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.