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

Summary Form only given, as follows. An FEL amplifier with a helical quadrupole wiggler has been investigated analytically and numerically. A three-dimensional simulation code has been 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 gain, 2% efficiency, and 8% bandwidth for the optimum TE/sub 21/ mode at 300 GHz for an untapered wiggler. Mode competition for an overmoded waveguide and a nonlinear wiggler taper for efficiency enhancement have been examined for a linear dipole wiggler in a rectangular guide. It has been found that the maximum TE/sub 01/ mode efficiency is 53% at 280 GHz and that the TM/sub 21/ mode power can be as high as 28% in the constant wiggler level region. The dipole wiggler case has been compared with that from the wiggler-averaged code WFEL.<>