Saturation behaviour of a waveguide optical klystron in the presence of electron energy spread

A waveguide optical klystron configuration has recently been suggested to e nhance the small signal gain of compact waveguide free electron lasers (FEL s) driven by low current electron beams. Based on an analytical formulation , it has been demonstrated that the small signal gain of waveguide free ele ctron lasers can be increased significantly when incorporated into the wave guide optical klystron configuration. However, this enhanced small signal g ain may be realised at the expense of achievable saturation power when the device is operated as an oscillator. In this study, a computer package was developed to investigate numerically saturation behaviours of waveguide opt ical klystrons taking into account the energy spread effects. It is shown t hat when the small signal gain of a waveguide FEL is only marginally greate r than the round-trip loss of the radiation power, the waveguide optical kl ystron arrangement can provide not only a much larger small signal gain but also a greater saturation power. (C) 1999 Published by Elsevier Science B. V. All rights reserved.