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The positively charged ions migrate toward grids that contain thousands of very precisely aligned holes (apertures) at the aft end of the ion thruster. The first grid is the positively charged electrode (screen grid). A very high positive voltage is applied to the screen grid, but it is configured to force the discharge plasma to reside at a high voltage. As ions pass between the grids, they are accelerated toward a negatively charged electrode (the accelerator grid) to very high speeds (up to 90,000 mph).

Taken from NASA website about ion thruster.

How could positively charged ion pass through negatively charged electrode (the accelerator grid) without sticking to the electrode.

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  • $\begingroup$ Can you also add the link for the source? $\endgroup$
    – Newbie
    Commented Jan 22, 2022 at 3:28
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    $\begingroup$ @Newbie suspect it is: NASA - Ion Propulsion $\endgroup$
    – GrapefruitIsAwesome
    Commented Jan 22, 2022 at 3:35
  • $\begingroup$ @GrapefruitIsAwesome Thanks, it is indeed from the link you shared. $\endgroup$
    – Newbie
    Commented Jan 22, 2022 at 3:36

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I'm posting my understanding of the article as it pertains to the question here. It seems that the screen grid which comes first in the path of the positive ions has higher potential compared to the accelerator grid which comes after the screen grid. This generates an electric field pointing towards the end of the thruster and the positive ions will be accelerated out of the exit. If I understand correctly, you're asking how come the positive ions passing the accelerator grid don't slow down and come back towards the accelerator grid since it has a negative potential. If that is your question, the following excerpt from the same source gives the answer:

The positively charged ions are accelerated out of the thruster as an ion beam, which produces thrust. The neutralizer, another hollow cathode, expels an equal amount of electrons to make the total charge of the exhaust beam neutral. Without a neutralizer, the spacecraft would build up a negative charge and eventually ions would be drawn back to the spacecraft, reducing thrust and causing spacecraft erosion.

As mentioned in the article, the electron beam and positive ion beam neutralize each other after the accelerator grid such that the created electric field won't affect neutral particles. Please let me know if this answers your question or if you have any other questions.

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