Design analysis of broadband output circuits for klystron amplifiers

Summary form only given. Klystrons are highly reliable, robust, and low cost sources of high power microwave radiation. They are traditionally used for narrowband or limited bandwidth (<1%) applications. However, various broadbanding techniques have enabled klystrons to achieve band-widths as high as 6 to 10%. In this paper we present analysis of output circuits of such broadband devices. The klystron output circuits must have the instantaneous bandwidth that matches the entire band of interest. Two types of commonly used broadband output circuits are analyzed:. (i) filter-loaded circuits, (ii) extended interaction circuits. It is very efficient to design and analyze such EM structures using equivalent electric circuit approach. The equivalent circuits are validated using extensive 3D simulations and the parameters are fine-tuned to get an accurate match between the two. It is important to get good matching for both the real and imaginary parts of the interaction impedance as a function of frequency. The equivalent circuit models are used to design 2 and 3 GHz klystron output circuits to achieve 7% bandwidth for the filter-loaded and 10% bandwidth for the extended interaction designs. The designs are validated using HFSS simulations and cold testing. The impedance response of the output circuit can be tailored to match the inverse profile of the bunching sections, so that the overall klystron displays good flat-band performance. The equivalent circuit approach can be further extended to the slow-wave structures such as coupled-cavity TWTs.