Controlling mode competition in a waveguide FEL with a two-sectioned wiggler magnet

In this paper, considered is a technique that can be used to control both the conventional and the operational mode competition in a waveguide free electron laser (FEL). The key of this technique is a non-standard wiggler magnet consisting of two cascade sections whose periods and field strengths are different. If one supposes the beam-wave interaction in a waveguide FEL to be strong at one desired frequency along with other unwanted resonance frequencies, with a two-sectioned wiggler, it is possible for its two constituent magnet sections to have their individual beam lines, of different slopes and different abscissa intersections on the dispersion diagram, both intersecting the waveguide's dispersion curve at the desired frequency. This would also lead to the two beam lines intersecting the dispersion curves at two different sets of unwanted resonance frequencies.