为了研制高功率长脉冲速调管放大器，对其输入腔进行了3维粒子模拟研究。建立了S波段速调管放大器输入腔开放腔的3维模型，计算了开放腔的谐振频率和有载Q值。将束压束流从原来的600 kV，5.4 kA提高到837 kV，12.48 kA，实现了长脉冲100 ns的稳定运行，调制束流与实验结果一致。在结构不变的情况下，将束压束流提高到900 kV，16 kA时出现了模式竞争，只能稳定运行到50 ns。为了克服模式竞争，将耦合孔改为对称的双耦合孔后，输入腔系统可以稳定运行到100 ns。

Three dimensional particle simulation of input cavity for S-band long-pulse high-current RKA is presented. First three-dimensional model of open cavity for input cavity is established and resonant frequency and loaded-Q of open cavity is calculated. Raising the beam voltage and beam current from 600 kV, 5.4 kA to 837 kV, 12.48 kA, three-dimensional Particle-in-cell(PIC) code can run stably for 100 ns.The modulated current of simulation is consistent with experiment result. With the structure unchanged, raising the beam voltage and beam current from 837 kV, 12.48 kA to 900 kV, 16 kA, the code can only run stably for 50 ns because of the mode competition. In order to conquer the mode competition, changing the coupling-hole to symmetric double coupling-hole, the code can run stably for 100ns.