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.
Bert
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