Traveling-wave tubes and backward-wave oscillators with weak external magnetic fields

Recent development of plasma-assisted slow-wave oscillators [Goebel et al. IEEE Trans. Plasma Sci. 22, 547 (1994)], microwave sources that operate without guiding magnetic fields, has stimulated interest in the theoretical analysis of such tubes. In principle, in the absence of guiding magnetic fields, due to the space charge forces and the radial electric field of the wave, the electrons may propagate radially outward which increases electron coupling to the slow wave whose field is localized near the slow-wave structure (SWS). This increases the wave growth rate and efficiency, and hence allows one to shorten the interaction region. So the radial electron motion can be beneficial for operation if it does not lead to interception of electrons by the SWS. To avoid this interception a weak external magnetic field can be applied. The theory developed describes the effect of weak magnetic fields on the operation of traveling-wave tubes and backward-wave oscillators with electrons moving not only axially but also transversely. This theory allows one to estimate the magnetic field required for protecting the SWS from electron bombardment at different power levels. Theoretical predictions of the efficiency enhancement due to the weak magnetic field are confirmed in experiments.