Equipment affiliated to Leo spacecraft can be used to accelerate the plasma naturally existed in space and then acquire thrust. A basic scheme about this idea is to put two grids supplied with different electric potentials to create an acceleration region for plasma on the orbit. We simulate the motion of ambient rare plasmas in accelerating grids by two di-mensional PIC method. The simulation results show that the ions first have been accelerated by the electron field between the grids and then would be decelerated by the electron field between the back grid and the plasma sheath,which means the ions would not be accelerated in the whole process. The problem would be solved if an ion potential raising region was added in front of the grids.%对于低轨（160~500 km）航天器，如果加上适当装置使其可以直接加速轨道环境中自然存在的等离子体，则航天器将受到一个反向推力。有关这个设想的基本方案是在航天器上添加一个加有电压的平行双栅结构，利用双栅之间形成的电场来加速轨道环境中的等离子体。采用二维PIC方法对轨道稀薄等离子体在加速双栅中的运动进行数值模拟，结果表明：离子在栅极间被电场加速后，到达下游，由于后栅和鞘层之间的电场作用，将被减速，离子在整个过程中并没有被加速。利用在平行栅前增加一个离子势能提升区的办法可以解决这个问题。
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