针对气体碰撞电离过程，介绍了蒙特卡罗碰撞（MMC）的处理方法，利用MMC方法编写了气体碰撞电离模块，将其移植到3维全电磁粒子模拟程序NEPTUNE之中，模拟了充有He气的磁绝缘线振荡器（MILO）。模拟结果表明：当He气密度较低时，电离的正离子由于较重无法自由移动，形成了正离子通道，可以有效中和电子束空间电荷场，有利于电子束传输和群聚，提高了束波互作用效率，微波输出功率得到了明显提高，起振时间也有所缩短；当进一步增加He气密度时，电离碰撞增强，电子和离子数目会雪崩式增长，电子束由于碰撞增强而导致能散度增大，其负效应已经远大于中和空间电荷场的正效应，反而不利于电子束的群聚和共振，从而导致输出微波功率降低乃至截断，起振时间缩短是由于其在非雪崩阶段的正效应积累所致，但是随着负效应的增强起振功率不能得以维持，二极管最终将闭合。另外，还模拟了MILO填充空气、水蒸气及二氧化碳等多原子、多组分气体的碰撞电离物理过程。模拟结果显示，同压强情况下，填充空气、水蒸气及二氧化碳的脉冲缩短现象要比填充He气等较低原子序数气体的情况严重得多。

The Monte-Carlo collision (MMC) method was introduced for description of the gas collision ionization course. The MCC gas ionization module based on the method was developed for 3D fully electromagnetic and PIC code NEPTUNE. The magnetically insulated transmission line oscillator(MILO) model with He gas was simulated by the developed NEPTUNE code. The simulated results indicate that, in the condition of lower He gas density, He+ ions produced by ionization collision can not move freely because of their weight, and form channels which can neutralize space-charge field perfectly and then improve the bunching and beam-wave interaction efficiency obviously, so the output power of microwave increases, and the oscillation forming time shortens markedly. When the density of He gas increases to a higher level, the numbers of electrons and He+ ions increase quickly because of stronger ionization collision, the energy of electrons is separated due to collision courses, which goes against the bunching and beam-wave interaction improvement, and then the output power of microwave is decreased or cut off. The shortening of oscillation forming time is induced by the initial ionization stage with lower electron and ion densities. With the increasing gas density, the output power pulse will be shortened, and the diode of MILO will be shut down. In addition, the MILO models with air, vapor and carbon dioxide gas were simulated, respectively. The simulated results indicate that, in these conditions, the shortening of output microwave pulse is more severe compared with that for the MILO filled with He gas at the same pressure.