Enhanced amplification of a waveguide free electron laser prebunched at multiharmonic frequencies

The mechanism of laser amplification is analysed for compact waveguide free electron lasers driven by prebunched electron beams of low energy and current. It is found that with a single sinusoidal bunching signal at the fundamental laser frequency, the conventional electron bunching method is intrinsically inefficient and as a result fails to achieve the maximum possible laser amplification. To optimise the electron bunching for a stronger beam-wave interaction, we propose a multiharmonic bunching technique with which the electron beam is bunched at both the fundamental frequency and its harmonics. By using this new technique, the laser amplification achieved with the conventional bunching technique can be increased by as much as 75%. Furthermore the new technique is shown to have the same beam quality requirement as its conventional counterpart.