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[TCML] Re: Slo-Mo Videos of Tesla Coil
Bert wrote:
Ed and all,
Even though positive and negative nitrogen and oxygen ions are
relatively slow to recombine, they really don't contribute much to the
spark breakdown processes. The heavy lifting is really done by free
electrons. However, these typically have a mean lifetime of only 11 nsec
in air. So why does the next spark retrace most of its predecessor's path?
It's most likely that hot air channels, left by the previous
discharge(s), provides the weakest link. Because of its lower density,
hot air has a significantly lower breakdown, or reignition, voltage.
Electrons have a longer mean free path, and can pick up more energy from
the ambient E-field before crashing into air molecules, thereby
enhancing the avalanche processes that culminate in breakdown. During
the next ringup, the left over hot channel is more easily reignited, and
the following discharge traces a similar path along the hotter portions
of the previous discharge (i.e., the arc root and main leaders). If the
bang rate is decreased sufficiently to allow these channels to cool to
the point where they recover most of their dielectric strength, (say 60
BPS or slower), the main paths will begin to change between bangs, even
though the ion density is almost the same.
The cooler streamer tips generally follow different paths, since the air
can more completely recover its original dielectric strength between
bangs, and the very streamer tips never rise much above ambient
temperature to begin with. However, uneven distributions of ions from
previous discharges (injected space charges) do come into play,
resulting in streamer tips that radically (even frantically) change
directions between successive bangs.
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
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