# Is there any possible reason to direct the electron gun specifically towards the ion trail behind an ion thruster?

This answer to Neutralization of ions in the ion thruster quotes the introduction to Wikipedia's Ion thruster:

Temporarily stored electrons are finally reinjected by a neutralizer in the cloud of ions after it has passed through the electrostatic grid, so the gas becomes neutral again and can freely disperse in space without any further electrical interaction with the thruster.

Question: Is neutralization of the ion trail by injecting an equal and opposite amount of charge from the electron gun really important? Does it need to be pointed specifically towards the ion trail behind an ion thruster, or would it be fine pointing in any direction? The light electrons will have such a higher speed than the spacecraft that it's not likely to re-encounter them again no matter where they're pointed

Unrelated gratuitous cropped screen capture from the YouTube video Star Trek - The Galileo Seven - visual effects comparison. Yes I know it's not ion propulsion, but I like to imagine that this is what the light from recombination in an ion plasma would look like.

• As the Author of the initial Answer, i like this question. I think, some parts are obvious: 1.) shooting the electrons in the wrong direction would produce thrust in the wrong direction. 2.) you need to put the electrons somewhere because otherwise your S/C will be charged .... but I think the most important (for me) part is: even if the ions would not be neutralized, would there be a problem with "returning" ions. – CallMeTom Jul 23 '20 at 7:01
• @CallMeTom yep, since momentum per particle is $p=\sqrt{2 m E}$ and the electrons are 2000 times less mass and probably emitted at one tenth of the acceleration voltage, it's like a 1% effect, but that's certainly still very important and worth using correctly. Please feel free to post an answer! – uhoh Jul 23 '20 at 7:07
• I think one significant factor may be that the positive ions pull the electrons, significantly helping the electron gun getting rid of them. – SF. Jul 23 '20 at 9:22
• actually I did that for protons, for ions the mass ratio is another 100 times larger so the electron thrust is less than 0.1% of the ion thrust. – uhoh Jul 23 '20 at 11:50