I recently saw an old thread, How do reaction engines create a force against the rocket?, get bumped up, and it asks a good question: in a chemical rocket, the fact that the rocket exhaust gets propelled away means that Newton's Third Law requires that there be some force acting on the rocket in the other direction, but the Third Law itself does not actually specify what that force is, with the answer being that it's the pressure of the gas in the combustion chamber and on the engine bell that produces an unbalanced force on the engine.
I'd like to ask exactly the same question, but for an ion thruster instead. As in the chemical rocket, the fact that there's an ion stream going away at high velocity implies that there needs to be a point at which the outgoing ions exert some form of electric force on the thruster. So: what is the nature of this force, and how does it work?