# Why can't we use charged fine metal dust instead of plasma for ion thruster?

One of the things that makes ion thrusters so bulky and problematic are the magnetic shields required to protect from high temperatures right? And we have high temperatures because we use plasma right? So why don't we just use fine metallic dust, charge it and feed it into an ion thruster to get rid of the temperature problem.

• The fact we use plasma because it can be ionized is half the reason. The other half is that any substance subjected to energies and accelerated to speeds like these in ion drive will turn into plasma. It's an unavoidable consequence which quite coincidentally is also an essential beneficial feature. – SF. Jan 21 '17 at 20:26
• @SF.: That doesn't seem right to me. I can pick a frame of reference in which my least favorite politician is moving at v=0.9999999c, but unfortunately that does not imply that he's vaporized into a plasma. What you're saying would certainly be correct for a thermalized exhaust flowing at velocity v out through a nozzle, but an ion beam can in theory be more like an optical beam. – Ben Crowell Jan 22 '17 at 4:03
• @BenCrowell: Can you pick a frame of reference in which he is accelerated from 0 to 0.01c over distance of 30 centimeters, and not turned into plasma by the force that accelerates him? ;) If temperature is understood as speed of particles, then particles accelerated to speeds which ion drive gives them, even if in coherent cloud, almost immobile relative to each other they would not behave as plasma (assuming the drive accelerated them all identically), the cloud will be very much plasma-like in contact with drive walls. – SF. Jan 22 '17 at 9:14
• @SF.: Your new version is partly improved, but still has a lot of wrong physics. I don't think we're going to clarify this in comments. – Ben Crowell Jan 22 '17 at 15:21

In an ion thruster, particles are accelerated because of their electrical charge. The force acting on them is proportional to the charge (and the external field applied, which we can treat as fixed for a specific engine design). Naturally, the heavier the particle is, the less it is accelerated by this force.

An extended particle we can describe as a capacitor and, as such it has a capacity given by $C = 4\pi\varepsilon_0 R$. If we put R as 10$~\rm\mu m$, the resulting capacity is $10^{-15}\rm F$. Now we can use an external voltage to charge this particle up. A reasonable voltage might be 100 kV - resulting in a charge of $10^{-10}\rm C$. As the elementary charge is $1.6\cdot 10^{-19}\rm C$, this means we are removing about 1.6 billion electrons from the particle. On the other hand, such a particle weighs about 10 ng and contains about $10^{14}$ atoms or $2.6\cdot10^{15}$ electrons.

That means, our engine can just remove about one in a million of all electrons available. Compare this to the plasma: Here we can remove a large fraction of electrons, the exact amount depends on the temperature reached, but is at least in the order of 5-10%. That means, the metal particles have a charge-to-weight ratio which is worse by a factor of 10,000x, giving a lot less thrust than the plasma.

Removing the same amount of electrons from the metal powder is just impossible: Imagine you were to remove 0.1% of the electrons of a larger object, such as the moon. This would exert such a high Coulomb force on the object, it would blow apart in a violent explosion, despite all its mass and gravity.

• I'm not sure that comparing the metal powder particles with the moon in the last paragraph is helpful for answering this question. – Paŭlo Ebermann Jan 21 '17 at 21:06
• @PaŭloEbermann I agree, the Moon is a bit of a red herring. Also, I think even the 100 kV potential on a 10 μm particle is unachievably high. If it didn't explore or discharge by ionized atoms spontaneously leaving the surface, it would discharge by ionizing gas molecules in anything less than a perfect vacuum. Overall it's a good answer though. – uhoh Jan 22 '17 at 3:23
• Nice answer. Now we can use an external voltage to charge this particle up. If you simply have a voltage available, that doesn't necessarily give you a way to charge your particle, does it? But for example if you try to charge up a dust particle by bouncing electrons off of it, the maximum voltage you can produce probably does correspond to the voltage you have available to accelerate the electrons. The argument given in the final paragraph seems more airtight. – Ben Crowell Jan 22 '17 at 4:00
• @BenCrowell If you have the voltage available, you can easily build an electron gun and aim it at the particle. Or you could make a design where the particles are stuck to a conductive surface, charge them up and release them (challenging to do, but should be possible). @ Paulo This statement holds for pretty much any macroscopic object, even for something as massive as a planet. The particle might be a bit more stable because of having atomic bonds instead of being a "pile of rocks", but most people (me included) don't have an intuitive feeling for these kind of forces. – asdfex Jan 22 '17 at 14:04

Ion thrusters work so well because they have a very high exhaust velocity. This is made possible by the fact you're using individual atoms as the propellant. Metal powder consists of much larger, heavier particles, and the speed they can reach would be much lower, making the thruster less efficient.

Electrical thrusters that use particles rather than ions exist. Most seem to be lower-performance (but higher-thrust) devices than the typical xenon ion thruster.

In general, these devices seem to use sprayed liquid droplets rather than solid particles.

https://en.wikipedia.org/wiki/Colloid_thruster