[TCML] Re: Spark models, revisited

["Udo Lenz" <udo_lenz@xxxxxxxxxxxxxx>]

Jim Lux wrote:

> You have to be careful when looking at some of the equations because they
> might be normalized for "constant density" or something, and, as you've
> noted there's lots of stuff going on.

I agree, but the assumption of power radiated being proportional to r and
the conductivity being proportional to r^2 is very seductive, since it leads
to 1/R ~ P^2.

I rather like that idea of the black body radiator you suggested.

> A lightning stroke is on the order of 20kA and has a diameter of about
> 0.5cm. 10-100kJ/meter dissipation. Yes, your sparks ARE thin, they just
> look bigger because of the high contrast. Also, recall that air at 2000K
> will be noticeably glowing and bright, so the "visibly glowing" size is
> bigger than the "conductive core" size.

That poses another question. A thin conductor does not have much
capacitance. The model calculations for a real arc indicate about
10 to 20 pF/m.
That is much more than a 0.2mm wire would have. Possibly a thin
conductor like that would be a perfect breakout point for sideways
arcs or create corona around the main arc, adding capacitance.

The long straight arcs, which can be achieved by slow voltage ramp up
don't look like that, though.

Another idea about this is, that due to the abundance of charge carriers
around the arc, they would drift in and out following the RF voltage.
Drift velocities for electrons is about 10cm/us at fields around 10kV/cm.
This would effectively thicken the arc and also increase its capacitance.
If this is the case, I would assume the capacitance to be lower
for higher frequencies, which would give the charge carriers less
time to drift and larger for higher voltage, which would push them more.

Udo


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