Study on rectangular waveguide grating slow-wave structure with cosine-shaped grooves

Recently, a new need for high-frequency, highpower sources has been emerging for advanced radar and communication needs, with frequencies in the band between 100 and 300GHz and peak powers as high as several hundreds of kilowatts, and with bandwidth of up to 10%. After investigation of the interaction mechanisms of different slow-wave structures, the rectangular waveguide grating SWS has attracted the scholar's interest due to some of its peculiarities: the scalability to smaller dimensions and shorter wavelengths, high precession of manufacturing and assembling, super thermal conductivity and low loss, which make it worth consideration, especially at millimeter-wave frequency-band. From the investigation of the different structures, such as V-type, cosine-type, dovetail-type, etc., the cosine-shaped groove SWS has the weakest dispersion and the widest bandwidth. In this paper, this type of structure was analyzed. A longitudinal cross section configuration is shown in Fig. 1. s is the width at the mouth of the groove, L is the period and L=s, b and p are waveguide height and groove depth, respectively. The width of the rectangular waveguide is represented by a. Expressed in terms of rectangular coordinates, the space of the rectangular waveguide grating SWS with cosine-shape grooves can be divided into two parts: interaction region I and groove region II. By means of an approximate field-theory analysis, the expressions of dispersion equation and coupling impedance are derived. In order to check the calculated results, the three dimensional electromagnetic simulation software CST was used for comparison. As shown in Fig. 2, the smooth curve represents the theoretical results, and the triangular points demonstrate the results given by CST simulations. It is obvious that the CST simulation data are in agreement with the theoretical results, which support our theory. Finally, numerical calculation shows that the bandwidth of the cosine-shaped groove SWS is much wider than that of rectangular-shaped groove one. And reducing the groove-width can broaden the frequency-band and decrease phase-velocity, while increasing the groove-depth can also decrease phase-velocity. For above cases, the coupling impedance is more than 16 Omega.