Abstract: In the wide application of electric thrusters, most of them work on the principle of accelerating positive ions only to provide moderate thrust and high specific impulse. The ion beam current and space charge need to be compensated by the additional neutralization systems in the downstream space to avoid selfcharging of the satellites, damage of the communication equipment and electronic devices as well as cancellation of the acceleration field. In order to simplify the system structure and prolong the life of thrusters, a space electric propulsion system where both positive and negative ions can be accelerated alternately to obtain the thrust was proposed. The background and present situation of the development of plasma propulsion with electronegative gases (PEGASES) were introduced. The way to generate and accelerate positive and negative ions simultaneously was also discussed, including how to choose electronegative gases, magnetic field filter design requirements, the applied bias voltage way for grids in which the positive and negative ions could be accelerated alternately and periodically, as well as the density diagnostics of the plume.

Key words: electric propulsion, acceleration of positive and negative ions, electronegative gas, electronic filter, period acceleration