Re: Brightness of streamers vs. ground strikes

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: Bert Hickman <bert.hickman@xxxxxxxxxx>

Tesla list wrote:

> Original poster: Steve Conner <steve@xxxxxxxxxxxx>
> Hi all
> I was puzzling over something recently. When messing around with my DRSSTC-
> http://www.scopeboy.com/tesla/drsstc/woot.jpg
> http://www.scopeboy.com/tesla/drsstc/zoot.jpg
> The ground strikes do not seem any brighter than air streamers. Now on 
> pictures of larger coils, ground strikes always seem brighter and a 
> different colour. I remember one picture by Peter Terren where the air 
> streamers are dim purple and the ground arcs are a lightning-like blue-white.
> I wonder if this has to do with the frequency of the coil? As the 
> frequency goes up, the current drawn by air streamers will increase and 
> they will get brighter. Ground arcs won't change since they are purely 
> resistive.
> If we thought about this- and it seems to happen about 200-250kHz for 3ft 
> streamers since my 260kHz spark gap coil behaves about the same- maybe it 
> would tell us something about the nature of streamer loading?
> To me it says that a 3ft streamer has a resistance about equal to or 
> greater than its capacitive reactance at 200kHz. Also that the resistance 
> of a 3 foot ground arc is much the same as that of an air streamer the 
> same length. But I don't know if I'm reading too much into it.
> Also it should be qualified by length- I imagine if I let the coil arc to 
> ground over a shorter distance it would be brighter.
> Steve Conner

Hi Steve,

That's an excellent observation. Although most ground strikes are indeed 
brighter than air discharges, there are some notable exceptions. These 
include coils with low topload C (most VTTC's, or low power disruptive 
coils with tiny or nonexistent toploads). However, this effect can also 
occur on large coils where discharges can just _barely_ connect to a 
ground, such as when coilers try to measure a coil's maximum spark length, 
and it's probably not frequency dependent.

The latter has sometimes been called a "weak streamer" on this list. Weak 
streamers have always counted as valid "hits" for spark length measurement 
purposes. Normal, bright ground discharges are the result of a transition 
of cooler leaders and streamers from relatively high impedance into a low 
impedance, arc-like path that can rapidly discharge the stored capacitive 
energy from the source and leader capacitance - resulting in a hotter, 
brighter, snappy capacitive discharge.

However, weak streamers to ground still have similar intensity as air 
streamers because they occur for discharges that are near their length 
limit. In this case, the overall channel resistance (particularly along the 
thinner outermost channels/streamers) limits the far-end current available 
for the final jump. Current limiting (by the long path itself) prevents 
sufficient additional heating (that otherwise would result in the path 
transitioning to a low impedance arc). The discharge path stays at a high 
impedance and the discharge path current stays about the same.

The resulting weak discharge is similar to [source] impedance limited 
discharges - these are discussed in the literature as "inhibited" discharges.

Best regards,

Bert
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