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Re: Leader Strike Photo



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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