction with electromagnetic field modes

“…The userdefined parameter L thresh limits the spot size variation along the undulator. Figure 4a shows a waterfall plot in the electron beam frame normalized at each z position to display the relative velocity of the radiation wavepacket, which is close to the beam velocity in the exponential regime and becomes superluminal in the non linear regime [30]. In Figure 4b, the spectrum just before saturation is shown as a function of L thresh normalized to the gain length.…”

High-gain harmonic generation with temporally overlapping seed pulses and application to ultrafast spectroscopy

“…In this section we analyze the modifications occurring to the field envelope during the harmonic conversion and the following FEL amplification processes, including the correction due to the onset of saturation. We exclude from our analysis short pulse effects and deep saturation effects, which would modify the dispersion during the interaction process and could not be modeled in terms of field amplitudes [29].…”