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This answer to What is the maximum speed an ion engine can propel a spacecraft at? mentions the following:

Lithium fed GIT (gridded ion thrusters) have demonstrated 50,000 - 80,000 seconds of ISP (490 km/s to 780 km/sec)

The second set of numbers is the exhaust velocity, and so it looks like those are for an acceleration voltage of about 20 kV.

Lithium has an advantage of being a light atom; for a given acceleration voltage and singly charged state, the specific impulse varies as $1/\sqrt{m}$. While iodine can be sublimated at a low temperature, I suppose the lithium would have to be heated significantly. Either way, solid sources don't require heavy pressure vessels like gas sources do (e.g. Kr, Xe).

But how does a lithium fed GIT (gridded ion thrusters) actually work? Is the source pure lithium metal or some compound? How is it fed into a plasma, or is it ionized in a different way? (e.g. field emission or contact with a different material that wants electrons more?)

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This seems like a relevant paper from 2017: Development of a 50,000s, Lithium-fueled, Gridded Ion Thruster.

Laboratory lithium feed system schematic

Diagram shows a laboratory setup, so not something that will actually fly. I found no examples of those. Details are not given for how an actually deployed system might differ. It looks pretty much like a lab setup described in 2001 paper, Lithium Mass Flow Control for High Power Lorentz Force Accelerators and so probably represents "state of the art" for lithium ion engines. In future there's intent to use a pistonless system and electromagnetic pumping: Electromagnetic Pumps for Conductive-Propellant Feed Systems which should avoid some of the issues with trying to wrangle liquid lithium which has a depressing tendency to corrode mechanical systems. Note the thermocouples on all the valves, which aren't just there to melt frozen lithium in a cold system, but also to deliberately freeze liquid lithium in closed valves to help stop the stuff oozing out and ruining everything.

So, lithium is stored as a solid, melted and fed into a controlled supply cylinder, which then feeds it into a vapourisor, which then feeds into what looks to my thoroughly untrained eye like a more-or-less standard electrostatic gridded thruster, using a ring-cusp discharge chamber, though the diagram quality is rather low so I won't repeat it here.

This design (apparently dreamed up in the 80s, so it is surprisingly old!) was used in the NSTAR engine (which propelled Deep Space 1 and Dawn) and NEXT design. Those used xenon propellant, but the design apparently works well enough for anything that you can pump in as a gas, as it has been used with argon and lithium-fuelled test models, too. I found a 2005 dissertation Dicharge Plasma Processes of Ring-Cusp Ion Thrusters which has this handy diagram:

Ring-cusp ion thruster cross section

Gaseous propellant is blown into the discharge chamber through a hollow cathode (and a couple of other places). Electrons travelling between the cathode and the anode formed by the chamber wall ionise the propellant. Apparently a potential difference of a mere 25 volts is enough for this. A larger voltage drop (1000V would be required for 50000 seconds performance) between the chamber and the grid accelerates the positive ions out of the business end of the thruster.

As this seems to be a fairly standard ion thruster design, there's lots of other papers out there about it, such as this one: Ion Propulsion: An Enabling Technology For The Dawn Mission with similar diagrams and explanatory text, if more information was needed.

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  • $\begingroup$ 1) but how is the lithium ionized? Is the lithium gas so dense that it can sustain a plasma by itself? 2) Does the vaporizer just thermally evaporate the liquid (some vaporizers work on other principles) 3) when are they planning on moving from lithium to dilithium? $\endgroup$
    – uhoh
    Jan 20, 2020 at 18:57
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    $\begingroup$ @uhoh in exactly the same way any other gaseous fuel is ionised in a GIT. You splurge it into your ring-cusp thingy, where it is hit by electrons coming out of a dicharge cathode flying towards the walls of the chamber and steered by magnetic fields. There's a description and some diagrams here, but as it seemed that most current electrostatic ion thrusters used this technique it didn't seem worth going into. Would you like me to splice this paragraph into the answer? $\endgroup$
    – Starfish Prime
    Jan 20, 2020 at 19:44
  • $\begingroup$ Since it's an explicit part of the question (last paragraph contains four question marks), it would be great to include in the answer, thanks! $\endgroup$
    – uhoh
    Jan 20, 2020 at 23:56
  • $\begingroup$ @uhoh eh, it seemed like "its a standard ion engine except for the fuel pump" was enough, but I've chucked in a diagram and some explanatory blurb, just for you. $\endgroup$
    – Starfish Prime
    Jan 21, 2020 at 10:05
  • $\begingroup$ no it's not just for me, it's for all the future readers who don't know what a "standard ion engine" is (is there really such a thing? I thought there's quite a variety of very different designs) and don't forget that visually impaired people use SE (e.g. 1, 2) and so answers that rely on reading images and text embedded in images are a real challenge for some users and doesn't show up in search results so typing things out and answering the question as asked is always the best way Thanks for the edit $\endgroup$
    – uhoh
    Jan 21, 2020 at 11:25

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