Nonlinear simulation of a high-power, collective free-electron laser

Results obtained with the three-dimensional nonlinear analysis and simulation code, ARACHNE, and a recent 33.4-GHz, collective, free-electron laser (FEL) amplifier experiment are compared. The experiment has demonstrated power levels of 61 MW (≈27% efficiency) without recourse to tapered magnetic fields, using a 750-keV/300-A electron beam with a nominal axial energy spread of 1.5% propagating through a cylindrical drift tube in the presence of a helical wiggler and an axial guide magnetic field. Significant differences in the character of the emission were found, depending on the direction of the guide magnetic field. When the wiggler and guide fields were parallel, observed power levels reached approximately 4 MW for both the strong and weak guide field regimes, but vanished in the neighborhood of the magnetic resonance. However, the maximum power was observed in the reversed field case when the wiggler and guide fields were antiparallel. In this case, no resonant enhancement in the transverse velocity is expected to occur; however, a significant reduction in the output power occurred in the neighborhood of the antiresonance. The ARACHNE simulation is in substantial agreement with the experiment