Analysis and nonlinear simulation of quadrupole and dipole wiggler FELs for millimeter waves

An FEL amplifier with a helical quadrupole wiggler is investigated analytically and numerically. A 3-D simulation code is utilized to examine the nonlinear evolution of circular waveguide modes as well as the characteristics of relativistic particle motion, which illustrate a strong focusing effect. The simulation predicts a 0.55 dB/cm, 2% efficiency, and 8% bandwidth for the optimum TE21 mode at 300 GHz for a untapered wiggler. Mode competition for an over-moded waveguide and a nonlinear wiggler taper for efficiency enhancement are examined for a linear dipole wiggler in a rectangular guide. It is found that the maximum TE01 mode efficiency is 53% at 280 GHz and that the TM21 mode power can be as high as 28% in the constant wiggler level region. The dipole wiggler case is compared with that from the LLNL wiggler-averaged code WFEL