Non-Axisymmetric Slowed Structure and Plasma Modes in a Waveguide with a Dielectric Rod and Magnetized Plasma

In the scope of the development of X-band antenna-amplifier, a Cherenkov maser with a rod slow-wave structure operating in the fundamental non-axisymmetric HE₁₁ mode of a dielectric rod antenna, an important task is the analysis of the possible influence of plasma that can appear at the dielectric surface on the beam-wave interaction. In this work, eigenmodes of a circular waveguide with a dielectric rod and plasma layer adjoining the rod surface have been considered in the general case of arbitrary number of RF field azimuthal variations. The plasma was assumed fully magnetized, uniform, and cold, with nonmoing ions, so that it was characterized by the electron density and laver thickness. The dispersion relation was derived and solved numerically for different eigenmodes, and the RF field profiles were calculated giving the Poynting flux, transmitted RF power and coupling impedances over the parameter space. The plasma influence on the structure electrodynamics is revealed, first, in the modification of electromagnetic, pure structure modes dispersion and field profiles and, second, in the appearance of the branch of plasma eigenmodes (Trivelpiece-Gould modes). We have studied in detail the dependences of the structure fundamental HF₁₁ mode phase velocity and coupling impedance on the plasma density and laver thickness, and parameters were identified that modified the mode dispersion to such a degree that prevents a Cherenkov interaction with an electron beam. From the standpoint of normal operation of an X-band antenna-amplifier device, the plasma formation at the rod surface is quite admissible while its density remains less than or of the order of 10¹² cm^-3. At higher densities, when the frequency is significantly less than the plasma frequency, the phase velocity rapidly increases with increasing density and becomes greater than c. However, the coupling impedance as a function of plasma density has a maximum (severa- l times greater than the no-plasma value) when the plasma frequency not much exceeds the frequency, so that the plasma presence in case of proper density may have even a positive influence on the antenna-amplifier. As to the plasma modes, it has been shown that they are insignificant in the antenna-amplifier as designed despite of higher coupling impedance because their phase velocities remain too low for given parameters.