As I read the grid voltage of an ion thruster is of the order of 1-4kV and to achieve that voltage they use electronic voltage boosters. Knowing that the thrust is proportional to the grid voltage(for accelerating the particles), why don't they just eject some electrons from the grid to increase the electrical potential? Ejecting 1 Coulomb of charge could obtain voltages of the order of 1e9 V
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7$\begingroup$ Do you realise just how much charge a coulomb is? It's a rather large amount $\endgroup$– ikraseSep 17, 2020 at 19:12
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$\begingroup$ @ikrase - well, a Coulomb in a second is an Ampere. Large, but not super large (well, at a MegaVolt getting an Amp means a MegaJoule of energy). A Coulomb over a year is 30 nanoAmps... $\endgroup$– Jon CusterSep 17, 2020 at 19:49
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3$\begingroup$ Yes, but that's in neutral wires. It's a whole other thing to have a whole coulomb of net charge. $\endgroup$– ikraseSep 17, 2020 at 19:54
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3 Answers
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Ion engines balance two different kinds of efficiency:
- $I_{sp}$ which is basically "reaction mass efficiency". For that you want the highest possible exhaust velocity, which will be helped by a higher voltage.
- Energy efficiency for which you want the exhaust velocity to be in the same range as the $\Delta v$ required for your mission. With a given power supply, reducing energy efficiency translates to reduced thrust.
If you make the voltage very high (and overcome some significant engineering problems in doing so) you get incredible $I_{sp}$, so you won't run out of reaction mass, but very low thrust for a given power consumtption.
If you make the voltage too low, you get great thrust, but run out of reaction mass.
An actual system is a tradeoff.
answered Sep 17, 2020 at 16:13
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$\begingroup$ why high voltage is a engineering problems? arcs occur in a vacuum? $\endgroup$ Sep 17, 2020 at 16:52
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8$\begingroup$ Yeah, to some degree. And an ion thruster... The propellant makes it not a vacuum any more. $\endgroup$– ikraseSep 17, 2020 at 19:13
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$\begingroup$ I plan to eventually answer For a DAWN-like solar-electric deep space mission, would using propellant particles much heavier than xenon atoms be beneficial? myself, but I'm really hoping someone beats me to it :-) ditto Why was circa 600 volts used for DAWN's ion propulsion? $\endgroup$– uhohSep 18, 2020 at 2:40
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$\begingroup$ @RicardoCasimiro high voltage engineering is the problem to avoid current flowing were it is not wanted, through isolators, on surfaces, through the air. Isolators may be destroyed by an inhomogenous electrostatic field. But how to get a homogenous field? $\endgroup$– UweSep 18, 2020 at 12:45
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1$\begingroup$ @RicardoCasimiro It's not a vacuum - you're pumping reaction mass (a gas) through the thruster, so the local pressure will be significant. You're forming a plasma, after all, and plasmas are conductive, so they have to be formed and shaped carefully to prevent the plume from collapsing into an arc, yes. The higher the voltage, the further the plume can 'reach' for a point to discharge to. $\endgroup$– J...Sep 18, 2020 at 16:02
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Knowing that the thrust is proportional to the grid voltage
Almost correct! All else equal, the thrust will be proportional to the ion velocity, which will vary as the square root of their kinetic energy, which is proportional to the acceleration voltage.
why don't they just eject some electrons from the grid to increase the electrical potential?
Well to accelerate the ions from the plasma volume and get them leave under high speed, the grid should be negative relative to the plasma in order to attract the ions and get them up to speed as they leave. I think in this case you want more electrons, not fewer.
Luckily you have plenty, because the process of producing ions removes electrons from atoms. The job of the power supply is then to put them into the grid where they now attract the same ions they were separated from a moment ago.
The hard part of the power supply's job is to push them there. After you put some electrons there and the grid is somewhat negative, it will take work to push more "up the potential hill" to join them because they repel each other.
Either way, it's easier to move electrons around in wires using electronics than it is to cause them to be ejected into, or accepted from space. "Electronic voltage boosters" in the kilovolt range are easy to make with modern high-voltage solid state electronics and an inductor. A string or group of LEDs for household or street lighting is wired in series, and a high voltage powers them all that way (so that the current in each one is equal). Random example: https://www.mouser.com/Search/Refine?Ntk=P_MarCom&Ntt=170342553
I see up to 400 Volts DC output voltage on that link, than there may be higher. And these may be overkill in terms of current or power delivered to the high voltage, Dawn only used 600 V and I'm still looking for the current but I think it's less than 1 amp.
See also how a Prius motor is supplied with 500 Volts from a battery of only 200 Volts.
update: I found Why was circa 600 volts used for DAWN's ion propulsion? and it estimates 2.2. Amps based on physics, but it's not sourced; its simply part of the (currently unanswered) question.
So I don't think there is any problem here that needs fixing, and if there was, I think what you've proposed would be harder. If you accept or reject electrons, they'll leak back and so you'll have to do it continuously. And to do that you'll still need the same power supply you're trying to replace.
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2$\begingroup$ High voltage DC power supplies as high as 50 kV are readily commercially available. $\endgroup$– ikraseSep 17, 2020 at 19:13
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1$\begingroup$ @ikrase Nice! For this I wanted to choose an example of something that's sort-of everyday COTS, so I thought that a string of LEDs fit the bill. $\endgroup$– uhohSep 17, 2020 at 19:18
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1$\begingroup$ @ikrase "High voltage DC power supplies as high as 50 kV" are available for use on Earth, but not for use in space and vacuum. $\endgroup$– UweSep 17, 2020 at 20:42
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1$\begingroup$ @Uwe let's see what is available: Highest DC voltage ever intentionally produced in space? $\endgroup$– uhohSep 17, 2020 at 21:21
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2$\begingroup$ @ikrase they're also large and heavy, and extra weight is the last thing you want on an ion thruster spacecraft $\endgroup$– llamaSep 17, 2020 at 23:02
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If you increase the grid voltage of an ion thruster, you need more, bigger and heavier isolation to avoid arcs and current flowing thru the isolation. The thruster gets much bigger and heavier, the reliability is lower. When thrust to weight ratio gets worse there is no benefit of higher voltage. To generate large DC voltages for the thruster you have to connect several recitifiers in series but this is difficult, tricky and less reliable.
Have a look at high voltage equipment on Earth, it is large and heavy. To avoid current flowing into the air you need large polished spheres without sharp edges.
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$\begingroup$ arcs may occur on the surface of an insulator too $\endgroup$– UweSep 17, 2020 at 17:28
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1$\begingroup$ @RicardoCasimiro I don't know the difference between arcs and sparks, but while there is no breakdown of vacuum like in air or other dielectrics, you can have discharge from pointy things (even roughness) and that can cause evaporation, sputtering or other processes that put matter into the vacuum so that locally it's no longer a vacuum, and that can sustain or enhance a discharge. Vacuum high voltage systems always arc or spark when you push them too hard, and by conditioning (sparking a lot, in a controlled way) you can slowly remove those pointy things and reduce further arcs/sparks. $\endgroup$– uhohSep 17, 2020 at 17:32
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$\begingroup$ @RicardoCasimiro the second tethered satellite mission flown from the shuttle failed because of an electrical discharge that melted the tether. ntrs.nasa.gov/api/citations/20160007056/downloads/… $\endgroup$ Sep 17, 2020 at 21:57
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3$\begingroup$ @RicardoCasimiro eventually with a high enough electric field between two metals, you can pull electrons out of the metal and into the vacuum to start an arc. See en.wikipedia.org/wiki/Vacuum_arc and en.wikipedia.org/wiki/Field_electron_emission $\endgroup$– llamaSep 17, 2020 at 22:56