Equivalent Circuit Analyses and Methods to Enhance Bandwidth of Klystron Reentrant Cavities

Abstract

Klystron is used as an amplifier at microwave frequency range. Klystrons use cylindrical or rectangular reentrant cavities in their interaction structure. Characteristics of reentrant cavity can be defined using parameters such as resonant frequency, characteristic shunt impedance and quality factor, which can be computed either using 3D electromagnetic field solver or using equivalent circuit analysis. An equivalent circuit analysis is often preferred to quickly arrive at initial dimensions of the cavity as well as to analyse the effect of various dimensional variations on the cavity parameters.

Equivalent circuit analysis for the calculation of these parameters is available in the literature for cylindrical reentrant cavity with single beam-tunnel and single constituent material. However, practical cavities are normally made with dissimilar materials and have multiple beam-tunnels as in the case of multiple-beam klystron. A new formulation has been proposed for accounting the effects of single and multiple beam-tunnels in the calculation of cavity gap capacitance. Proposed formulation provides better estimation of resonant frequency for the cavities with multiple beam-tunnels as compared to that from the existing analysis. A new analysis has also been proposed for the calculation of unloaded quality factor of cylindrical reentrant cavity using Wheeler’s incremental inductance rule. Analysis provides estimation of quality factor within 15% deviation compared with simulations. In addition to analysing the cavities with dissimilar materials, the proposed approach provides better estimation of quality factor compared with existing approaches.

Since a similar equivalent circuit analysis was not available for the rectangular reentrant cavity, the same is proposed here through geometric approximation and analytical approaches. Both the analyses have been proposed for a rectangular reentrant cavity with circular cylindrical ferrule and operating in the TM110 mode. Unloaded quality factor has been obtained using Wheeler’s incremental inductance rule. Analyses, through geometric approximation and analytical approaches, estimate resonant frequencies within 3% and 4.5% deviations, characteristic shunt impedance within 17% and 13% deviations, and unloaded quality factor within 15% and 14% deviations, respectively, compared with 3D electromagnetic simulations. The proposed analyses have also been validated against measurements and a good agreement has been obtained.

Klystrons inherently suffer from bandwidth limitation and a lot of research is going on to enhance its instantaneous bandwidth. Bandwidth of klystron depends primarily on the quality factor of the output cavity circuit. Here, three methods for enhancing the bandwidth of a klystron have been proposed. The first method utilises tailoring the surface roughness, by which the unloaded quality factor of the cavities could be reduced by a maximum of 48%. In the second method, a ridge-loaded rectangular and cylindrical reentrant cavity has been proposed where the quality factor could be reduced by maximum of 80%. In the third method, a post-loaded rectangular reentrant cavity has been proposed to have even higher reduction in the quality factor (upto 90%). Measurements have been carried out on unloaded, ridge-loaded and post-loaded rectangular reentrant cavities for the validation of proposed methods. Measured unloaded quality factors have been found within 10% deviations compared with those obtained through the simulations.

Equivalent circuit analyses and methods for reducing the quality factor of reentrant cavities proposed in the thesis will be useful for klystron designers to arrive at preliminary geometrical parameters of cylindrical or rectangular reentrant cavities with dissimilar constituent materials and to bolster their efforts to meet broadband operation.