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



Greg Leyh wrote:
<snip>


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.

Bert
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