On Oct 11, 9:10 pm, Robert Clark wrote:
In researching the amount of energy required to ionize gas for ion
drives I was surprised by the total amounts of energy that would be
required to *fully* ionize the gas. This amount of energy is quite
large, actually huge, and so for actual ion drives the gas is only
minimally ionized.
Some examples of the amount of ionization energy energy can be found
he

Ionization energies of the elements.http://en.wikipedia.org/wiki/Ionizat...f_the_elements

You see for hydrogen it's 1312 kilojoules per mole. Since the atomic
weight of hydrogen is 1, this is 1,312,000 joules per gram or 1.3
billion joules per kilo. Note that this amount of energy that needs
to be added to ionize the gas will conversely be released when the
electrons are recombined with the ionized gas. Then this is several
times higher than the maximum energy density of chemical reactions on
a per weight basis such as by chemically oxidizing neutral hydrogen:

Energy density in energy storage and in fuel.http://en.wikipedia.org/wiki/Energy_...ity_in_energy_...

Other elements can produce even higher amounts. By and large, the
energy density gets higher for the heavier elements. For instance you
can find the total for copper by adding up the amounts given on the
"Ionization energies of the elements" page. You get 4,345,619.4 in kJ/
mol. Then since the atomic weight of copper is 64, this amounts to 68
billion joules per kilo.
On the "Energy density in energy storage and in fuel" page, there is
a huge gap in energy density between the chemical reactions to the
nuclear reactions. Then these "electron recombination" reactions, if
you will, would provide an intermediate level in energy storage
density.
However, for getting these amounts note that the element has to be in
gas form since the energy required to release the electrons from orbit
is different for solids, called the "work function", usually smaller.
So the released amount of energy on recombination would also be
smaller. Then for some elements such as metals you would also have to
supply high heat to get the element in gas form. Then this energy
storage method would probably be better in heavy gases, such as
xenon.
The ionization energy of xenon is incomplete on the "Ionization
energies of the elements" page. A more complete list can be found on
the page:

NIST Atomic Spectra Database Levels Form.http://physics.nist.gov/PhysRefData/...vels_form.html

by typing in for example Xe 53 to get the last (54th) electron
ionization energy. However, not every ionization level for xenon is
given on this page either. After a web search, I found the total
amount of energy required to fully ionize xenon is about 200 keV.
Since 1 eV is about 100 kJ/mol , this is about, 2 x 10^10 J/mol. Since
the atomic weight of xenon is 130 this comes to 154 million joules per
gram, 154 billion joules per kilo.
...

This report gives the total ionization energy for uranium as 762.9
keV:

Electron Emission Following the Interaction of Slow Highly Charged
Ions with Solids.
http://www.osti.gov/bridge/servlets/...ble/301182.pdf

Since 1 eV is about 100 kJ/mol and the atomic weight of uranium is
238, this amounts to 320 billion joules per kilogram. Other elements
with high total ionization energies are given in Fig. 1 in this
report.
To put this in perspective, the energy density of hydrogen burned
with oxygen is 140 million joules per kilo of hydrogen. So the
electron recombination reaction of uranium results in more than 2000
times the energy per kilogram.
The space shuttle external tank contains about 100,000 kg of hydrogen
and 600,000 kg of oxygen. Then the energy content here would be
equivalent to only 50 kg of fully ionized uranium. (Note this is *not*
a nuclear reaction.) And the oxygen also would not be required.
Note this is only in regards to the energy content. It does not
consider how the thrust would be generated.


Bob Clark