Dear Christopher M. Jones:

"Christopher M. Jones" wrote in message
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Or more massive ions. Less massive ions increase the charge
density per thrust, increase the amount of overhead in
ionization per thrust, which leads to lower efficiency. We're
probably at the limit there atom-wise since Xenon is pretty
massive and pretty easy to handle (the only other good options
would be Radon, which is even rarer than Xenon, and Uuo, which
is even rarer than monkeys flying out of my ... well, anyway).
More massive molecules or "mesoscopic" particles (i.e. dust)
would lead to yet higher efficiencies but they're a lot more
difficult to use in an electric rocket without it getting all
gummed up in about two seconds. There's some research on
using C60, for example, in ion engines but it's still a
loooong way from workable. But if they ever get it to work
then it should lead to much higher efficiencies (since C60
is about 5.5x as heavy as Xe).

I'd worry about "selectively ionizing" a molecule for propulsion. Since
the number of electrons stripped off provides the handles for accelerating
the mass, the more electrons removed means the faster you can accelerate
the molecule. But the more electrons you remove the weaker (or smaller)
the molecule fractions become. So your C60 becomes just 60C, and you are
back to accelerating a bunch of light nucleii.

David A. Smith