🚀 go-pugleaf

Scott Chase has already posted that a plasma is an ionised gas. That is
the usual approximation that those of us who do fusion theory take:
purely ionised gas. But there is more to it that that. The density of
charged particles must be such that a phenomenon known as "electron
screening" is important. This means that each particle interacts mostly
with the electric field set up by all the other particles of the gas.
Only rarely during the flight of any given particle does it come so
close to another that direct, two-body electric forces enter play. These
are the short-lived affairs we call "collisions", and do not actually
mean that the two particles touched (the plasma collisionality is
usually much greater than what would obtain if the particles were not
charged).  "Electron screening" means that if I place a test charge in a
plasma, the background electrons will move in response to the charge's
own electric field such that the new charge will not feel direct,
two-body force from any other charge (both positive ions and electrons)
if it is more than a certain distance away (called the "Debye length",
after its discoverer). The ensemble is a "plasma" if the number of
*charges* in a Debye sphere (sphere of radius Debye-length) is large
compared to 1. This means that there is a lot of screening, and two-body
forces are dominated by collective effects (a fancy way to say that the
electric field to which each charge reacts is from the ensemble and not
from only one or two of the other charges).

Now, I've said "charge" rather than "particle", because in general a
plasma need not be fully, or even dominantly, charged. Both a flame and
the glow discharge in a flourescent light are examples in which the
charge density is very small compared to the neutral density. However,
there are in both cases enough charges at high enough temperature to
satisfy the above condition.

In an industrial place like Germany, most plasma research (by number of
researchers rather than money) is on glow discharges, and most new PhD's
go to places like Siemens and Phillips.

In a defense-oriented place like the US, most work on things like beam
physics and high-performance lasers.

Fusion is the wild card, requiring much money (for the big experiments)
and comparatively fewer people.

Read what you want into that.

Gruss,
Dr Bruce Scott                             The deadliest bullshit is
Max-Planck-Institut fuer Plasmaphysik       odorless and transparent
bds at spl6n1.aug.ipp-garching.mpg.de                 -- W Gibson

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