Like charges repel, and as long as you've ionized the air/medium inside
of a balloon, it will float.  If any of you are buffs of the Inertial
Electrostatic Confinement Reactor, don't think too far into fusion.

Think about this, the Thermionic Emission, used in a Vacuum Tube
Diodes, Triodes, Quadrodes, Pentodes, etc. Amplifiers, was first
discovered without any vacuum at all.  But, in open air with a hot
cathode, and a cold anode.  The Vacuum Tube, design allowed for less
current to be used to generate more heat with less power concering the
filiment, and an improvement on the device, that was initially used at
atomospheric pressure, in open air.

With that they discovered that under specific conditions that electrons
could be acellerated, they use opposite charges to attract a beam of
electrons in the direction they want.  The Inertial Electrostatic
Confinement Reactor, does that by placing an energized grid in front of
a cloud of electrons, and until they reach an energy level high
enough/voltage, they cannot get past the grid.

http://fusor.net/

This does the same thing as a step up transformer, for the most part.
But, it also shows that you can pass a static charge onto any gas, at
any desired energy level.

This will make any atom, act like a pith ball.
http://www.ap.smu.ca/demos/content/e_and_m/pith_balls/pith_balls.html

And the greatest achievement of them all, they wind up placing a
distance, like heat would between atoms and molecules of the air.  The
gas is not techically lighter than air, it's more like hot air, but
instead of heat causing the distance between atoms, electrostatic
charge does.

Now, if you have ever seen a burner on a hot air balloon, the Fusor or
Inertial Electrostatic Confinement Reactor, is actually just a model of
the burner you would need.

Here's the great part of it all, a balloon only needs picoamperes to
remain adhered to the ceiling when you rub the balloon on your head.
It could require less.  In humid coniditions it could require more.

A Van De Graaff, generator is designed based upon the principal that
the center of a conductor is always at a potential of zero.  This is a
very common fact of most conductors, but the fusor/Inertial
Electrostatic Confinement Reactor, shows us that common, also has
common variables that allow for control or contradictory circumstance.
If that were not the case, there would be no confinement of the
electrons, and the metal sphere would be charged, and the centeral grid
at a potential of zero.

This is what you need to understand, and see:
http://www.lbl.gov/abc/wallchart/chapters/11/2.html

Did you see how the Van De Graaff is used to acellerate ions?

Let's look at more complete block diagram:
http://omega.ujf.cas.cz/vdg/graaff-principle.html

Basically, that is like placing a flyback transformer inside the metal
sphere, with a small Cockroft Walton Voltage Multiplier on the inside.
http://home.earthlink.net/~jimlux/hv/cw1.htm

At the supercolliders, and acellerator labs, they like to control which
gases go in, and base the calculation on the predictability of what
comes out.  It's not used for ballooning, but for the most part, is has
been done at atmospheric pressure.  The 25MeV result, is from the
acelleration of the ion, not the voltage of the Van De Graaff.  The Van
De Graaff, is really at about 1.4, to 1.5 MeV.  If an electron has a
voltage of 1 million volts, and it's just drifting around slowly, and
you acellerate it to a velocity that is 25 times faster than that, it
will produce a voltage that is 25 times more.  In vacuum tube physics,
this is considered the Kinetic value, or K.

Basically, a voltage multiplier will always work in respect to ground.
If ground were the balloon, and it was air born.  The surface charge
may be 1,000,000 volts, but the circuit is going to look at the surface
of the balloon as if it were ground potential, and the multiplied
voltage will always be in respect to ground.

So, we go back to the Cockroft Walton Voltage Multiplier, and when we
place it inside of the sphere, and it is powered from the inside, all
we can possibly do is increase the static potential, or kinetic
potential of every electron present in that material.  So, the voltage
go racing through the roof, and the current remains at about nill,
except a minor bleed off.  So, as long as we produce a molecular beam
inside that fills a chamber that looks just like the fusor's
electrostatic confinement chamber, the balloon places a greater and
greater distance between atoms and molecules, just like pith balls.
Every time an atom tries to stablize, the voltage of the electron is
jacked way up, and in fact it's the sum of the number of cycles, for
example 45KV + 45KV +, +,, etc... and the beam is forced into that
spherical cage.  The electrons are then acellerated and produce a
voltage around 1.5MeV to escape the cage.  Since, you voltage
multiplier is resting on the sphere's externally,  it has that much
more energy to help it, and call ground level.  So, that's 1.5 MeV +
45KV which was enough to produce a molecular beam and fill the fusor
grid.  So, that will bring the gases energy level to 3MeV, and the
distance between atoms and gaseous molecules is growing.  But, the
current required is very low.  The thing starts to glow, and float.

I can promise you that it will require a certain current in, and
certain current will be lost to the air surrounding the craft, but that
should still be in uA, mircoamperes, or milliamperes.  Very low for a
very large area of balloon.

Now, wouldn't that just beat the hell out of a payload of gases to
burn, hydrogen or helium?  Why the hell would aliens, use probes with
massive tanks, when a 500 or 1000 Watt Solid State Voltage Multiplier
could do the same thing as heat, or a nobel gas like helium?  If the
environment of another world included a helium atmosphere, how in the
world could you get it to fly?  If a balloon is designed right, it can
take you to the edge of space.