What do scanning electron microscopes and satellites have in common? On the face of things, not much, but after seeing [Zachary Tong]’s latest video on liquid metal ion thrusters, we see that they seem to have a lot more in common than we’d initially thought.
As you’d expect with such a project, there were a lot of false starts and dead ends. [Zach] started with a porous-emitter array design, which uses a sintered glass plate with an array of tiny cones machined into it. The cones are coated in a liquid metal — [Zach] used Galinstan, an alloy of gallium, indium, and tin — and an high voltage is applied between the liquid metal and an extraction electrode. Ideally, the intense electric field causes the metal to ionize at the ultra-sharp tips of the cones and fling off toward the extraction electrode and into the vacuum beyond, generating thrust.
Getting that working was very difficult, enough so that [Zach] gave up and switched to a slot thruster design. This was easier to machine, but alas, no easier to make work. The main problem was taming the high-voltage end of things, which seemed to find more ways to produce unwanted arcs than the desired thrust. This prompted a switch to a capillary emitter design, which uses a fine glass capillary tube to contain the liquid metal. This showed far more promise and allowed [Zach] to infer a thrust by measuring the tiny current created by the ejected ions. At 11.8 μN, it’s not much, but it’s something, and that’s the thing with ion thrusters — over time, they’re very efficient.
To be sure, [Zach]’s efforts here didn’t result in a practical ion thruster, but that wasn’t the point. We suspect the idea here was to explore the real-world applications for his interests in topics like electron beam lithography and microfabrication, and in that, we think he did a bang-up job with this project.
Thrust is an interesting figure to have, but was the specific impulse measured? That’s the important metric for an ion engine
It was, it ended up around 3000 seconds which is obviously low for this particular type of ion engine but still way better than chemical engines.
It was not measured. It was inferred from the current drawn and the mass consumed, and making a host of assumptions. The key assumption being the mass actually got ejected and didn’t just land on the electrode.
Remember: The ions are ripped off the tips because they are attracted to the extraction electrode. It takes careful field arrangement and prompt cancellation of that charge to NOT have those ions simply fall into the electrode, promptly cancelling the thrust.
Failure to cancel the ions’ charge (by injection of electrons after the ions have been accelerated) will simply cause them to be attracted right back to the electrode (or to your spacecraft in general). It will cause net zero thrust, but will make your spacecraft glow faintly in the x-ray spectrum.
It’s kind of like learning to fly by falling yourself toward the ground, but missing.
Yep, all of this! Proper thrust testing is done with a calibrated thrust balance that can measure the direct force. This method is more of a rough guess at thrust (and specific impulse) based on indirect metrics.
But should be noted that the current measured was at the faraday cup not the extractor electrode. So the current tracks relatively well with the amount of emitted ions that managed to escape the thruster (although there are some assumptions ignoring any secondary electrons at the faraday cup which might be inflating the numbers)
Ah, yes, the Faraday cup does mitigate a lot of those concerns. Thanks for pointing that out.
“What do scanning electron microscopes and satellites have in common?”
Vacuum?
And both are pretty expensive
An interesting project.
I note that water glass (sodium silicate solution) will combine with CO2 in the air to form a layer of silicon dioxide. Fuzed quartz (silicon dioxide) has a dielectric strength of roughly 500 Volts per micron.
Using a base metal layer for charge, a thin layer of silicon dioxide for insulation, and a 3rd layer to wick up the propellant might solve or mitigate some of the arcing issues.
Just a thought.
Pretty sure that any layer of silicon dioxide precipitating out from water glass will be very, very far from FUSED silica.
How about a solid metal ion thruster? Basically it’s a Metal Inert Gas Welder (MIG). The fuel, a spool of metal wire. Use high current arcs to vaporize, and ionize the metal. Much easier to manage. Just feed in the metal wire at the right rate, and ion thrust is possible. It’s also off the shelf hardware available for a few hundreds of dollars.