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#1 2007-09-22 17:11:29
Re: Storing highly ionized plasma at low density for ion drives.
Below is a post to sci.physics. It proposes just storing ionized gas for fuel for ion drive engines rather than using the power on board for ionizing the gas as well as accelerating the ions.
My question is what kind of power would you need to contain a fully ionized gas using magnetic or electric fields? If the gas was low density could you just use light weight permanent magnets?
Bob Clark
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Newsgroups: sci.space.policy, sci.astro, sci.physics, sci.physics.relativity, sci.physics.fusion
From: Robert Clark
Date: Thu, 20 Sep 2007 13:47:28 -0700
Local: Thurs, Sep 20 2007 4:47 pm
Subject: Stored ionized gas for ion drives.
This page gives a formula for the exhaust speed of an ion engine in
terms of the charge on the ions and the voltage driving the ion flow:
Ion thruster.
http://en.wikipedia.org/wiki/Ion_thruster#Energy_usage
The exhaust speed increases with the charge on the ions and decreases
with their mass. You would think then that a light gas like hydrogen
would be ideal since heavier gases even when fully ionized would still
contain approximately equal numbers of neutrons as protons which would
not contribute to the charge but would approximately double the mass.
Yet it is the heavier gases like cesium and more recently xenon that
are used. The explanation is that of the energy it takes to ionize the
gas used as fuel. The figure on this page shows the energy to ionize a
light gas such as hydrogen is relatively high compared to the heavier
gases:
Ionization Energies.
http://hyperphysics.phy-astr.gsu.edu/hb … onize.html
The figure gives the energy per mole which is high in itself. It is
even worse when you consider this on a per mass basis since the mass
amount of hydrogen would be so small compared to the amount of energy
needed to ionize it.
So could we instead store the hydrogen or some other light gas
already in ionized form so we would not have to supply power to ionize
the gas, only to accelerate it?
If you used ionized hydrogen, so you would be accelerating protons,
then using 6 x 10^18 protons to make one 1 Coulomb, and a mass of 1.6
x 10^-27 kg for a proton, and V representing the voltage in volts, the
speed on the ions (protons) would be about (10^4)sqrt(2*V) in meters/
second.
If we made the voltage be 5,000 V we would get 1,000,000 m/s speed
much higher than any current ion drive. Also, there are power supplies
that convert low voltage high amperage power into high voltage, low
amperage power, even up to 500,000 V. The we could get 10,000,000 m/s
= 10,000 km/s exhaust speed.
The question is could we get light weight means of storing large
amounts of ionized gas? Note that is this for space based propulsion
not launch from Earth. You would have a possibly large energy
generating station that remained in low Earth orbit to supply the
power to ionize the gas once the spacecraft was placed in orbit. The
power generator would be left behind in orbit. Then the volume of the
gas container could be large to keep the density of the gas low. This
would allow very thin container walls. Note the low density would also
allow the electrostatic repulsion of the positively charged ions to be
more easily constrained.
A possible problem though is the charged ions contacting the walls
could lead to a loss of ionization. You might be able to use a low
level magnetic field to prevent the ions contacting the walls. Low
density of the gas would insure the strength of the magnetic field
required would be low. It might even be accomplished by thin permanent
magnets so you would not need to use extra power.
Some questions: what would be the electrostatic pressure produced by
a low density highly ionized gas? What strength magnetic field would
you need to contain it?
Note that with an exhaust speed of say 10,000 km/s, by the rocket
equation we could get the rocket itself up to relativistic speeds with
acceptable mass ratios.
Then this would provide a means of testing relativistic effects on
macroscopic bodies.
Bob Clark
==================================================
Old Space rule of acquisition (with a nod to Star Trek - the Next Generation):
“Anything worth doing is worth doing for a billion dollars.”
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#2 2007-09-22 22:59:25
- cIclops
- Member
- Registered: 2005-06-16
- Posts: 3,230
Re: Storing highly ionized plasma at low density for ion drives.
The ionization potential of Xenon is quite low (12 V) compared with the typical 5000 V needed to accelerate the ions. It would seem unlikely that the cost in extra mass of the containment field would offset the power savings, power that is provided essentially "free" from solar panels. Xe atoms are 131 times the mass of H, and momentum is what counts. The trades with Hydrogen would be the higher mass due to the larger tanks, higher voltages would also require more mass for the power converters and insulation. Checkout the size of the 425 kg Xe tank on the Dawn spacecraft (about 0.4 m³), containing the gas with a magnetic field that large would need a lot of magnets, the problem would be worse for Hydrogen.
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#3 2007-09-26 16:19:11
- Nik
- Banned
- From: UK
- Registered: 2007-08-26
- Posts: 18
Re: Storing highly ionized plasma at low density for ion drives.
Um, look at the problems of low-pressure plasma containment for fusion research...
My guess on the internal pressure for this scheme would be combustion chamber of rocket engine-- Just before it spontaneously disassembled.
Mind you, if he cares to try it, I'd be interested in the numbers.
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#4 2007-09-28 18:20:52
Re: Storing highly ionized plasma at low density for ion drives.
The ionization potential of Xenon is quite low (12 V) compared with the typical 5000 V needed to accelerate the ions. It would seem unlikely that the cost in extra mass of the containment field would offset the power savings, power that is provided essentially "free" from solar panels. Xe atoms are 131 times the mass of H, and momentum is what counts. The trades with Hydrogen would be the higher mass due to the larger tanks, higher voltages would also require more mass for the power converters and insulation. Checkout the size of the 425 kg Xe tank on the Dawn spacecraft (about 0.4 m³), containing the gas with a magnetic field that large would need a lot of magnets, the problem would be worse for Hydrogen.
It is true that only a relatively low proportion of the power goes to ionizing the gas, such as xenon, in an ion drive. But the reason is that the xenon gas is only being partially ionized, perhaps only one or two electrons being knocked off.
Then a gas that is fully ionized would undergo higher velocity since it would have a higher positive charge being acted on by the electrostatic forces.
That the xenon is actually only minimally ionized with the current ion drives is illustrated by the energy required to remove the electrons more tightly bound to the nucleus. See this web site for the data on the ionization energies of the elements:
NIST Atomic Spectra Database Levels Form.
http://physics.nist.gov/PhysRefData/ASD … _form.html
Enter Xe 53 into this form to get the last (54th) electron ionization energy,
41,299.7042eV.
1 eV counts as about 100 kJ/mol. So this means removing that final electron would have required 4 billion joules per mol of xenon, about 30 million joules per gram.
This is just for that single last electron. The total energy required for removing all of them would be in the hundreds of millions of joules per gram.
My plan is to have a possibly large generating station in orbit to supply this energy. Note since the gas is being stored as it is being ionized you might not have to have a very powerful generator, just one operating for a long period of time.
You can calculate the exhaust velocity possible for a fully ionized Xenon atom using SI units at say a 5,000 V voltage and using the number I gave for the charge on a single proton in coulombs, C, from this formula:
Note that since this is in SI units you must give the mass of the Xenon atom in kilograms, as I did in my calculation in giving the mass of the proton in kilograms.
Bob Clark
Old Space rule of acquisition (with a nod to Star Trek - the Next Generation):
“Anything worth doing is worth doing for a billion dollars.”
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#5 2007-09-28 18:40:39
Re: Storing highly ionized plasma at low density for ion drives.
Um, look at the problems of low-pressure plasma containment for fusion research...
My guess on the internal pressure for this scheme would be combustion chamber of rocket engine-- Just before it spontaneously disassembled.
Mind you, if he cares to try it, I'd be interested in the numbers.
Since this is for a propulsion system operating only in space I could make the pressure of the gas low by using a very large volume.
I am looking up references on this on the net. Some keywords to use if you want to look up some references are "non neutral plasma" and "Brillouin limit".
The Brillouin density limit is a limit on the number of ionized particles that can be contained on a magnetic trap based on the strength of the magnetic field.
Bob Clark
Old Space rule of acquisition (with a nod to Star Trek - the Next Generation):
“Anything worth doing is worth doing for a billion dollars.”
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