Re: EMI and Tesla Coils, calculation of

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Malcolm Watts by way of Terry Fritz <twftesla-at-uswest-dot-net>" <m.j.watts-at-massey.ac.nz>

Hi Jim, all,

On 5 Mar 01, at 22:13, Tesla list wrote:

> Original poster: "Jim Lux by way of Terry Fritz <twftesla-at-uswest-dot-net>"
> <jimlux-at-earthlink-dot-net>
> 
> >
> > Modelling a (vertical) two metre "spark" as a wire of ten 20cm
> > segments 2mm diameter using NEC2 is quick and simple and takes less
> > than one second on a celeron 300A.  I have used a series loading of
> > 722k-ohms per metre (220k per foot), 1uH per metre and 10pF per
> > metre. At 11Mc/s and with an exciting voltage of 500kV, the other
> > end of the spark being grounded in a perfectly-conducting ground
> > plane, the input power (reckon all this on a cw basis) is 8,0610E5
> > watts, the structure loss is 8,0609E5 watts, and just 6,5W <0,001%
> > of the input power is radiated.
> 
> I think 722k per meter might be a bit high... At 10,000 Amps, the
> voltage drop in a free burning arc is around 1000 V/meter... i.e. 1
> ohm per meter. We're probably not up to that current, but, if you
> figure 1000 amps, a voltage drop of 10 kV/meter might be reasonable,
> or even 100 kV/meter, for 10-100 ohms/meter.  Ionized gas is a pretty
> good conductor.  Data from Goncz indicates resistivity is proportional
> to the inverse of the square root of current density at values of 400
> to 10k A/sq cm.  Edgerton (who cited the data from Goncz) gives values
> of 0.01 to 0.02 ohm cm for the resistivity of the plasma in a
> flashlamp with a variety of gases and pressures.

I've long thought that rather than trying to pin down a specific 
resistance per metre that the spark will actually create a resistance 
that the charge behind it will allow. After all, we know that 
resistance in an ionized gas is dependent on current (and some 
characeristics of that gas such as ionization potentials, pressure 
etc.) and not the other way around. I suppose I'm rehashing old 
ground here but I think it matters.

Regards,
malcolm