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

Summary form only given. Mode competitions in waveguide free electron lasers (FELs) are complex because the beam-wave interaction can grow from noise into different waveguide modes which may be spaced very closely in the spectrum. Even when a FEL system is operated in one single waveguide mode, it may still resonate at two frequencies, one higher and one lower, because of waveguide dispersion, and this leads to an operational mode competition. Without an appropriate control, this operational mode competition would result in an eventual oscillation of the FEL system at the lower frequency because the lower frequency mode usually receives a higher small signal gain than the higher one. This of course is not desirable since FELs need to be operated at a frequency as high as possible. In this paper, we consider a technique that can be used to control both the conventional and the operational mode competition. The key of this technique is a non-standard wiggler magnet consisting two cascaded sections whose periods and field strengths are different.