NASA Successfully Tests Ion Engine

I wrote:
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...

I should qualify that first bit, as an afterthought. As I understand it,
the FEEP thruster and similar concepts manage to achieve fairly efficient
ionization. Unfortunately, they do it by using an emission process which
(in its current implementations, anyway) is quite slow, so beam currents
are very low and it's hard to get significant thrust that way.
--
MOST launched 30 June; first light, 29 July; 5arcsec | Henry Spencer
pointing, 10 Sept; first science, early Oct; all well. |

Mary Shafer wrote in
:

I could never be a writer for an anchorperson; I'd be fired after
about a week. Cf. "Chuckles the Clown Is Dead", an episode of the
Mary Tyler Moore Show. Ted Baxter is, as far as I can tell, smarter
than many anchorpersons, by the way. Or cf. "Dirty Laundry" by some
group or another.

Don Henley, I think.

--
JRF

Reply-to address spam-proofed - to reply by E-mail,
check "Organization" (I am not assimilated) and
think one step ahead of IBM.

Mary Shafer wrote:


Or cf. "Dirty Laundry" by some group or another.

Don Henley

Paul

I'm interested in seeing materials other than the well tested xenon
gas being used to form the plasma. There seems to be some uncertainly
in exactly how the grapes generate the plasma under the microwaves.
How large can the plasma pulse get for one grape? It seems to be
rather large in the video demonstrations available online. Could
several of these plasma pulses lift a 2 oz. weight as has been
demonstrated by Myrabo using a much more powerful optical laser? This
is a rather easy thing to test.
You mentioned in this thread that C60 has been investigated as a
possible material. That reminds me of a surprising discovery involving
single walled carbon nanotubes. Researchers found that under light
from an ordinary photographic flash the SWNT's burst into flame:

Carbon Nanotubes Ignite When Exposed to Flash
"To the best of our knowledge, no other material emits such a loud
sound and ignites spontaneously when exposed to unfocused low-power
light; this adds to the long list of unique properties of carbon
nanotubes," said Ramanath. "From an applications perspective, our work
opens up exciting possibilities of using low-power light sources to
create new forms of nanomaterials, and will serve as a starting point
for developing nanotube-based actuators and sensors that rely on
remote activation and triggering," he added. "
http://www.rpi.edu/web/News/press_re...002/flash.html

Light flash restructures carbon nanotubes
By Chappell Brown
EE Times
April 30, 2002 (11:44 a.m. ET)
"He found that the light restructured the tubes into horn-shaped
configurations, surprising since carbon bonds need temperatures
between 1,500°C and 2,000°C to break and reform. Also, multiple-walled
carbon nanotubes, carbon soot and samples of Buckminsterfullerene
(C60) were not affected by the light, which came from an ordinary
photographic flash lamp. Ignition in the presence of oxygen required a
local temperature of 600°C to 700°C, which could be achieved at a
threshold illumination of 100 milliwatts/cm2. "
http://www.eetimes.com/story/OEG20020430S0014

The full text Science article is available he

Nanotubes in a Flash--Ignition and Reconstruction.*
P. M. Ajayan, M. Terrones, A. de la Guardia, V. Huc,
N. Grobert, B. Q. Wei, H. Lezec, G. Ramanath, T. W. Ebbesen
Science, Vol. 296, No 5568, p. 705, 26th of April 2002
http://www.ece.lsu.edu/bwei/webpage_...e2002flash.pdf

It is surprising that rather low-intensity, unfocused light caused
them to ignite and reach such high temperatures. Then increasing the
intensity further, and not even using lasers, might allow them to
reach the temperatures required to form a plasma. I was intrigued to
see in one of the descriptions of the experiment that it was explained
as due to a highly efficient conversion of light into heat. (Other
obvious applications of this would be their use for efficient solar
power heating systems.)
Also would other wavelegnths such as microwaves be subject to this
efficient light to heat conversion?

Bob Clark

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(Henry Spencer) wrote in message ...
In article ,
Robert Clark wrote:
Would this provide a low energy means of creating the plasma required
for ion engines? One means of creating the required plasma is by
irradiating the propellent gas with intense laser or x-ray beams to
strip off the electrons of the atoms of the gas, producing an ionized
plasma. However, these are both high-frequency forms of EM radiation
and therefore require high energy to produce. Microwaves being longer
wavelengths require less energy to produce.

There are already ion thrusters that use microwaves for ionization, and
also some that use lower-frequency radio waves. No actual thruster that
I'm aware of uses lasers or X-rays.

Another means that is actually used for the Deep Space 1 probe is to
use electrons emitted by a cathode to irradiate the gas, ionizing it.
How does the energy requirement for the heating element of a cathode
compare to the energy requirement for producing the microwaves?

Both are relatively efficient processes, in themselves. Unfortunately,
that doesn't imply that you get efficient ionization as a result. In
either case, much of the energy gets used unproductively.

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).

(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.