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Re: Streamer modeling

Original poster: Terry Fritz <teslalist@xxxxxxxxxxxxxxxxxxxxxxx>

Hi Antonio,

At 08:14 PM 6/30/2005, you wrote:
Tesla list wrote:
Original poster: Terry Fritz <teslalist@xxxxxxxxxxxxxxxxxxxxxxx>
Hi All,
I had a chance to look through the book "Spark Discharge" and it appears they have things pretty much figured out!! They come up with most of the same numbers we do and add some wonderful things like "optimal rise time" :-)))) 111uS for a 2 meter streamer ;-))
They get about 1.9pF/foot for streamer capacitance which is pretty close to our "average" number taking various factors (slow resonant rise) into account. It is pretty straight forward adding a dynamic model to ScanTesla...
They have some pretty good math behind their numbers but they obviously really "checked it" too ;-))
The book is actually worth $130! It is from CRC Press too (which apparently has presses made of gold, diamond, and platinum....) so it is cheap for their books!
I will do some test (or maybe E-Tesla can do it) to better lock down the numbers for our case and see if I can get it into ScanTesla... 

It's simple to change the code to allow time-varying elements. I am testing
now a version of the calculation engine where the capacitances and resistances
can change. The present model for the streamer load, C2 (part), R3, and C3,
can then be made to change to simulate breakout and streamer growth.
The problem is how they change.

I am working on that function now. Actually the computer is ;-)) But it is a simple capacitance vs streamer length function. Apparently about 1.8pF per foot is standard, but in our case, we need to take into account the field effects near the big top terminal.

In a first approximation I am trying to
leave R3 constant and increasing C2 and C3 when the terminal voltage
exceeds a certain breakout voltage, by equal amounts that return the terminal
voltage to about the breakout voltage.

A streamer does not present enough load to the top terminal to limit its voltage much. It really is just an increase in loss. Apparently, the resistive part is fairly fixed. Our old 1pF+220k thing was not very far off. But now we can actively vary the capacitance with streamer length during the firing oscillations in real time...

The result is that the load capacitance
increases at the voltage peaks, and more power is dissipated in the fixed
streamer resistance R3. Detuning limits the increase at some point.
If this makes sense, streamer length can be estimated by the increase in
the streamer capacitance.

Makes some sense. The streamer travels at "above" 1m/uS! so the time it takes for the streamer to reach a distance is almost not a factor. Path heating does stretch that distance some for the streamer that is following and already heated path. Most of the power is dissipated at the less than 1mm "tip" of the streamer which is a fairly constant resistive load regardless of length. Most of the streamer is a good conductor.

I need to study it more though....

Cheers,

        Terry



Antonio Carlos M. de Queiroz