Nonlinear analysis and simulation of a millimeter range free electron laser with a helical quadrupole wiggler

A free electron laser (FEL) with a helical quadrupole wiggler and guide field is studied via analytic theory and computer simulation in the Compton regime. Analytic work on the stability of ideal orbits in the quadrupole wiggler with axial guide field has been carried out. An NRL code which was developed to examine helical dipole wiggler amplifiers is modified to treat both a helical quadrupole wiggler and the resulting relativistic orbit equations. Earlier work which examined the effects of near-axis self-fields on the stability of betatron oscillations and linear gain is extended to include waveguide modes, electron orbits far off aixs, axial guide field and the 3-D nonlinear gain to saturation. The numerical simulation of nonlinear gain for the TE21 mode without wiggler taper yields moderate efficiency (5%), high gain (66 dB) and broad bandwidth (50%) at 300 GHz with an annular beam. (orig.)