Open Comb-shaped Groove Waveguide Slow-wave System

The developing trends of the traveling wave tube (TWT) are wider bandwidth, higher output power, higher efficiency, better linearity, higher reliability and longer life. As the key component of the beam-wave interaction of a TWT for exciting microwave energy, the slow-wave structure (SWS) basically determine the performance of the TWT. Due to the advantages of easy being micro-machined, wide horizontal size, good heat dissipation etc, the open rectangular grating SWS is a potential high frequency system working in the millimeter-wave band and sub-millimeter band (even THz band). So it becomes a hot topic at present.In this paper we have investigated the open rectangular grating wave guide SWS theoretically firstly, and then studied the SWSs with dielectric loading and arbitrary grating shape. The dispersion equations and coupling impendence expressions are obtained by means of field theory, and the linear beam-wave interaction theory for three SWSs are built. Furthermore, the calculation programs in Matlab environment are developed to solve above equations. Based upon a large amount of numerical calculations, the dispersion characteristics, the coupling impendence characteristics and the small signal gains of the SWS are obtained; and the influences of the structure parameters, the dielectric parameters, the grating shapes and the electronic beam parameters are discussed for increasing the bandwidth or gain. Several important and valuable results which bring forth new ideas are achieved and listed as the followings:1. By using field-matching method and taking effects of fast and slow wave into account, the dispersion equation and the coupling impendence expression of an open rectangular grating waveguide SWS are obtained. The influence of various structure dimensions on the dispersion relation and the coupling impendence are discussed though numerical calculation. It shows the structure may work at higher frequency range though reducing the length of period and the height of rectangular groove.2. The beam-wave interaction of an open rectangular grating waveguide SWS is analyzed. The 'hot' dispersion equation of the SWS is obtained according to the self-consistent field theory. The numerical computation results of the 'hot' dispersion equation give the influence of the structure dimensions and the electron beam parameters on the small signal gain.3. The dispersion equation and the coupling impendence expression of a dielectric loaded, open rectangular grating waveguide SWS are derived. The numerical computation results of the dispersion equation and the coupling impendence are carried out. It shows that when introducing the dielectric layer into the SWS the phase velocity of the structure is obvious reduced and the frequency band is wider, while the coupling impendence is reduced.4. The 'hot' dispersion equation of a dielectric loaded, open rectangular grating waveguide SWS is obtained by using the self-consistent field theory. The influences of the dielectric constant and dielectric thickness on the small signal gain are calculated. The theoretical results indicate that the frequency band becomes wider while the small-signal gain is reduced as introducing the dielectric layer into the SWS.5. An approach to the analysis of the arbitrary-shaped grating structure is presented. The unified dispersion equation is obtained by means of an approximate field-theory analysis, in which the profile of the groove is replaced by a series of rectangular steps. The dispersion characteristics and the coupling impendence of five different grating structures are compared, and it is found the triangle-groove SWS has the widest bandwidth and the smallest coupling impendence and the swallow-tailed groove SWS has the narrowest and the biggest coupling impendence among these structures.6. The unified 'hot' dispersion equation of an arbitrary grating-shaped, open rectangular waveguide SWS is obtained based upon the self-consistent field theory. By numerical calculation the small signal gains of five different shape grating structures are investigated, and the results indicate the triangle-groove SWS has the smallest gains and widest bandwidth and the swallow-tailed groove SWS has the biggest gain and the narrowest bandwidth among these structures.