An improved X-band relativistic triaxial klystron amplifier with active suppression of asymmetric TM mode self-excitation

The self-excitation of asymmetric modes is one of the main factors limiting the long-pulse operation of the relativistic triaxial klystron amplifier (TKA). Research studies have revealed that the asymmetric TM modes with negative beam loading conductance (Ge/G0) in the bunching cavities are the main culprits of self-excitation. The existing studies focus on passively destroying the growth of asymmetric TM modes, but we attempt to actively suppress their self-excitation in this paper. Investigation results demonstrate that the employment of single-gap bunching cavities can significantly reduce the risk of self-excitation of asymmetric TM modes, as the Ge/G0 of asymmetric TMn11 modes in single period structures are positive. The current modulation capacity of a single-gap bunching cavity, however, is not sufficient, so a cascade structure consisting of two groups of bunching cavities is employed to compensate for this deficiency. Based on the above conclusions, an X-band TKA device with four single-gap beam–wave interaction cavities is proposed and investigated in this paper. The three-dimensional particle-in-cell simulation results demonstrate that there is no self-excitation of asymmetric modes in the proposed X-band TKA device.