Spectral behavior of high-power Compton free-electron lasers. II. Effect of filtering and tapering on sideband generation

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Abstract
We show via a numerical study of the asymptotic spectral behavior of filtered and tapered free-electron-laser (FEL) oscillators that tapering must in general be assisted by filtering to prevent spectral broadening. To interpret these results, we present a theoretical analysis of the sideband instability, including the transient regime. For long amplifiers, an analysis of trapped particles appears to be relevant. This is not the case for an FEL oscillator where the standard ‘‘deeply-trapped-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 mechanisms involved in FEL dynamics.

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