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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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