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Lifters generate lift through ionizing air molecules and then attracting those ions toward an oppositely charged surface to create thrust. are constructed on a balsa frame with a 1 inch wide aluminum foil strip positioned vertically 1 cm above a parallel wire. The strip and wife are attached to opposite outputs of a van de graft generator.

What molecular species in our atmosphere are susceptible to ionization in such an ion engine?

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It is funny that it doesn't matter (except minor effects) which surface is the cathode. – arivero Sep 12 '15 at 22:21

I would guess that the ions are generated by a corona discharge. If there are any free electrons in the vicinity of the wire they are accelerated by the electric field. The ionisation energy of N$_2$ is about 16eV and oxygen is about 12eV, so you don't have to accelerate an electron very much to give it enough energy to ionise a nitrogen or oxygen molecule. Ionisation frees more electrons, and they in turn accelerate and ionise more molecules, and the end result is an avalanche.

You still need the original electrons. Where these come from depends on the system. They may come from background radiation,or possibly from field emission from the wire though I'm not sure this would be significant at the voltages being used in the lifter.

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