Analysis of the FEL-RF interaction in recirculating, energy-recovering linacs with an FEL

Recirculating, energy-recovering linacs can be used as driver accelerators for high power FELs. Instabilities which arise from fluctuations of the cav ity fields are investigated. Energy changes can cause beam loss on aperture s, phase oscillations and optical cavity detuning. These effects in turn ca use changes in the laser output power through a time-varying FEL gain funct ion. All three effects change the beam-induced voltage in the cavities and can lead to unstable variations of the accelerating field and output laser power. We have developed a model of the coupled system and solved it both a nalytically and numerically. It includes the beam-cavity interaction, low l evel RF feedback, and the electron-photon interaction. The latter includes the FEL gain function in terms of cavity detuning, energy offset, and is va lid both in the small signal gain and in the saturated regimes. We have dem onstrated that in the limit of small perturbations, the linear theory agree s with the numerical solutions and have performed numerical simulations for the IR FEL presently being commissioned at Jefferson Lab. (C) 1999 Elsevie r Science B.V. All rights reserved.