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Re: [TCML] Re: Slo-Mo Videos of Tesla Coil

Greg Leyh wrote:
Bert wrote:

Greg Leyh wrote:
> > Hi Bert,
> > I remember reading that an arc discharge maintains a roughly constant > current density as it grows. This implies that a higher current arc > would leave a larger diameter column of hot air in its wake. Given the > squared/cubed cooling effects on the hot air channel, would you imagine > that a coil that launches higher current arcs might have a lower optimum > BPS for arc length vs. power? GL >

Very interesting question! I think that Jim Lux is correct - the rapidly
changing voltage and current envelopes of a Tesla Coil are pretty far removed from free-burning arcs in equilibrium.

However, TC arc channels do indeed get fatter (and hotter) with
increasing branch current, reaching maximum diameter in the main current
carrying arc root. Reviews of high speed TC video footage show that decay of the plasma afterglow begins at smaller, lower current branches, disappearing last in the main discharge root.

Measurements of the free-air recovery time (i.e., the time to recover
90%+ of the initial dielectric breakdown strength) for both short and
long air gaps tends to be in the range of 5 - 10 ms from current zero. Surprisingly, this is fairly independent of prior arc current or gap spacing. (See the discussion on pages 292-294 in "Gas Discharge Closing Switches" by Schaefer and Guenther, ISBN 0306436191). Full dielectric recovery in free air is normally complete after 20 ms. This implies that the minimum bang rate that supports spark growth from bang to bang lie somewhere between 50 - 100 BPS... irrespective of available discharge current.

While faster break rates may permit easier reignition of increasingly smaller current channels, it's not clear that this will contribute much to actually increasing spark length. I suspect that increasing topload capacity and peak output voltage are considerably more important for obtaining maximum spark length. These would indeed increase maximum channel current, but I suspect that maximum spark length would otherwise be be relatively independent of break rate as long as you were significantly higher than ~100 BPS.


Hi Bert,

Interesting, that recovery time is fairly independent of prior arc current or gap spacing. It makes sense that increasing topload capacity and peak output voltage are important towards obtaining maximum spark length. So, given a fixed energy per bang available to the secondary, how might you imagine trading off voltage and topload C? GL

Hi Greg,

I'm not sure there is a clear answer to this question. My 2 cents worth for large coils...

Let's assume that your topload is sized so that that it provides sufficient electrostatic shielding for your secondary, the minor ROC is large enough to prevent initial breakout for the highest expected secondary voltage, and the topload has an adjustable breakout bump/projecting rod to tweak initial breakout voltage and direction.

One could argue that a topload that satisfies the above criteria may provide the optimal trade off between topload C and output voltage for a large system. The "larger toroid is better" strategy that worked so well for smaller systems may reach a point of diminishing returns on large systems. Since large systems already use physically large toploads to satisfy shielding requirements, the resulting topload C already provides a sufficient reservoir of charge to support efficient leader/streamer propagation. Further oversizing the topload lowers peak voltage but may not significantly improve spark growth processes. Spark propagation will cease when the field at streamer tips drops below about 5 kV/cm. Spark length is (ultimately) constrained by maximum terminal voltage. The objective should be to maximize V (commensurate with a topload sized to meet shielding and breakout restrictions).

Your thoughts?

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