From: "Paul M. Koloc" <pmkoloc@erols.com>
Newsgroups: sci.physics.plasma
Subject: Re: Plasma questions
Organization: Prometheus II, Ltd



Arthur Carlson TOK wrote:
>
> fodder@ix.netcom.com (Mike Greenberg) writes:
>
> > 1)What part of a flame is plasma? Does it have a magnetic field?
>
> Roughly speaking, a gas which is hot enough to glow is hot enough to
> be significantly ionized. I wouldn't expect a magnetic field
> appreciably stronger than the ambient.
>
> > 2)Does the earth's magnetic field affect fire or plasma?
>
> I would expect the collision frequency to be much larger than the
> gyrofrequency, so, No.

But Art, consider that this is a function of scale. (distance).
Consequently, if one observes the Aurora, we see a rather spectacular
effect generated by the earth- field's ability to trap and vector plasma
into the upper atmosphere over the poles.

> > 3)Since electric current is random in a plasma, there should be no
> > field. But can you make an diode plasma? Or what about lighting a
Ø > diode on fire? If you ignite an LED, will it light?

A switch composed of a mercury (Hg), an ignitron, may be used to crowbar
(short) an underdamped (oscillating) L-C circuits, and this switch tends
to behave as a poor diode, with higher conductance in one direction than
the other, especially during the period that the currents nearly die out
at cross-over.

> I can't make any sense out of this question. There can certainly be
> a net current in a plasma. A plasma surface interface acts very much
> like a diode.
>
> > 4)What is the resistance of plasma when conducting electricity, and
> > what factors influence it?
>
> The conductivity in ohm-m of a fully ionized plasma (which a flame is
> not) is
>
> 5.2e-5 * Z * ln lambda / T_e^(3/2)
>
> where Z is the charge of the ions, ln lambda, known as the Coulomb
> logarithm, is a weak function of density and temperature usually equal
> to 10 or 15, and T_e is the electron temperature expressed in electron
> volts (eV). Thus a plasma is a poorer conductor than copper unless it
Ø is hotter than about 1 keV (10 000 000 deg).

Another rule of thumb for radiating plasmas, is that its conductivity
varies with the current to the 3/2 power I^(3/2a).
However, if really, really high voltage impulse is applied about a torus
of plasma, the current electrons can be accelerated (run-away) to
essentially light speed, and the conductivity of such plasma circuits
may become several orders higher than ordinary copper conductivity.

Another interesting point is that such plasmas can confine magnetic
fields. Usually only the converse is employed as the principle to
build the crudely magnetic coil confined plasmas such as the tokamak.
.

Plasma physics is a young science, and little of its potential is known
and understood. Perhaps in a few more centuries, it might be better
established.

> --
> To study, to finish, to publish. -- Benjamin Franklin
>
> Dr. Arthur Carlson
> Max Planck Institute for Plasma Physics
> Garching, Germany
> carlson@ipp-garching.mpg.de
> http://www.rzg.mpg.de/~awc/home.html
>
> As usual, if I am caught or killed, the Institute
> will disavow any knowledge of my actions.