Energy spread analysis of a waveguide optical klystron

Waveguide free electron lasers (FELs) based on electrostatic accelerator have many advantages including CW operation and high wall-plug efficiency, and therefore are an excellent candidate for many potential applications in medicine, science and industry. One drawback of electrostatic accelerators is that their beam current is usually small and this results in a low beam-wave interaction gain. To reduce this problem, one possible approach is to employ an FEL configuration having an intrinsically higher gain such as a waveguide optical klystron configuration. However because of the enhanced electron bunching process, the interaction gain of waveguide optical klystrons may become more susceptible to electron energy spread. To assess this quantitatively, energy spread effects in a waveguide optical klystron are analysed and formulated in the small signal limit. Through numerical examples, it is shown that the higher interaction gain of waveguide optical klystron devices can be realistically achieved with the nominal electron beam quality of electrostatic accelerators.