为了进一步提高S波段高功率强流长脉冲相对论速调管放大器(RKA)的重复频率稳频稳相运行的性能，采用实验结合理论和模拟的方法，分析了其主要制约因素，特别分析了造成器件脉冲缩短和重复频率运行不稳定等问题的根源。研究结果表明，RKA中的中间腔和输出腔的电子反射、电子散焦轰击腔体鼻锥是造成脉冲缩短、重复频率运行不稳定的主要根源。通过采用大漂移管半径的器件结构、在漂移管中加载吸波材料以及引导磁场位形、采用电子发射较均匀的碳/碳复合阴极材料等措施，使杂频振荡、脉冲缩短和重复频率工作不稳定性等问题得到了明显减轻，输出微波相位稳定性得到显著提高。采用电压830 kV、束流7.7 kA、脉宽190 ns的环行电子束驱动S波段3腔RKA，重复频率25 Hz运行得到了峰值功率1.55 GW、脉宽163 ns、相位抖动18°（rms）的输出微波。

To improve the stability of frequency and phase of the output microwave on condition of repetitive operation, an S-band, long-pulse relativistic klystron amplifier (RKA) is investigated by theoretical modeling, numerical simulation and experimental measurement. Description and analysis are made on the physical origins of self-oscillation, output microwave pulse shortening, the instability in the repetitive operation and the phase fluctuation of output microwave in the RKA. Measures are taken to solve or alleviate these problems, which are demonstrated by the simulation and experiment. It is confirmed that the electrons reflected from the gaps of idler cavity and output cavity of RKA result in the self-oscillation and the output microwave pulse shortening, and the electron dispersion and bombardment on the noses of the output cavity and idler cavity affect the stability of repetitive operation. It is validated that enlarging the drifting tube radius, adding some microwave absorber in the drifting tube between those cavities and optimizing the structure of the input cavity, output cavity and collector of the electron beams can alleviate the problem of self-oscillation and pulse shortening obviously and improve the stability of repetitive operation and phase fluctuation of output microwave. Using an 830 kV, 7.7 kA, 190 ns electron beam and an input microwave of 80 kW, an output power of 1.55 GW, pulse width of 163 ns, and phase fluctuation of 18° are generated by the S-band three-cavity RKA operating at 25 Hz.