Spectral dynamics of a collective free electron maser

A study of the nonlinear spectral dynamics of a Free Electron Maser (FEM) is reported. In the FEM for fusion applications [1], the electron beam is modulated by a step-tapered undulator consisting of two sections with different strengths and lengths. The sections have equal periodicities and are separated by a field-free gap. The microwave beam is guided in a rectangular corrugated waveguide. The electron energy is rather low and the current density is large, therefore, the FEM operates in the collective (Raman) regime. namics of the spectrum is calculated with a code that is based on a multifrequency model in the continuous beam limit with a 3D description of the electron beam. Space-charge forces are included. They strongly influence the behaviour of the generated spectrum. In the saturated regime the gain is still relatively high because of the large outcoupled power fraction. It is discussed that quite robust parameter regions exist, where single-peaked frequency spectra are excited.