Amplification of ultrashort electromagnetic pulses propagating along quasi-continuous electron beams

Specific features of amplification of short electromagnetic pulses propagating along steady-state nonequilibrium electron flows with a group velocity differing from the translational velocity of particles are analyzed. It is shown that an amplitude level substantially higher than the saturation level in amplification of quasi-continuous signals can be attained by permanent injection of electrons without initial modulation to one of the pulse fronts. The effective duration of the pulse being amplified is reduced simultaneously. The Cherenkov and undulator interaction mechanisms are considered. Analysis is carried out using a simple 1D model based on the averaged description of the electron-wave interaction, as well as direct numerical simulation based on the KARAT code taking into account the parameters of planned experiments on observation of this effect.