Nonlinear electron interaction with TM modes in a low-voltage waveguide FEL

Summary form only given. A parallel approach to reduce the size of these compact waveguide FEL systems is to minimize the electron beam voltage required to generate a strong radiation at a given frequency. In a recent work, we have demonstrated that the electron interaction with the longitudinal electric field component of TM modes leads to an electron velocity component oscillating at a period half as long as the wiggler period, /spl lambda/w, in the longitudinal direction. Therefore the electron beam experiences an effective wiggler magnet of /spl lambda/w. For a waveguide FEL based on such a longitudinal interaction mechanism, the required electron voltage for a strong radiation at a given frequency is therefore less than that for a waveguide FEL based on the conventional transverse interaction mechanism. In this contribution, we extend our previous results and consider the nonlinear behaviour of the electron-wave interaction. Since the space charge effects are negligible for these low power waveguide FELs, the electron interaction with TM modes are simulated in the Compton regime. It is shown that a previously ignored non-resonant term in the energy conservation equation could be significant in the FEL interaction, and under certain operation conditions its contribution may be adjusted to enhance the interaction gain. This new interaction feature may be unique to low voltage waveguide FELs for which the wavelength of the radiation field is in the same order as the wiggler period. Numerical simulation is then extended to the nonlinear interaction regime and similar saturation behaviours to those in a conventional waveguide FEL are found although their saturation onsets are different. The numerical work reported in this contribution may be used as a design base for TM mode based low voltage waveguide FEL systems.