Re: Leader Strike Photo

["Tesla list" <tesla@xxxxxxxxxx>]

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

Tesla list wrote:
> Original poster: Vardan <vardan01@xxxxxxxxxxxxxxxxxxxxxxx>
>  > Original poster: Vardan <vardan01@xxxxxxxxxxxxxxxxxxxxxxx>
>  >
>  > Hi All,
>  >
>  > This high-speed streak cam photo pretty much sums it all up!
>  >
>  > http://drsstc.com/~streakcam/instruments/SET-14/strike.JPG
>  >
> It would be interesting to see if there is some difference in the
> propagation of the streamer when the polarity is positive or negative.
> In the picture, I would say that the streamer that almost touches
> the target is positive, by the appearance of the tip with those
> branches. Lightning also shows these branches, but with the
> clouds negative and the ground positive, with the branches going
> down. Maybe branches appear with any polarity, always pointing
> in the direction of the target, where the electric field is smaller.
> Antonio Carlos M. de Queiroz

Antonio, Terry, and all,

It has been amply demonstrated that leaders and streamers are 
initiated, and propagate, at lower E-fields within a diverging 
positive E-field than within a diverging negative E-field. And, the 
underlying propagation mechanisms are significantly different for 
positive leaders versus negative leaders. Under sufficient fields, a 
positive leader will grow more or less continuously, while a negative 
leader proceeds in discrete jumps, forming bright, isolated, 
leader-like "space stems" ahead of the negative leader. Each space 
stem has streamers coming off BOTH ends - positive streamers towards 
the advancing negative leader and negative streamers on the other 
end. (BTW, this is also observed during stepped leader growth during 
negative lightning propagation). And, some of Peter's streak photos 
actually appear to show space stems. For example:
http://tesladownunder.com/HVRotMirrorPolarityNegLeader.jpg

Both Peter and Terry are presently using "rod-rod" electrode 
configurations which complicate the shape of the initial E-field in 
the gap. It would be interesting to see if they see different results 
when using a rod-plane or a rod-wire configuration instead - theory 
predicts that this should cause significantly more streamer and 
leader branching in the gap. Of course, once leaders and streamers 
start growing within the gap, previously injected space charge and 
topload voltage reversals dramatically complicate things.  =<:^O

At higher average E-fields and longer sparks, the propagation 
"advantage" of a positive source tends to disappear, and negative 
leaders appear to grow (macroscopically, at least) at fields similar 
to positive leaders. And, branching occurs with either polarity as 
the discharge propagates into weaker field areas. Similar branching 
behavior is readily observed for both positive and negative lightning.

Branching of streamers and, (for long gaps) leaders, appears to be 
inherent to dielectric breakdown of gaseous, liquid, and even solid 
dielectrics (electrical treeing and Lichtenberg figures). Branching 
angles fall between 30 - 40 degrees (i.e., 15 - 20 degrees on either 
side of the previously unbranched leader direction). And there are 
lots of interesting theories (and various simulations) that hint as 
to why this is the case.

Bert
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