[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: Leader Strike Photo

Original poster: "Peter Terren" <pterren@xxxxxxxxxxxx>

Thanks Bert for your perceptive comments
I regard myself a a noob doing play physics with a TC, a motor and a few LED's.
I would prefer a GHz rated Rogowski coil integrator, digital storage and spectrum analyser coupled to an image intensifier with field plates that allow image shifting in the nanosec range. Somehow I wonder if that would not be quite as much fun but more a job to fulfil the requirements of a grant and with pressure from supervisors to publish. 

Comments interspersed.


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.
You are of course correct. For example streamers enlarge to the naked eye on a second timeframe. I have wondered about doing some shots from a slowly rotating mirror. It is only recently that I have had a full view of a 2 foot spark but I need a better mirror.
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.
The multistage LED's are a poor man's approach to the problem but may provide some interesting information over a very wide current range - potentially microamps to kiloamps ie 9 orders of magnitude or more. I hope to get some shots this weekend. Beyond that a Rogowski is possible but I have zero experience at MHz plus design. I wonder about a passive integrator and my 100mHz CRO though. That shouldn't be too hard. Or just simple voltage drop across a 50ohm resistor at the ground end. Coordinating the image and CRO would be needed. 


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...
Single shots are not a problem but if you want to see consecutive bangs then a slower running mirror is needed otherwise some complicated triggering of the coil when the mirror is in position is required.
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. 

A rod/plane gap is something I want to try.
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? 

Refer to my wish list...
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.
I agree that TC's have more of interest in the accessible timeframe with greater safety.
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. 

Lots of interesting stuff to explore.


This is really some fantastic work!

Best regards,

Bert