Analysis of aftercavity interaction in gyrotrons

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Abstract
This paper is devoted to the analysis of a so-called “aftercavity interaction” in gyrotrons. The term “aftercavity interaction” is used for defining the cyclotron resonance interaction between electrons in a spent beam exiting the cavity and traveling electromagnetic waves propagating from the cavity exit into the output part of a tube. A simple theory allowing one to analyze the role of aftercavity interaction in gyrotron operation has been developed. With the use of only two parameters characterizing the profile of the waveguide wall and the profile of the magnetic field, one can find conditions resulting in weakening this parasitic effect. Also, the simulations for the gyrotron efficiency in the presence of aftercavity interaction are performed for an existing device. Results of these simulations agree with analytical predictions. They also show how the axial distribution of the magnetic field in an existing cryomagnet should be modified in order, first, to use the aftercavity interaction for improving the interaction efficiency and, second, for weakening the role of this interaction in degrading the wall-plug efficiency in gyrotrons with depressed collectors. Results of such analysis can be beneficial for the gyrotron efficiency enhancement.

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