Sideband elimination and high efficiencies in a strongly tapered free-electron laser amplifier

It is demonstrated by numerical simulation that strongly tapered free-electron laser amplifiers exhibit negligible sideband growth, consistent with the predictions of a recent analytical theory [R.P. Pilla and A. Bhattacharjee, Phys. Plasma 1 (1994) 390]. One-dimensional simulations are presented for a short-wavelength (∼10 μm) amplifier with a quadratically tapered wiggler. For pre-bunched electrons with a phase-spread in the range (13–12) mod 2π and thermal spread in the range (0.5–1)%, the extraction efficiency lies in the range (50–33)%.