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Recently,I've posted a question on ion thrusters.

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I thought that it would be enough for the grids to be charged to accelerate ions. If I have a positive plate and a negative plate, separated by a distance 'd', a positively charged particle placed in the space would migrate towards the negatively charged plate.

-But a few of you guys pointed out that just 'Charging the plates' wouldn't be enough to accelerate the ion beam. There needs to be a current passing through them. -I don't understand why there has to be a current passing through them and how is the current supposed to pass?? (Please draw the circuit diagram showing the direction of the current if possible)

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  • $\begingroup$ You should believe in energy conservation, it is valid for electrical equipment too and also for ion thrusters. The ions have mass, the acceleration of ions needs energy. Acceleration of a mass is impossible without energy. There is a current flowing between the positive and negative grid. The current is necessary to hold the voltages of the grids while ions are flowing through those grids. Without current, the grids will have the same voltage very soon. $\endgroup$ – Uwe Feb 3 '17 at 17:51
  • $\begingroup$ But if the ion optics and the grids are designed properly, ions wouldn't hit the grids. Hence, the charge on the grids doesn't change. Doesn't this mean that the potentials do not change too? $\endgroup$ – Chandrahas Feb 4 '17 at 1:57
  • $\begingroup$ The plasma consists of positive ions and negative electrons, But the ion beam leaving the negative grid contains only positive ions. The electrons did hit the positive grid and flow as a current to the source of the positive grid voltage. The ion beam is neutralized by the injection of electrons from the neutralizer. The electrons hit the grid and thus change the charge of the grid. A current must flow to maintain the grid voltage. $\endgroup$ – Uwe Feb 4 '17 at 9:13
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    $\begingroup$ This is a good question, and deserves a good answer. There is a lot of hocus-pocus baloney here, but a good answer should be more than a few catchy sentences that more than just sound not necessarily incorrect. As long as there are not electrons or ions striking or being emitted by the grids, neither will source;sink current, and to prevent degradation that might be a good thing. @Uwe you should believe in conservation of charge as well. This is not an easy question, and it should not be dismissed so lightly. $\endgroup$ – uhoh Mar 17 '17 at 9:39
  • $\begingroup$ I see a current of + charges from the plasma (a conductor) out the back, and a current of - charges out the gun. I'd add a bonus but I've got my maximum of three running already. Anybody else feeling charitable, or willing to spot me a +100? $\endgroup$ – uhoh Mar 17 '17 at 9:40
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There are many things going on, some of which depend on the exact variety of gridded ion thruster. I'm referencing chapter 6 of Spacecraft Systems Engineering 4th Edition by Fortescue, Swinerd, and Stark. It's a very affordable textbook which gives a good overview of just about any common system found on unmanned spacecraft. It is not a textbook on electric propulsion.

The first thing to realize is that the ionized propellant is electrically conductive. This is true for the same reason that ion-bearing liquids are electrically conductive. This details of this conductivity are not important to my answer. http://onlinelibrary.wiley.com/doi/10.1002/ctpp.19840240304/abstract

The conductivity of the ionized propellant allows current to flow from the negative grid to the positive grid. This is not the whole story though. The propellant becomes ionized within the chamber, and the resulting free electrons are attracted to the positive grid, which creates a source of inflowing current. These electrons need to be continually removed to keep the grid positively charged. There are also electrons which are carried from the negative grid into space by the propellant.

The complete story of how the ions move around is more complicated, but I think this answers your question.

In summary, the current flow is not a requirement, it's a consequence.

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  • $\begingroup$ To double check - if acceleration grids are initially charged to produce a high acceleration potential, and then disconnected, they would slowly discharge due to unwanted collisions by the small fraction of the charged particles that hit the grid instead of passing through its openings? And so this small current, and the power needed to supply it, does not actually contribute significantly to the job of accelerating the spacecraft? $\endgroup$ – uhoh Mar 20 '17 at 4:30
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    $\begingroup$ That is correct, though there is ongoing current flow used in a separate process to ionize the propellant which is completely necessary for the propulsion of the spacecraft. $\endgroup$ – Schlusstein Mar 20 '17 at 4:47
  • $\begingroup$ So.. there will be no current passing through the grids if the ion optics are perfect? $\endgroup$ – Chandrahas Mar 21 '17 at 2:28
  • $\begingroup$ Well, even capacitors discharge slowly over time. The ion stuff is a much bigger contributor in operation though. Like I said, there's a lot going on. $\endgroup$ – Schlusstein Mar 21 '17 at 2:48
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The current isn't flowing through each grid, but is flowing from one grid to the other. A current is a flow of charge. The propellant ions are charged particles. They are moving, therefore there is a flow of charge, therefore there is a current.

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  • $\begingroup$ How would that affect the system? $\endgroup$ – Chandrahas Feb 4 '17 at 2:23
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Like was said before, the electricity is running through the grids to each other, one being positive one being negative. The negative electrons are attracted to the positive grid, absorbed, and the ions are attracted to the negative grid and accelerated, creating the thrust that the engine produces.

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