Theoretical research on TEM mode feedback for compact design of an X-band triaxial klystron amplifier

Abstract

A relativistic triaxial klystron amplifier (TKA) is considered to be one of the most promising high power microwave (HPM) sources for coherent power combination in high frequency bands. In general investigations, the TEM mode leakage and asymmetric modes' competition are considered to play unfavorable roles in the normal operation of TKA. Therefore, mode reflectors with high reflection efficiency in both the TEM mode and the asymmetric modes are employed and expected to significantly suppress the transmissions of the microwaves between the cavities. So the cavities can be separated from each other, while the long drift length is necessary to achieve the deep modulation. Therefore, the size of TKA is noticeably larger compared to other HPM oscillators, which requires large magnetic coil and huge energy consumption. In our present research, it is found that the TEM mode leakage can be utilized to form positive feedback between the cavities, which is significant for electron beam modulation and furthermore to make the devices more compact. In this paper, the mechanism of TEM mode feedback between the cavities is discussed and the conditions of positive and negative feedback are achieved. Besides, an especially designed reflector with high reflection coefficients in asymmetric modes and low reflection coefficients in the TEM mode is designed to utilize the TEM mode and suppress the asymmetric modes. Based on the above research, a TKA adopting positive TEM mode feedback, whose total length is decreased by 23.6% compared to the previous design, is presented. A high power microwave with a power of 2.25 GW and a frequency of 8.4 GHz is generated with a 690 kV, 9.3 kA electron beam excitation and a 40 kW radio-frequency signal injection. Meanwhile, there is insignificant self-excitation of the parasitic mode in the proposed structure in three-dimensional particle-in-cell simulation, and the relative phase shifts between the injected signals and the output microwaves are kept locked after the amplifier becomes saturated.