[TCML] Streamer development recorded with high speed camera

[Bert Hickman]

Hi David,

Interesting videos - thanks for sharing them. I suspect there are at 
least two effects at work.

First, for streamers that almost or just barely connect, you'll notice 
short, seemingly isolated, brighter segments with more diffuse 
discharges coming off each end. These brighter regions are called "space 
stems" and "space leaders". They appear as sharply defined bright 
segments that are actually isolated bright leaders that are supplied bu 
current from burst corona coming off each end (negative corona on the 
end closest to ground, and positive corona on the end facing the 
approaching main negative leader). These are a normal part of the 
propagation process for negative leaders (where the HV 
terminal/discharge is negative wrt ground).

As the leader propagates, the space leader becomes longer by expanding 
in both directions, finally merging with the main leader from the HV 
terminal. If the Tesla coil reached its maximum negative potential 
before the space leader merged with the main leader, further propagation 
stops and we can easily the stranded space leader. Although space 
leaders are more readily visible with high speed equipment, they can 
also become noticeable with the unaided eye when the spark is nearing 
its limit for the applied voltage, or when the capacitance of the 
voltage source is comparatively low, limiting follow-through current. A 
description of them can be seen here (part of an excellent paper, 
"Fundamental Processes in Long Air Gap Discharges" by I. Gallimberti, 
et. al.):

http://www.capturedlightning.com/temp/Longgap1_5.pdf

If a leader fully connects, you may see part, or all, of the "return 
stroke" as a low impedance path to ground rapidly removes space charge 
from the newly connected leader. There is a sudden jump in current at 
the far end beginning from ground, and up the channel, making the return 
stroke significantly hotter (and brighter) than the previous connecting 
leader. Given sufficient time and voltage, the return stroke will flow 
all the way back to the topload, forming a bright and evenly lit spark 
or arc. However, if the topload voltage just peaked when the final jump 
occurred, and the connection is weak, the total leader channel 
resistance and declining topload voltage may prevent the return stroke 
from progressing very far up the leader channel. In this case, you'll 
see localized brightening for only a short distance from the ground 
while the rest of the spark retains the dimmer appearance of lower 
current air leaders - just the effects seen in the video clip when the 
leaders are just barely connecting.

Bert
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David Kronstein wrote:
> Hey guys,
> 
> I took an interesting video today, running my DRSSTC and high speed 
> camera at the same rate, 120Hz. This allows you to see the development 
> of the streamers burst by burst.
> 
> Youtube video:
> http://www.youtube.com/watch?v=nUkeVj8WTQ0
> 
> Higher quality AVI:
> http://4hv.org/e107_files/public/1235979158_347_FT0_tesla.avi
> 
> It takes a few pulses to grow the streamer enough to make contact with 
> the duct on the left (~1m distance). I also notice a brighter, frantic 
> arc action near the duct. I've seen this brighter section near the end 
> on Van de Graaff sparks too. Any idea what causes it?
> 
> David
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