SPECTRAL BEHAVIOR OF HIGH-POWER COMPTON FREE-ELECTRON LASERS .2. EFFECT OF FILTERING AND TAPERING ON SIDE-BAND GENERATION

We show via a numerical study of the asymptotic spectral behavior of f iltered and tapered free-electron-laser (FEL) oscillators that taperin g must in general be assisted by filtering to prevent spectral broaden ing. To interpret these results, we present a theoretical analysis of the sideband instability, including the transient regime. For long amp lifiers, an analysis of trapped particles appears to be relevant. This is not the case for an FEL oscillator where the standard ''deeply-tra pped-electrons'' and ''exponential-regime'' assumptions are most often irrelevant. Moreover, spectral broadening is not related in a simple way to a sideband inhibition, due to the multiplicity of nonlinear mec hanisms involved in FEL dynamics.