Analysis and design of the taper in metal-grating periodic slow-wave structures for rectangular Cerenkov masers

Abstract

The hybrid-mode dispersion equation of the metal-grating periodic slow-wave structure for a rectangular Cerenkov maser is derived by using the Borgnis function and field-matching methods. An equivalent-circuit model for the taper of the groove depth that matches the smooth waveguide to the metal-grating structure is proposed. By using the equivalent-circuit method, as well as the Ansoft high frequency structure simulator (HFSS) code, an appropriate electromagnetic mode for beam-wave interaction is selected and the equivalent-circuit analysis on the taper is given. The calculated results show that a cumulative reflection coefficient of 0.025 for the beam-wave interaction structure at a working frequency of 78.1 GHz can be reached by designing the exponential taper with a TEz10 rectangular waveguide mode as the input and the desired TEx10 mode as the output. It is worth pointing out that by using the equivalent-circuit method, the complex field-matching problems from the traditional field-theory method for taper design can be avoided, so the taper analysis process is markedly simplified.