Newsgroups: sci.physics.plasma
From news@lace.colorado.edu Fri Jul 26 20:49:00 1996
From: fcrary@rintintin.Colorado.EDU (Frank Crary)
Organization: University of Colorado, Boulder
Subject: Re: A "cold" dense plasma?

In article <4taokg$31s@mojo.eng.umd.edu>,
Arthur Carlson TOK <carlson@ipp-garching.mpg.de> wrote:
>> My question is: what kind of density would be possible at low
>> plasma temperatures in a simple mirror or Tokomak at say 5 Tesla?
>> Back of the envelope calculation assuming near ideal gas behaviour
>> would suggest a 3000 K plasma at 5 Tesla would result in
>> 10 times atmospheric density...

>Due to various instabilities, the plasma pressure in a tokamak cannot
>be more than about 5-10% of the magnetic field pressure, so you lose
>another order of magnitude on the back of your envelope. At 3000 K,
>the plasma will not be fully ionized, so you might want to go up a
>factor of 3 to 10, at least, in the temperature, which brings your
>density down again. A more serious problem is that a plasma at such
>low temperatures will radiate like mad, hydrogen less than other
>elements, but still. The electrical resistance will also be very high,
>so you will have to work hard to keep your current flowing. I don't
>know what you want to do with this plasma (no good for fusion, e.g.),
>but I don't see any fundamental reason you cannot create a tokamak
>plasma with a substantial fraction of atmospheric density.

It would be an excellent device for studying MHD and other low
frequency waves. The usual problem is that these waves have
very long wavelengths, so unless you have a huge machine,
you are only going to get a few wavelengths worth of propagation.
The only machine I know of, doing this sort of work, if the
LArge Plasma Device (LAPD) at UCLA. Let's see... For a 0.01 kg/m^3
density (about 1% of atmospheric pressure), and a 5 T magnetic
field, I get an Alfven velocity of around 45 km/s. With that kind
of magnetic field, the proton gyrofrequency would be in the
10 MHz range, so the wavelength of an Alfven wave could be as
little as a centimeter. That gives lots of room for propagation
experiments. Of course, you probably wouldn't want to go that
far: Some of the really interesting effects come from non-ideal
MHD, when the perpendicular length scales are of order c/w_p
where w_p is the plasma frequency. If you drive the mass density
up to 0.01 kg/m^3, that length scale would be far too small
for convenient work.

Frank Crary
CU Boulder