[TCML] Spark gaps (again)
dexterlabs at dcemail.com
Tue Mar 10 12:32:48 MST 2009
--- jimlux at earthlink.net wrote:
From: jimlux <jimlux at earthlink.net>
To: Tesla Coil Mailing List <tesla at pupman.com>
Subject: Re: [TCML] Spark gaps (again)
Date: Mon, 09 Mar 2009 20:37:06 -0700
Dex Dexter wrote:
> I'm not so sure about thin that much final diameters for a Tesla gap spark.
> Read here (section 3.2) about diameter of steady state and low frequency arc:
> I took in my estimate 1000 A primary current arc to expand to about d=10 mm.
> 100 A current is probably closer to low powered coil,but it depends also
> on operating frequency.
> What is radial average velocity of expanding column of such arcs it's hard to tell.
> I don't have data and I'm interested to learn.
The diameter data they give is 3cm at 7.8kA. Also a cathode spot
current density of 2.4kA/cm2. At a primary current of 100A, that energy
density works out to .04 cm2, a radius of about 1mm
If you consider a typical medium sized coil with 130 uH and 0.01 uF
primary charged with 15kV, the current is about 130A. (based on LI^2=CV^2)
I'll have to track down the paper by Latham, since it apparently talks
about lightning, which is a fast transient, probably more representative
of what's going on in a TC primary gap.
I'm afraid that a direct comparison with lightning can introduce large errors.
Tesla primary gap:
But if we know time-like dendencies among column expansion rate,conductivity,and cross-sectional current distribution
than that Paper could be of use.
Measurments of voltage drops on primary gaps during transients and in D.C. arc regime will be the best ,and ultimate,judge.
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