Simulations of the build-up of transverse and longitudinal structures of the microwave field in the Fusion FEM

The interaction of longitudinal and transverse modes in the Dutch Fusion FEM is studied, taking the characteristics of the feedback system, provided by quasi-optical stepped reflectors, into account. The simulations predict that the stable single-frequency operation is possible basically due to the frequency dispersion of the feedback system. As for the transverse mode interaction, the selective properties of the feedback system, as well as the effect of the suppression of parasitic modes by the operating mode at the output of the interaction region lead to a very pure transverse structure of the output microwave radiation.