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Re: Leader Strike Photo

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

Hi Terry,
You and Peter are really breaking some new ground in TC research! However, the phenomena we're interested in span many orders of magnitude (in light intensity, current, and time), and optical measurements alone don't tell us the complete story. Also, there's no synchronization between the TC and the rotating mirror, leading to many more "misses" than successes.
And, although using LED's to show polarity is a great advance, it doesn't provide an accurate measure of current magnitude versus time. Although peak brightness does scale relatively linearly to LED current, no intensity measurement are presently being done (via a fiber optic coupling and PIN optical detector or phototransistor). The bar display approach was ingenious - too bad it is too slow. And, although using a series of LED's to fire at various current levels is another excellent idea, even this does not provide the accuracy that's available via Rogowski or Pearson type current transformer. 

Some thoughts going forward:
1. Add triggering capability to your research coil in order to allow you to sync on either single shots, or a sequence of multiple discharges. The idea is to trigger the TC at a consistent point relative to mirror position using a consistent bang size. A less attractive alternative (since it limits mirror speed) would be to synchronize the mirror relative to the incoming line waveform (i.e., near the voltage peak) to better synchronize the coil and mirror. Directly triggering the research coil would the better approach in order to minimize jitter and maximize flexibility. This would also become essential if you wanted to try to capture actual streamer growth using higher mirror speeds. Your DRSSTC may actually provide better research coil for these tests...
2. Float a battery powered DSO at the top of the toroid (in an appropriate shielded enclosure) and use one of your Pearson wideband current transformers to capture the actual current waveforms exiting from the HV electrode (sort of a miniature version of Greg Leyh's Electrum measurements). Or, use your optically isolated current measurement hardware to do a similar thing. The floating DSO approach may also require an optically-coupled trigger circuit that will allow you to sync the scope's trigger with the TC "ringup".
3. Use a comparatively large suspended ground plane above the TC toroid and an upward pointing rod for a rod-plane gap. This may help in correlating observations with those in the literature. By increasing distance or decreasing output power, both incomplete and complete leaders/sparks can then be studied.
4. It may be impossible to capture actual streamer and leader growth without using considerably higher mirror speeds, much better synchronization, and (perhaps) an image intensifier. Maybe an inexpensive night vision scope could be interposed between the mirror and camera to obtain a poor man's version of an image intensifier to capture lower light events?
Most of the technical literature focuses spark propagation during unipolar HV impulses (typically lightning-like long-tail waveforms generated by Marx generators). Although a small Marx generator could also be used for these experiments, I would fear for the safety of your test equipment (and, potentially, the experimenter).
Using a TC is also of more direct interest since it's quite likely that periodic voltage reversals, coupled with the growing output voltage during ringup, and residual space charge the discharge immediately preceding the current one, all conspire to aid in TC streamer/leader growth (versus a unipolar waveform having a similar rising envelope). And then we add bang-bang growth. The interplay ultimately explains why TC leaders are so much longer than expected versus output voltage.
Comparing sync'ed measurements of terminal voltage and current with the optical measurements should shed much more light on the macroscopic and dynamic behavior of TC leaders and streamers. By first using single shots to characterize growth during ringup, we can then use a sequence of successive shots to shed more light on bang-bang growth. At this stage getting repeatable, synchronized single shots is probably the most important next step. 

This is really some fantastic work!

Best regards,

Bert


Tesla list wrote:


Original poster: Vardan <vardan01@xxxxxxxxxxxxxxxxxxxxxxx>
Hi Bert,
I think our "newfound" high-speed streamer photo advantage is now in day 19 (much spent in ordering special parts!)... Here, things or in total chaos and confusion %;-)) The LM3914 thing fell flat, and now LM319 things are on the mind. Long since forgot which polarity is which anyway... @:D I think both Peter and I stare at "hundreds" (approaching thousands) of photos and we "select" which ones to tell about... My coil is a solid state DC disruptive coil with very reproducible polarity and power ringups... Almost "cheating"... The whole polarity could be easily reversed by switching primary leads... But we have "too many" variables"... To many things "to do"... We can provide the data, but there is so much!! We can't figure it all out... So tell us what "you want"? We have more knobs than we have fingers now... Maybe more folks will hook up mirrors to get a more divergent view on all this!! "Great pictures" are running 1/50 now... I need to order more NiMH batteries to keep up ;o)) 
Higher RPM on the mirrors seem to really help now to streeeaaatch
things out...   We have more answerers than we have questions...
Cheers,
        Terry


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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