为了有效提高器件输出微波相位的稳定性，对S波段强流相对论速调管放大器输出微波相位特性开展了理论研究和优化设计。理论分析结果表明，束流发射和传输均匀性、腔内杂频和强流脉冲电压波动是影响器件输出微波相位稳定性的重要原因，通过优化二极管结构，增加吸波材料以及优化腔体结构和参数等手段可以有效地减小上述因素对输出微波相位稳定性的影响，提高器件工作的稳定性。对优化后的器件开展了实验研究，得到单台单次输出功率GW量级基础上，100 ns内相位标准差约10°，单台重频5 Hz工作输出微波相位标准差约20°，两台同时工作输出微波相位差抖动约20°的结果，满足了实际应用的需求。

Phase characteristic is one of the key parameters which limit the promotional applications of intense relativistic klystron amplifier. In order to reduce the phase jitter and improve the stability of device, an S-band intense relativistic klystron amplifier (RKA) with three cavities and annular beams is studied theoretically. The theoretical results indicate that fluctuation of intense pulse, self-oscillation, non-uniform of beam emission and transmission are the main reasons for phase fluctuation. Then some optimization works are done as follows: (1) The structure of diode are optimized to improve the uniformity of electric beams; (2) Microwave absorbing materials are added in the drift to suppress the self-oscillation; (3) On the condition of output power greater than GW level, the parameters of cavities are optimized to reduce the phase jitter caused by fluctuation of pulse voltage. At last, an experiment research is carried out on the LTD accelerator to confirm the effectiveness of optimization. The experiment results show: on the condition of output power greater than GW level, the standard deviation of output microwave phase is about 10° within 100 ns in single shot and about 20°in 5 Hz repeat. The phase difference jetter between two RKAs is about 20°.