NONLINEAR SIMULATION OF A HIGH-POWER, COLLECTIVE FREE-ELECTRON LASER

A comparison is described between the three-dimensional nonlinear analysis and simulation code, ARACHNE, and a recent 33.4-GHz, collective, free-electron laser amplifier experiment at MIT. The experiment has demonstrated power levels of 61 MW (almost-equal-to 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 (B(w) less-than-or-equal-to 1.8 kG, lambda(w) = 3.18 cm) and an axial guide magnetic field (B0 less-than-or-equal-to 12 kG). Significant differences in the character of the emission were found based upon 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 (when the Larmor and wiggler periods are comparable). In this case, resonance refers to the enhancement of the transverse wiggle-induced velocity, and the reduction in the emission is due to the fact that the electron beam cannot propagate in this regime due to orbital instabilities. 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 was found to occur in the neighborhood of the antiresonance. The ARACHNE simulation is in substantial agreement with the experiment. In the reversed field case, the simulation shows peak power levels of 60 MW at the nominal axial energy spread of the experiment, as well as providing good correspondence with the power reduction at the anti-resonance. The source of this power reduction appears to be a previously unsuspected effect on the electron orbits due to the wiggler inhomogeneity. Agreement with the much lower power levels found when the wiggler and guide fields are parallel, however, requires the assumption of a substantial increase in the energy spread of the beam.