Suppression of Mode Competition in a Triaxial Klystron Amplifier With an Improved Three-Gap Bunching Cavity

An improved three-gap bunching cavity is proposed for the suppression of mode competition in a triaxial klystron amplifier (TKA). With the theoretical analysis and simulation verification, we find that the asymmetric TM612, TM712, and TM812 modes of the initial bunching cavity are easily excited by the electron beam due to their negative beam-loading conductance ratios and small frequency interval between TM $_{\textit {n{12}}}$ and TM $_{n{13}}$ modes. To solve this issue, an improved three-gap bunching cavity is designed on the one hand to change the beam-loading conductance ratios of the three modes into nearly positive and, on the other hand, to increase the frequency interval between TM $_{n{12}}$ and TM $_{n{13}}$ modes to over 1 GHz. In addition, a reflector loaded before the bunching cavity is removed to eliminate the self-oscillation of two reflectors by increasing the external quality factor of the bunching cavity and choosing a specific drift tube length between the input and bunching cavity. The improved bunching cavity is further examined in an ${X}$ -band TKA by 3-D particle-in-cell simulation, which shows that high power microwave (HPM) with a peak power of 1.2 GW is generated corresponding to an efficiency of about 33%, and there is no mode competition during 300-ns simulation time.