Newsgroups: sci.physics.plasma
From news@CUBoulder.Colorado.EDU Wed Feb 1 23:18:09 1995
From: fcrary@benji.Colorado.EDU (Frank Crary)
Organization: University of Colorado, Boulder
Subject: Re: onset of plasma effects

In article <3goegg$rhf@mojo.eng.umd.edu>,
Gerald Luther Graef <ggraef@alpha1.csd.uwm.edu> wrote:
>Suppose we could fill a box with ionized hydrogen. Should we expect
>to see plasma effects at room temperature?

Possibly. You can certainly see plasma effects in a vacuum tube
or a neon light at room temperatures. But in a random
box of hydrogen, with no wires attached, you would not.

>...If not, does anyone
>have a general idea of the temperature necessary for macroscopic
>effects to manifest? Or do we have to wait until kT = binding energy?

Temperature is not the only factor. At room temperatures, only
a completely trivial fraction of the gas would be ionized.
But, as someone else pointed out, in a low density gas, it
might take a long time for the ions to recombine. What
you really have to look at is the source of ions and
the recombination rate. If the densities are low enough,
then it's possible (or even easy) to pump up the
number of ions and sustain them against recombination.
That's exactly what the cathode in a neon light does,
creating a plasma despite the low temperatures. Also,
water can be an exception: Put a salt into the water,
and it will ionize, regardless of the temperature.
Salt water can be affected by plasma processes.
Specifically, it follows the governing laws of magnetohydrodynamics.
If memory serves, the Japanese have actually built
experimental boats powered by a MHD drive.

Frank Crary
CU Boulder