FEL amplification by a multiharmonically prebunched electron beam

Summary form only given. In this paper, we consider a multiharmonic bunching technique in which the electron beam is prebunched at both the fundamental laser frequency and its harmonics. To demonstrate its principle, the new technique is applied to a waveguide free electron laser. It is shown that by optimizing the electron bunching, this multiharmonic bunching technique is capable of producing a greater enhancement of laser amplification than what is achievable with the fundamental frequency bunching technique. This comparison is further explored by considering the energy spread effect in prebunched waveguide FEL devices. An analytical formulation of the energy spread effect suggests that the beam quality requirement is essentially the same no matter whether a waveguide FEL is prebunched at the fundamental frequency or at multiharmonic frequencies. Therefore the projected enhancement of the laser amplification can be achieved without imposing a more stringent requirement for electron beam quality. With some straightforward modifications to an existing prebunched FEL experiment apparatus, for instance adding harmonic bunched cavities, the beam-wave interaction should be enhanced using the same waveguide and magnet system.