Excitation of electromagnetic waves by an axial electron beam in a slow wave structure

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Abstract
An axial electron beam passing through a slow wave structure is unstable to an electromagnetic perturbation whose phase velocity equals the velocity of the beam. This phenomenon of Čerenkov emission is the basis of all traveling wave tubes. In this paper a simple treatment of excitation of electromagnetic waves in a slow wave structure, viz., two parallel planes partially loaded with a dielectric is presented. When the effects of a guide field or a background plasma are ignored, the dispersion relation is quite simple. It is in the form of a coupling between a TM mode of a stationary electron plasma-filled slow wave structure and the beam mode. In the presence of a guide magnetic field, the electron motion is predominantly one-dimensional and the growth rate of instability is considerably modified. At relatively higher gas pressures, when a background plasma is created by the beam, the mode structure is considerably modified, leading to a reduction in the growth rate, hence to the efficiency of the device.

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