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We have substantially increased the output power and extraction efficiency of a free-electron laser operating at 34.6 GHz by tapering the wiggler magnetic field. In the exponential-gain regime, the laser exhibited a measured gain of 34 dB/m. With a 50-kW input signal, the amplifier saturated in 1.3 m with a 180-MW output signal. By using a taper that brought the magnetic field at the end of the wiggler down to 45% of its initial (peak) value, we increased the output signal to 1.0 GW. This corresponds to an extraction efficiency of 34%.
Orzechowski et al. (1986) studied this question.