###### Abstract  
Ka-band frequencies are becoming increasingly popular due to their application potential in high-data-rate wireless communications relevant to 5G applications, satellite link establishment, etc. High-power amplifiers in this frequency band, offering several tens of watts of output RF power, are one of the main enabling components of these communication systems. This article reports the design studies and analysis of the radio frequency (RF) section of a multiple beam-extended interaction klystron (MB-EIK). The proposed multiple beam RF section with a ladder-type EIK structure offers several crucial features, such as a low-voltage operation, moderate operational bandwidth, and high output power. Starting from the design of the intermediate cavities, the input and output sections and the overall RF section are presented. The proposed RF section supports the operation at 28.5 GHz center frequency with about a 500 MHz 3 dB bandwidth employing four electron beams with a 4 kV DC accelerating field. extended interaction klystron (EIK); Ka-band; multibeam klystron (MBK); multi-gap; quality factor; RF cavity resonator; shunt impedance +
Footnote †: journal: Accepted: 11 November 2022
Klystrons with extended interaction RF cavities (EIKs) are designed with multiple gaps to achieve larger shunt impedances for an efficient beam-wave energy transfer and the proper synchronization of the electron beam with the RF field [10; 11; 12; 13; 14]. The MBKs with microfabrication-compatible extended interaction cavities, a lower operating voltage, and a (few hundred mA) beam current can provide a moderate operating bandwidth and RF output power in Ka-band frequencies. These characteristics offer cherished alternatives to solid-state-based RF amplifiers in this frequency range, which may be applied for long-distance high-data-rate wireless communications, such as 5G wireless back-haul/front-haul, wireless middle-mile connectivity reconfigurable 5G deployments, etc. Because the usage of these amplifiers enables achieving a high data rate without the deployment of fibers, it is suitable for difficult terrains and/or sparsely located rural populations where fiber laying is commercially not viable [15; 16]. In this work, we report a compact, low-voltage-operated (-4 kV) MBK employing a microfabrication-compatible novel ladder-type interaction structure operating at a 28.5 GHz frequency. An axial magnetic field of 0.35T is employed for focusing the electron beams, and a 500 MHz (-3 dB) bandwidth has been obtained, with a peak power level of 375 W. Some other klystrons (single beam or multiple beams) reported in this frequency band offer higher output powers--but the operating voltages are higher (9.5 kV to 500 kV), form factors are larger, and offered bandwidths are less [17; 18; 19]. Although the estimated bandwidth of the device is less than that of the traveling wave tubes operating in these frequency bands, the lower operating voltage and relatively simple (than the TWTs) fabrication possibilities make these devices attractive for high-data-rate communication applications [20; 21]. In comparison with the state-of-the-art semiconductor power amplifiers, the output power and efficiency of the proposed device are far superior, but the available bandwidth is less. Although there have been remarkable advancements in the solid-state technology in recent times (especially the GaN technology), the output power level provided by a single microwave monolithic integrated circuit (MMIC) around 28 GHz is still limited to around 16 watts (Max). The output power level of about 100 W with an efficiency of 19% can be achieved using power-combining techniques [22; 23]. Besides the communication applications, a similar concept may be adopted to design the devices operating in other frequency bands and/or power levels. The applications of such devices include particle accelerators, industrial and plasma heating, RADARs and active denial technologies, deep space communication, etc. [24; 25; 26; 27; 28].