Newsgroups: sci.physics.plasma
From: fcrary@benji.Colorado.EDU (Frank Crary)
Organization: University of Colorado, Boulder
Subject: Re: What precisely is plasma ?
Approved: eastman@glue.umd.edu

In article <2vjpga$7vf@mojo.eng.umd.edu>,
Seng Woon Lim <amoured@u.washington.edu> wrote:
>Since plasma is made up of a collection of ions and neutral molecules,
>how does one determine what makes up a 'glob' of plasma material
>experimentally? some kind of mass spectroscopy, or on
>photoemission spectroscopy??

I think it really depends on what kind of plasma you are thinking
of. A mass spectrometer or a Fariday cup will give you
the charge density, but that just tells you if there is some
plasma in the "glob" not if the "glob" as a whole is a plasma.
Looking for photoemission would tell you if it's a hot and dense
(or large) plasma but you wouldn't see a cold plasma or a
one with a low column density. Looking at the low frequency
cut off at the plasma frequency is another technique, but again
it doesn't say much about the amount of neutral material
mixed in. The best test might be to impose an electric field on
the plasma and see if the charged particles move around to
cancel out the field. That ought to work on any plasma;
even a non-neutral plasma still shields out external fields,
even though it also creates its own, internal field. If you
use an AC field, you'd even find out how many neutral particles
are out there: They would give you a resistance dependent on the
electron-neutral collision frequency.

Frank Crary, CU Boulder