Re: Quenching Theory Question (fwd)

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Original poster: List moderator <mod1@xxxxxxxxxx>



---------- Forwarded message ----------
Date: Fri, 18 May 2007 12:04:11 -0700
From: Jim Lux <jimlux@xxxxxxxxxxxxx>
To: Tesla list <tesla@xxxxxxxxxx>
Subject: Re: Quenching Theory Question (fwd)

At 08:34 AM 5/18/2007, you wrote:
>Original poster: List moderator <mod1@xxxxxxxxxx>
>
>
>
>---------- Forwarded message ----------
>Date: Fri, 18 May 2007 11:19:35 -0400
>From: "Breneman, Chris" <brenemanc@xxxxxxxxxxxxxx>
>To: tesla@xxxxxxxxxx
>Subject: Quenching Theory Question
>
>Hello,
>
>I'm trying to figure out some simulation parameters for a spark gap, 
>and had a few questions about arc formation and quenching.

This can be a fairly complex topic.  You might want to get a book 
called Gaseous Conductors by John Cobine. It was published by Dover 
press in a softcover edition and might be still available.  Or, 
interlibrary loans can get it.

>  Is it correct that the point at which an arc forms is entirely 
> dependent on the potential difference between the contacts?

No.. there's lots of other factors that come into play, especially 
things like temperature.  If you have a hot spot, it will tend to 
emit more electrons, and becomes a more likely spot for future arcs.


>  I've seen a lot of tables that relate arc length to voltage, so if 
> this is correct, I could find out all of the arc formation parameters myself.


Are you looking at ARC length or SPARK length.. they're very 
different.  The latter is typically what's found in breakdown tables 
(e.g. http://home.earthlink.net/~jimlux/hv/sphgap.htm ).  once the 
gap is bridged, and current is flowing, you have an ARC, which has 
very different properties.  For instance, you can start with a short 
arc and "draw" it much longer (something familiar to anyone who has 
done arc welding or operated a carbon arc lamp, or for that matter, 
watched a jacob's ladder)

A free burning arc looks kind of like a fixed voltage (called the 
cathode drop) plus a voltage drop along the arc column.  The drop 
along the column scales with length (roughly) and also with the 
current in the arc (which sets the arc diameter, and hence, the 
resistivity, since most free burning arcs run about the same 
temperature.. some 7000K)


>   Also, I was wondering what determined when an arc quenched.

When the thermal balance is such that the energy dissipated in the 
arc isn't sufficient to keep the air ionized.

>   I know that once a gap fires, the voltage can drop and/or 
> oscillate significantly without the arc extinguishing, but how low 
> can the voltage/current drop before a typical arc extinguishes for 
> a given distance?  The distance involved here is going to be only a 
> few millimeters, so that's what I'm interested in.

Depends a lot on the electrode materials and the temperatures 
involved.  If you get the electrode good and hot, you can have 
significant current flow just from thermionic emission. (like in a 
vacuum tube diode)


>   Like, how low could the voltage/current across a gap of a few 
> millimeters get before quenching, with moderate air flow?

ALmost impossible to predict from first principles.  You have to 
build a test setup and try it.  There's a huge number of variables.

>I also had a few questions about resistance encountered in a 
>gap.  Is the resistance generally constant with a given gap width?

No

>  And what kind of resistances are usually encountered?

The resistance is somewhat nonlinear.  The voltage drop is 
proportional to 1/sqrt(current).

What sort of currents and arc lengths are you looking at, and we can 
probably give you a reasonable number.

If you check the literature on flash tubes (by Edgerton and Goncz, 
among others) you can find some empirical data for common 
situations.  Cobine's book (mentioned above) has lots of data too.


>Thanks a lot,
>Chris