Re: [TCML] Spark gaps (again)

["Dex Dexter" <dexterlabs@xxxxxxxxxxx>]

--- jimlux@xxxxxxxxxxxxx wrote:

From: jimlux <jimlux@xxxxxxxxxxxxx>
To: Tesla Coil Mailing List <tesla@xxxxxxxxxx>
Cc:
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:
> http://www.mintek.co.za/Pyromet/Files/1997Curr.pdf
> 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.
> 
> Dex 
> 
> 
Interesting paper.

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.
Lightning:
I=20-100 kA
dI/dt=10^10-10^12 A/s

Tesla primary gap:
I=100-2000 A
dI/dt=10^7-10^9 A/s

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.

Dex  

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