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_..._an d_in_fuel

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.
There are table top instruments available for producing and studying
these highly ionized plasmas:

Highly Ionized Plasmas.
http://www.llnl.gov/str/Schneider.html

The problem of course is storing them for long periods. If they
contact the walls of a container then they will lose their ionization.
Some possibilities would be to use Penning or Paul traps used to store
non neutral plasmas for fusion research:

Penning trap.
http://en.wikipedia.org/wiki/Penning_trap

Quadrupole ion trap (Paul trap).
http://en.wikipedia.org/wiki/Paul_trap

The amount of energy available from the ions is so high it's possible
we could siphon off a small portion of them to use their energy to
maintain the containment of the rest.
The Penning trap uses in part magnetic fields and there is a limit to
the number of particles such a trap will contain called the Brillouin
limit depending on the strength of the magnetic field. Since there is
a limit to the strength of *stable* magnetic fields that so far can be
maintained in the range of perhaps 50 T, this puts a severe limit on
the density of fully ionized particles that could be contained.
However, some researchers claim the Brillouin limit can be exceeded:

Confinement Of Pure Ion Plasma In A Cylindrical Current Sheet.
http://www.pppl.gov/pub_report//2000/PPPL-3403.pdf

Even the density achieved here though is still quite low at 4×10^14
particles per cm^3. This is at nanogram levels per cubic centimeter,
milligrams per cubic meter.
Since the Paul trap does not use magnetic fields it is unclear to me
if there is a limit to how many particles it can contain.
There would need to be quite a bit more research on how to contain
these plasmas at high densities if this is to be an energy storage
method in common use on Earth. However, it is possible that they could
be used to provide energy for space missions in deep space where
volume is not as big a concern, only mass. For instance, even at
milligrams per cubic meter this could provide kilograms of storage if
kept within a volume a hundred meters wide. For ion drives that
typically use fuel at rates of milligrams per second this could
provide fuel and the energy to power the drive over several days.

cf.:

From: Robert Clark
Newsgroups: sci.space.policy, sci.astro, sci.physics,
sci.physics.relativity, sci.physics.fusion
Date: Thu, 20 Sep 2007 13:47:28 -0700
Local: Thurs, Sep 20 2007 4:47 pm
Subject: Stored ionized gas for ion drives.
http://groups.google.com/group/sci.s...4c75eb5630f41d


Bob Clark