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Ramo
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Austin, Texas, USA
Oct 1999
time: 23:34
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Since this thread clearly ain't going to get any posts till pchang gets here, I'll start with an overview of plasma physics.
What plasma is, is a bunch of really fast ions and electrons flowing together. Essentially, it's a fluid with electromagnetic forces.
So the first step is some statistical description of this ****.
You start with the principle of phase-space conservation. Say, you have a six dimensional space - 3 spatial and 3 velocity coordinates. And you have a distribution function, F, of ions (or electrons) in this space. And you have a region in this space. So, the time-rate of change of f plus the flux through the surface (the time derivitive of the 6-vector dotted with the 6-gradient of F) is 0 in the case of particle conservation.
Now, you break up the flux term, and you get dF/dt + v.grad(F) + a.dF/dv = 0 (bolded are R3-vectors, . is dot product). a is the Lorentz force, (E + vxB)q/m.
The problem with F is that it's a bunch of spiky dirac-delta functions. So we'd like to take "ensemble averages" of it.
Define f = .
Nother problem is that EM fields depend on the path, so that there's some correlation that needs to be accounted for. IOW:
<a.dF/dv>=<a>.df/dv-C(f), where C's something called the colission operator (in general nonlinear) that accounts for correlations.
Putting it all together and writing as v for simplicity:
df/dt + v.grad(f) + q/m(E + vxB).df/dv = C(f)
If you assume phase-space incompressibility:
div(v) = d/dv.a = 0
Then, you get:
df/dt + div(vf) + q/m*d/dv.((E + vxB)f) = C(f)
Collision operators are really hard to figure out, so what people do are just to take moments of that thing (wrt products of v) to get fluid equations. But that's enough for now. Clear as mud, eh?
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DanS
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Kickball Capital of the World
Jan 1970
time: 00:34
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quote:
Well, we do know a fair amount. It is hard to judge because we don't know a lot of what we don't know. However, as fusion research has shown, controlling a high energy plasma is pretty much impossible. |
I was thinking along the lines of the electric plasma propulsion devices that I asked about a couple of weeks ago to extrapolate to the basic science. As we discussed, there's not much available yet beyond relatively low energy ion thrusters and Hall Effect thrusters. Not a lot of effort went into this kind of research until the late 90s when the Russkis turned over some science to us.
Of course, I wouldn't be surprised to learn that this might not be a valid extrapolation.
(Sorry about the minor threadjack, Ramo.)
Last edited by DanS on 05-04-2005 at 22:48
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05-04-2005 08:40
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