Re: 'Glow' discharge

To: tesla-at-pupman-dot-com
Subject: Re: 'Glow' discharge
From: "Jim Lux" <jimlux-at-jpl.nasa.gov> (by way of Terry Fritz <twftesla-at-uswest-dot-net>)
Date: Thu, 17 Feb 2000 17:10:26 -0700
Approved: twftesla-at-uswest-dot-net
Delivered-To: fixup-tesla-at-pupman-dot-com-at-fixme

-----Original Message-----
From: Tesla List <tesla-at-pupman-dot-com>
To: tesla-at-pupman-dot-com <tesla-at-pupman-dot-com>
Date: Thursday, February 17, 2000 5:40 AM
Subject: 'Glow' discharge


>Original Poster: "Sarah Thompson" <sarah-at-telergy-dot-com>
>
>Hi,
>
>I'm new to the list. I've not yet built a TC, so I'll refrain from asking
>lame questions until such time as I'm half way through it and can't get it
>working. :)

No no no.. ask the lame questions now, if only for our entertainment
purposes so we can all laugh at you <grin>...

Really, the time to ask questions is before you break something.. that
is,... now..  Then, when something goes wrong, you can post what happened
and why ("Well, perhaps winding the secondary on the cat's scratching post
wasn't such a great idea, because.." or alternately, "Hey, guys, winding the
secondary on a cat scratching post is really great, because the cat's fur
helps reduce the corona..." )

More below...


>
>The reason I want to make a TC is for the purpose of creating ionised gas
>for experiments in MHD. Before I start figuring out how big a coil to make,
>I was wondering if people out there can answer a couple of questions about
>the kind of voltage levels that will be necessary.
>
>One experiment I want to do is related to MHD power generation. I'd like to
>be able to ionise the air moving along a non-conductive pipe, whilst also
>causing a hefty current to flow across it (through the gas). The electrodes
>need to be longitudinal with respect to the pipe, and maybe 1 - 2" apart
and
>parallel. If the theory works, it should be possible to blow gas through
the
>pipe and extract power from a coil wound around the pipe. I suspect that a
>neon transformer on its own might be enough, but I'm willing to accept
>advice on this.

It takes a fair amount of power to ionize large amounts of air at
atmospheric pressure.  The trick is to create a plasma, without creating an
arc.  As far as voltage goes, you need 70 kV/inch to break down air in a
uniform field.   You can get noticeable ionization by using sharp points and
a few kV, because the field is very high on the points.  You might want to
experiment with a set up that can run at reduced pressure (say a few tens of
torr), which you can get to with a water aspirator, and then run Argon as
your working gas. (Cheap and available and, because it is monoatomic, it
stays ionized a while)

A very popular way to generate an ionized flowing plasma is to use a
microwave oven.  Run the tube through the oven compartment.  Even fancier is
to make a cylindrical cavity with the magnetron sticking off the side, and a
tube with the working gas through the central axis of the cavity.  You can
email me off list if you need more details on this approach, which is cheap,
easy, and very efficient.
>
>The other experiment is a little weirder. What I'd like to be able to do is
>create an even 'glow' discharge around a spherical or dome shaped
conductor,
>such that the air around the conductor gets ionised, but streamers or
sparks
>are avoided. Is this even possible with a TC? If so, what kind of voltage
>would be necessary for a moderate sized dome or sphere? I'd guess that a
>pretty high voltage will be necessary, but avoiding arcing would seem to be
>the hardest bit.

More a pressure and voltage issue to get a glow discharge.  Getting an even
glow discharge but no sparks is a bit of a challenge, as once the glow
starts, it wants to form arc channels.  Low pressure helps a lot.
>
>Thanks in advance,
>Sarah Thompson
>
>
>