MOPA optical klystron FELs and coherent harmonic generation

We analyse the behaviour of a master oscillator power amplifier FEL, where the amplifier operates on a harmonic of the oscillator. The premature saturation of the amplifier output power, due to the induced energy spread of the oscillator, is studied. The effects of saturation in the oscillator can be eliminated when the modulating section is provided with an optical-klystron configuration. We show that this configuration ensures a stable output power after the amplifier section.