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Basically, if the ions are accelerated due to their attraction to a negatively charged gird at the back end of the thruster, why don't the ions just stay by the grid? How are they ejected out of the back at high speeds? Do the ions simply have a high oscillation amplitude, and are neutralized before they would 'want' to return to the negatively charged grid?

I think I may be misunderstanding a more fundamental piece of the physics? Thanks for any help.

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  • $\begingroup$ @Woody In a CRT, the fluorescent screen (anode) is at a high positive potential. $\endgroup$
    – John Doty
    Feb 19 at 1:54
  • $\begingroup$ Good question. I have the same problem understanding cathode ray tubes. I get it that the anode accelerates electrons towards it, but why doesn't it decelerate the same electrons once they have passed through the anode grid? I look forward to answers. $\endgroup$
    – Woody
    Feb 20 at 5:02

4 Answers 4

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There are multiple electrodes and grids. The propellant is first ionized between a positive and negative grid. Just on the other side of the positively charged grid is a high voltage negatively charged grid, as soon as they are through that they are rapidly accelerated away. an electron gun fires electrons into the rapidly accelerating positive ion beam to ensure the ion engine does not become highly charged with respect to the beam. https://i.ytimg.com/vi/grU8g9jnS4w/maxresdefault.jpg

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why don't the ions just stay by the grid?

https://www.nasa.gov/wp-content/uploads/2015/08/ionpropfact_sheet_ps-01628.pdf 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

How are they ejected out of the back at high speeds?

https://www.nasa.gov/wp-content/uploads/2015/08/ionpropfact_sheet_ps-01628.pdf 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... 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).

enter image description here https://en.wikipedia.org/wiki/Gridded_ion_thruster

The electric field lines adjacent to a flat plate are perpendicular to that plate. Positive ions between the two grids are accelerated axially. Once they exit from the negative grid, they are neutralized by the electron gun. If they remained positive, they would return to the negative grid and cancel out the thrust.

You may wonder why positive ions would approach the positive grid and pop through the grid holes ??? Because the chamber is full of hot, high pressure plasma. Plasma pressure is produced by heat and magnetic confinement. Also, the chamber walls have a high positive charge which attracts electrons and repels positive ions.

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To accelerate positive ions, the grid must be negatively charged relative to the ion emitter. However, it need not be negatively charged relative to the spacecraft: instead, the ion emitter may be positively charged with respect to the spacecraft.

Still, if you just accelerate positive ions, the spacecraft will accumulate negative charge and attract them back. So, you must balance the positive ion current by with an equal current of negative ions and/or electrons. You need to keep the spacecraft potential close to the potential of the surrounding plasma.

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  • $\begingroup$ Hmmm I'm specifically thinking about how the positive ions accelerate. I understand this is due to their attraction to the negatively charged grid at the end of the thruster. However, if the positive ions are attracted to the thruster, how are they emitted? Wouldn't they just stay at the negatively charged grid, which they are attracted to? $\endgroup$
    – lewellyn
    Feb 19 at 2:22
  • $\begingroup$ @lewellyn The ions accelerate toward the grid, but the electric field doesn't point directly at the grid mesh until they are very close. So, most of them shoot through the holes in the grid on momentum: they don't follow the tightly curving field lines . Then, if there's no field on the far side, they keep going. $\endgroup$
    – John Doty
    Feb 19 at 13:03
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if the ions are accelerated due to their attraction to a negatively charged gird

That they are attracted to the grid is a simplification. The potential on the grids set up an electric field that is very strong between the grids and very weak outside.

During the passage between the grids, the charges are accelerated. Once it passes through the grid, the field is very weak and there is only a small deceleration.

If the negative grid were in isolation, you would get the oscillating effect you presume. There would be a reasonable attraction on both sides of the grid. But the presence of the positive grid prevents that.

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  • $\begingroup$ Thank you, this was the source of my confusion $\endgroup$
    – lewellyn
    Feb 21 at 2:59

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