Optimization of optical klystron for coherent harmonic generation FEL

The optimization of an optical klystron is discussed for coherent harmonic generation (CHG) FEL. A simple general formula is given for optimal Nd (dispersive section parameter). Its relations with the optical field and electron beam energy spread were analysed. We proposed and analysed a non-symmetric optical klystron configuration in which the second undulator is resonant on the higher harmonic. It is shown that by proper choice of the magnetic field parameters the harmonic emission can be enhanced compared with that of symmetric configuration.