NASA Successfully Tests Ion Engine

(Henry Spencer) wrote in message ...
To date, nobody has an *efficient* method of ionizing the plasma in an ion
thruster. The result is that ion thrusters have unimpressive efficiency
numbers, unless you run the exhaust velocity up to the point where the
efficient acceleration process dominates the inefficient ionization... but
most real-world applications optimize at quite low exhaust velocities, to
minimize the mass of the power source (higher exhaust velocities need lots
more power).

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).


(Published numbers on efficiency need to be scrutinized very carefully,
because there is a lot of specsmanship -- often what is quoted is *not*
overall, end-to-end, low-voltage-DC-to-jet-power efficiency, but the
efficiency of some better-looking subset of the process.)

One reason for interest in Hall-effect thrusters and other plasma
thrusters, as alternatives to ion thrusters, is that they don't need high
ionization percentages and hence can avoid most of the efficiency penalty.

And run in a more useful range of power/thrust levels, at
present.

Dear Christopher M. Jones:

"Christopher M. Jones" wrote in message
...
....
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

On Sat, 29 Nov 2003 07:58:47 -0700
\(formerly\)" dlzc1.cox@net wrote:

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.

C_60 is pretty stable, though, and, like any molecule, will hold on tighter to its remaining electrons once it's already lost some. Some quick googling suggests C_60 can lose at least 3 electrons without breaking up, but will start shedding C_2 ions at some point after that. I don't know much about ion drives, but I'd think that'd be good enough.

What I'd be more worried about is carbon buildup on the grids. If even a small fraction of the molecules sticks to the charged surfaces (and those C_2 fragments are likely to be particularly sticky) the resulting soot buildup might well become a problem over time.

--
Ilmari Karonen
If replying by e-mail, please replace ".invalid" with ".net" in address.

Dear Ilmari Karonen:

"Ilmari Karonen" wrote in message
...
On Sat, 29 Nov 2003 07:58:47 -0700
\(formerly\)" dlzc1.cox@net wrote:

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.

C_60 is pretty stable, though, and, like any molecule, will hold on
tighter
to its remaining electrons once it's already lost some. Some quick
googling suggests C_60 can lose at least 3 electrons without breaking up,
but will start shedding C_2 ions at some point after that. I don't know
much
about ion drives, but I'd think that'd be good enough.

I wouldn't expect that to be of much help. That is a lot of mass, and very
little net charge. The accelerator path would have to be "long".

What I'd be more worried about is carbon buildup on the grids. If even a
small fraction of the molecules sticks to the charged surfaces (and those
C_2 fragments are likely to be particularly sticky) the resulting soot
buildup
might well become a problem over time.

If you strip electrons, just leave a net positive charge on the "nozzle"
(or an AC with a positive bias to invoke any diamagnetism).

I would wonder if you couldn't *add* a few electrons, to either C60, or
some long polymer chain. In fact, I seem to recall that some ions were
trapped *inside* C60 (how they'd stay ions seems the $64 question). In
other words, shoot the cat and then the rubber rod out the back...

David A. Smith

In article FFyyb.24793$Bk1.5331@fed1read05,
\(formerly\) dlzc1.cox@net wrote:
I would wonder if you couldn't *add* a few electrons, to either C60, or
some long polymer chain.

Unfortunately, good negative-ion sources are even harder to build than
positive-ion sources.
--
MOST launched 30 June; first light, 29 July; 5arcsec | Henry Spencer
pointing, 10 Sept; first science, early Oct; all well. |

In article ,
Ilmari Karonen wrote:
C_60 is pretty stable, though, and, like any molecule, will hold on
tighter to its remaining electrons once it's already lost some. Some
quick googling suggests C_60 can lose at least 3 electrons without
breaking up...

Unfortunately, preliminary experiments with it for ion propulsion were not
favorable -- in the sort of environment needed to quickly ionize significant
flows of material, it's not stable *enough*. The more determined folks have
not entirely given up, but it's not going to be easy.
--
MOST launched 30 June; first light, 29 July; 5arcsec | Henry Spencer
pointing, 10 Sept; first science, early Oct; all well. |

Henry Spencer opined

In article ,
Ilmari Karonen wrote:
C_60 is pretty stable, though, and, like any molecule, will hold on
tighter to its remaining electrons once it's already lost some. Some
quick googling suggests C_60 can lose at least 3 electrons without
breaking up...

Unfortunately, preliminary experiments with it for ion propulsion were not
favorable -- in the sort of environment needed to quickly ionize significant
flows of material, it's not stable *enough*. The more determined folks have
not entirely given up, but it's not going to be easy.

Given all the symmetries with the C60 molecule, could a tuned laser do the
ionization?

-ash
for assistance dial MYCROFTXXX

"Ash Wyllie" writes:

Henry Spencer opined

In article ,
Ilmari Karonen wrote:
C_60 is pretty stable, though, and, like any molecule, will hold on
tighter to its remaining electrons once it's already lost some. Some
quick googling suggests C_60 can lose at least 3 electrons without
breaking up...

Unfortunately, preliminary experiments with it for ion propulsion were not
favorable -- in the sort of environment needed to quickly ionize significant
flows of material, it's not stable *enough*. The more determined folks have
not entirely given up, but it's not going to be easy.

Given all the symmetries with the C60 molecule, could a tuned laser do the
ionization?


Yes, but it would almost certainly have to be a tuned ultraviolet laser,
and those are pretty hard to come by. Harder still if you demand even
five percent efficiency, which you need to be competitive with the more
conventional propellant ionization systems.

Two-photon processes could theoretically get you ionization using only
tuned visible lasers, but such processes are inherently inefficient,
demand very high laser intensities, and are generally not worth the
bother unless the universe obliges you with a metastable state halfway
between ground and first ionization.


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